250 not provide for exceptions from emissions standards basedare not relying on de minimis principles where a MACT floor exists.'" ( See National Lime at 640.) We recently reaffirmed our position on the unavailability of de minimis exemptions from the MACT floor in the final rule regulating organic liquids distribution. ( See 69 FR 5038, 5048 49 ( February 3, 2004).)

We see no reason to revisit this fundamental issue.

Section 112 of the CAA is replete with careful definitions of mass or effect based limitation on regulation,

indicating that Congress has already defined what amounts of HAP emissions are too small to warrant MACT standards or other controls under section 112. The requirement to adopt MACT emission limitations, for example, applies without exception to " each category or subcategory of major sources... of [ HAP]." ( See CAA section 112( d)( 1).) For sources below the major source threshold, however, we have discretion to require " generally available control technologies or management practices." ( See CAA section 112( d)( 5).) Congress has, thus, itself defined which sources' emissions are small enough not to warrant mandatory MACT standards.

Congress likewise defined several MACT exceptions applicable where emissions have de minimis health effects.

Section 112( d)( 4) of the CAA allows us to establish 251 standards less stringent than MACT for HAP with an established health threshold, so long as we set a standard below the health threshold with " an ample margin of safety."

Section 112( b)( 3)( C) of the CAA directs us to delist HAP   precluding section 112( d) MACT standards   if we determine that " there is adequate data on the health and environmental effects of the substance to determine that emissions,

ambient concentrations, bioaccumulation or deposition of the substance may not reasonably be anticipated to cause any adverse effects to the [ sic] human health or adverse environmental effects." Section 112( c)( 9)( B)( i) of the CAA ( discussed further below) lets us delete source categories and subcategories from the category list   the consequence again being no MACT control   if we determine that, for emissions of carcinogenic HAP, " no source in the category... emits such [ HAP] in quantities which may cause a lifetime risk of cancer greater than one in one million to the individual in the population who is most exposed to emissions of such pollutant from the source." For noncarcinogens, we may delete source categories and subcategories if we determine that " emissions from no source in the category or subcategory... exceed a level which is adequate to protect public health with an ample margin of safety and no adverse environmental effect will result from emissions from any source." ( See CAA section 252 112( c)( 9)( B)( ii).) Moreover, in defining which source modifications trigger additional regulatory standards, CAA section 112( g)( 1)( A) mentions a " greater than de minimis increase in actual emission of a [ HAP]." This shows that Congress knew how to use the de minimis concept when it considered it appropriate in section 112, and the fact that Congress did not use it in section 112( d)( 3) supports our   and the D. C. Circuit's   conclusion that it is unavailable to support an exception to a MACT floor that is not otherwise authorized under section 112.

We do not find persuasive the proposition that the overall purpose of CAA section 112 is protecting human health and the environment, and that, therefore, as long as this general purpose is met, we may fashion de minimis exceptions from MACT beyond those allowed under section 112.

First, this position appears to assume that the issue is to be drawn on a clean slate, while the D. C. Circuit has affirmed our view that section 112( d)( 3) provides no discretion to use a de minimis rationale to avoid MACT.

Second, the commenter appears to give prominence to an overarching statutory goal over the specific language of the statutory provisions themselves, in assessing whether those provisions are extraordinarily rigid regarding EPA's otherwise inherent de minimis authority; the logical extension of such an approach would be to find that no 253 single provision in the CAA could restrict our de minimis authority, in light of the CAA's over arching purpose " to promote the public health and welfare." ( See CAA section 101( b)( 1).) Third, the commenter does not present any persuasive statutory arguments to overcome those that we presented to the court   and which the court affirmed   in National Lime. Fourth, we are unable to discern the basis for the commenter's suggestion that we have in fact been relying on de minimis authority in the MACT program for several years in establishing applicability thresholds, and we are not aware of any instance in which we have explicitly justified an exception from an applicable MACT floor based on a de minimis rationale that would be, like the commenter's requested exemption, in contravention of the Court's ruling in National Lime. Fifth, notwithstanding any asserted overall risk based goal of CAA section 112, CAA section 112( d)( 2) repeatedly directs us to eliminate emissions, where feasible, and CAA section 112( d)( 6) imposes an ongoing obligation for us to review and revise standards as necessary to account for developments in technology, and neither of these specific goals is restricted to situations where health targets have not yet been reached. Finally, to the extent the commenters believe such a de minimis exemption is justified by the wish to reduce the costs of the PCWP rule as proposed, we are not free to grant a de 254 minimis exemption to account for costs. Congress already took cost into account in section 112( d), relying on prior business judgments by the best performing sources to substitute for the judgment of the rest of the PCWP industry, therefore, denying us the leeway to consider costs as a factor to modify the MACT floor. Only in considering more stringent beyond floor standards may we consider costs in the MACT context. Therefore, we do not feel it is appropriate or necessary to revisit our and the D. C.

Circuit's prior conclusions regarding the availability of the de minimis principle in today's final PCWP rule.

for today's action, and therefore, do not need to respond to these comments.

Section 112( c)( 9) of the CAA Comment: Two commenters opposed using subcategorization as a mechanism to exempt facilities. One of the commenters stated that subcategorization is a tool that should be used in the standard setting process, and using it to exempt facilities would have a detrimental effect on the stringency of the MACT floor and would generally degrade the standard.

According to the commenter, the two step subcategorization proposal is inconsistent with how subcategorization has been done in numerous previous NESHAP.

The other commenter argued that EPA's subcategorization theories are unlawful. According to the commenter, CAA 255 section 112( c)( 9) does not authorize EPA to separate identical pollution sources into subcategories that are regulated differently to weed out low risk facilities or reduce the scope/ cost of the standard. The commenter stated that subcategories based solely on risk do not bear a reasonable relationship to Congress' technology based approach or the statutory structure and purposes of CAA section 112, and are not authorized by the CAA. According to the commenter, categories and subcategories are required to be consistent with the categories of stationary sources in CAA section 111. The commenter was not aware of any instance in which EPA has established categories or subcategories based on risk. The commenter stated that EPA routinely defines subcategories based on equipment characteristics ( e. g., technical differences in emissions characteristics, processes, control device applicability, or opportunities for P2). According to the commenter, EPA has not offered any explaination for why reinterpreting the statute to ignore nearly 12 years of settled practices and expectations under the MACT program is reasonable, nor why reducing the applicability of HAP emission standards serves Congress's goals in enacting the 1990 CAA Amendments.

The commenter noted that EPA's discussion of the risk based exemptions was contained in a preamble section entitled, " Can We Achieve the Goals of the Proposed Rule in 256 a Less Costly Manner," which strongly suggests that EPA's motivation for considering these risk based approaches is consideration of cost. The commenter cited prior EPA documentation and stated that EPA in the past has rejected the notion that cost should influence MACT determination,

and this prior, consistently applied interpretation better serves the purposes of CAA section 112. The commenter argued that subcategorizing to set a no control MACT floor is the same as refusing to set a MACT standard because the benefits would be negligible, which is unlawful.

The commenter also stated that CAA section 112( c)( 9)( B)( i) does not authorize EPA to delist subcategories. According to the commenter, section 112( c)( 9)( B) contains two subsections: subsection ( i)

refers only to categories, and subsection ( ii) refers to both categories and subcategories. The commenter argued that the absence of the term " subcategories" in section 112( c)( 9)( B)( i) indicates a Congressional choice not to permit the Administrator to delist subcategories of sources under section 112( c)( 9)( B). The commenter stated that this is consistent with Congress' decision to require a higher standard to delist categories that emit carcinogens.

According to the commenter, the section 112( c)( 9)( B)( ii)

requirement of less than one in a million lifetime cancer risk for the most exposed individual is a higher and more 257 specific standard than the standard for other HAP.

To the contrary, two commenters stated that EPA has ample authority under CAA sections 112( c)( 1) and 112( c)( 9)

to create and delist low risk categories or subcategories.

According to the commenters, section 112( c)( 1) provides the Administrator with significant flexibility to create categories and subcategories as needed to implement CAA section 112. One commenter stated that there is nothing in the statute that limits the criteria the Administrator can use in establishing categories and subcategories. The commenter added that there is also nothing in the history of EPA's interpretation of section 112( c) that precludes subcategorization based on risk. In addition, EPA has stated that emission characteristics are factors to be considered when defining categories.

The commenter stated that application of statutory authority to exclude sources from regulation under section 112( d)( 3) is also supported by relevant case law, e. g., in the Vinyl Chloride case. ( NRDC v. EPA, 824 F. 2D 1126 ( D. C.

Cir. 1987)) According to the commenter, the court in that case established a range of acceptable levels of risk in establishing limits under prior language in section 112, and the establishment of an acceptable level of risk could be used to create a low risk subcategory that could be delisted. The commenter stated that technological or 258 operational differences among sources may also help discriminate between low risk and high risk sources. The commenter stated that effective use of section 112( c)( 1)

authority to create risk based subcategories would significantly improve the cost effectiveness of the section 112 program without undermining its role in protecting public health and the environment.

Both commenters noted that CAA section 112( c)( 9)( B)

provides EPA with broad authority to remove from MACT applicability those categories and subcategories of facilities whose HAP emissions are sufficiently low as to demonstrate a cancer risk less than one in a million to the most exposed individual in the population ( for non threshold carcinogens) and no adverse environmental or public health effect ( for threshold HAP). ( The commenter asserted that Congress used the terms category and subcategory interchangeably, indicating that either one can be delisted.) One commenter suggested that sources able to demonstrate a basis for inclusion in the delisted category on a case by case basis would then be exempted from the MACT, subject to possible federally enforceable conditions designed by EPA. The commenter stated that the new category could include the following: all low risk facilities,

facilities producing wood products found to pose no expected risk to human health ( i. e., fiberboard, medium density 259 fiberboard and plywood), facilities with acrolein emissions below a certain threshold, or facilities selected on the basis of some other risk criterion. The commenter suggested that the low risk category be included in the final rule and delisted within 6 months following publication of the final rule. The delisting notices would designate health benchmarks and facilities would be required to submit evidence ( e. g., tiered dispersion modeling) demonstrating that their emissions result in exposures that fall below the benchmarks. Following delisting of the category, an affected source could apply to EPA for a determination that it qualifies for inclusion in the low risk category. After evaluating the source's petition, EPA would issue a written determination of applicability based on the petition that would be binding on the permitting authority ( unless the petition was found to contain significant errors or omissions) and appealable by the affected source or interested parties. The EPA could require all facilities that qualify for inclusion in the delisted category to comply with federally enforceable conditions, similar to the conditions established in permits for synthetic minor sources ( e. g., limits on potential to emit, production limits).

The commenter also responded to objections regarding the subcategorization and delisting of low risk facilities. The 260 commenter stated that the contrasting of the terms category and subcategory offered a distinction that in no way limited EPA's authority to delist low risk facilities. According to the commenter, the argument that EPA cannot create subcategories based on risk is contradicted by the statutory language, which expressly states that the categories and subcategories EPA creates under CAA section 112 need not match those created under CAA section 111. Furthermore,

prior EPA statements do nothing to detract from EPA's broad discretion to establish categories and subcategories. The subcategorization factors previously discussed by EPA justify subcategorization based on risk. The authority cited by one commenter does not establish that EPA's discretion to alter subcategorization is limited in any way,

and even if it were, EPA is not bound by any prior position.

The arguments that EPA may not delist subcategories for carcinogens ( or sources emitting carcinogens) rest on a formalistic distinction that EPA previously has rejected as meaningless, and that, at any rate, can be remedied with a simple recasting of a subcategory as a category. The commenter stated that doing so is undisputedly within EPA's authority.

Three commenters addressed the issue of subcategorizing PCWP facilities based on characteristics other than risk.

One commenter stated that the only option that appears 261 consistent with the CAA, does not create excessive work for State and local agencies, and may be able to be based on science, is the subcategorization and delisting approach.

However, the commenter added that the subcategories should be based on equipment or fuel use, not risk. The commenter stated that a subcategory based on site specific risk creates a circular definition and does not make sense. The commenter also stated that subcategory delisting should occur before the compliance date so that facilities do not put off compliance in the hope or anticipation of delisting.

The second commenter stated that EPA requested comment on the establishment of PCWP subcategories ostensibly based on physical and operational characteristics, but in reality based on risk. According to the commenter, this indirect approach is just a variation on the approach ( direct reliance on risk) that it claims EPA itself notes would disrupt and weaken establishment of MACT floors, and is accordingly unlawful. The commenter stated that, even if these approaches were lawful, to the extent that EPA's proposal could be read to suggest that facilities could be allowed to become part of the allegedly low risk subcategory in the future without additional EPA rulemaking, this too would be unlawful. According to the commenter, CAA section 112( c)( 9) provides the EPA Administrator alone the authority to make delisting determinations, and such authority may not 262 be delegated to other government authorities or private parties. The commenter stated that EPA's proposal suggests an approach entirely backward from the statute   allowing sources to demonstrate after the fact that they belong in a subcategory that has been delisted under section 112( c)( 9),

when the statute requires that EPA determine that no source in the category emits cancer causing HAP above specified levels, or that no source in the category or subcategory emit non carcinogenic HAP above specified levels, by the time EPA establishes the standard. The commenter stated that EPA has provided no explanation of how the suggested approaches would be lawful or workable.

The third commenter indicated that low risk is an adequate and appropriate criterion for categorization. The commenter disagreed that EPA should create and delist categories on a technology basis when the intent is delisting of low risk facilities. The commenter believed that seeking a technology based surrogate for risk is unnecessary within the statutory framework. The commenter noted that the Congressional intent was " to avoid regulatory costs which would be without public health benefit." ( S.

Rep. No. 228, 101st Cong., 1st. Sess. 175 6 ( 1990))

Nevertheless, the commenter described some technology based criteria that they believed could be used to develop lowrisk groups of PCWP facilities. 263 Four commenters addressed the impact that creation of a low risk subcategory under CAA section 112( c)( 9) could have on the establishment of MACT floors for the PCWP category.

Two commenters argued that such subcategorization would have a negative effect. One commenter stated that this situation provided a valid reason for EPA not to mix risk based and technology based standards development. The commenter added that EPA also did not address how the " once in, always in"

policy would apply in such a situation. The other commenter stated that this situation was another compelling reason why the suggested section 112( c)( 9) subcategorization approach was unlawful and arbitrary. The commenter stated that the flaw was so obvious, inherent, and contrary to the MACT floor provisions of CAA section 112 and its legislative history, that it proves the undoing of the suggested section 112( c)( 9) exemption. According to the commenter, EPA cannot simultaneously exercise its source category delisting authority consistent with section 112( c)( 9), establish appropriate MACT floors under CAA section 112( d), and establish subcategory exemptions in the manner suggested by EPA, because the latter approach contravenes both section 112( c)( 9) and the section 112( d) floor setting process. The commenter stated that CAA section 112' s major source thresholds and statutory deadlines make clear that sources meeting MACT by the time EPA is required to issue MACT 264 standards must install MACT controls and may not subsequently throw them off or be relieved from meeting the MACT level standards. While the CAA section 112( f) residual risk process allows EPA to establish more stringent emissions standards, there is nothing in the CAA that suggests EPA possesses authority to relax promulgated MACT standards.

The third commenter indicated that dilution of the MACT floor would not occur if low risk category delisting occurred as follows: ( 1) propose low risk category with final PCWP rule, ( 2) promulgate low risk category 6 months after proposal, and ( 3) delist facilities prior to MACT compliance deadline. If EPA issued the final PCWP rule   thereby setting the MACT floor   before it allowed affected sources to apply for inclusion in the low risk category to be delisted, then every affected source would be considered in the establishment of the MACT floor. Thus, as a result of this timing, the MACT floor could not be diluted because no sources would be exempted from MACT before the MACT floor is set.

The fourth commenter believed that a MACT floor reevaluation would be appropriate and would further ensure that only facilities posing significant risk are required to install expensive controls.

Response: We feel that establishing a low risk PCWP 265 subcategory under CAA section 112( c)( 1) and deleting that subcategory under CAA section 112( c)( 9) best balances Congress' dual concerns that categories and subcategories of major sources of HAP be subject to technology based ( and possible future risk based) emission standards, but that undue burdens not be placed on groups of sources within the PCWP source category whose HAP emissions are demonstrated to present little risk to public health and the environment.

We do not contend that the CAA specifically directs us to establish categories and subcategories of HAP sources based on risk, and we recognize that, at the time of the 1990 CAA Amendments, Congress may have assumed that we would generally base categories and subcategories on the traditional technological, process, output, and product factors that had been considered under CAA section 111.

However, when properly considered, it becomes apparent that Congress did not intend the unduly restrictive   and consequently over regulatory   reading of the CAA that some commenters urge regarding low risk PCWP facilities.

Numerous CAA section 112 provisions evidence Congress'

intent that we be able to find that sources, such as those in the PCWP category whose HAP emissions are below identified risk levels, should not necessarily be subject to MACT. These provisions, together with other indications of Congressional intent regarding the goals of section 112, 266 must all be considered in determining whether we may base a PCWP subcategory on risk and delist that group of sources,

without requiring additional HAP regulation that would be redundant for purposes of meeting Congress' risk based goals.

While it is true that CAA section 112( c)( 1) provides that "[ t] o the extent practicable, the categories and subcategories listed under this subsection shall be consistent with the list of source categories established pursuant to section 111 and part C[,]" the provision also states that "[ n] othing in the preceding sentence limits the Administrator's authority to establish subcategories under this section, as appropriate." Therefore, by its plain terms, section 112( c)( 1) does not preclude basing subcategories on criteria other than those traditionally used under section 111 before 1990, or those used after 1990 for sections 111 and 112. Moreover, while after 1990 we have principally used the traditional criteria to define categories and subcategories, such use in general does not restrict how we may define a subcategory in a specific case,

" as appropriate," since each HAP emitting industry presents its own unique situation and factors to be considered.

( See, e. g., Sierra Club v. EPA, D. C. Cir. No. 02 1253, 2004 U. S. App. LEXIS 348 ( decided Jan. 13, 2004).)

Even assuming for argument that the language of section 267 112( c)( 1) may initially appear to restrict our authority to define subcategories, section 112( c)( 1) cannot be read in isolation. A broad review of the entire text, structure,

and purpose of the statute, as well as Congressional intent shows that, applied within the context of CAA section 112( c)( 9), our approach of defining a low risk subcategory of PCWP facilities is reasonable, at the very least as a way to reconcile the possible tension between the arguably restrictive language of section 112( c)( 1) and the Congressional intent behind section 112( c)( 9). ( See, e. g.,

Virginia v. Browner, 80 F. 3d 869, 879 ( 4th Cir. 1996).)

Alternatively, even if the language is clear on its face in restricting our ability to define subcategories, we feel that, as a matter of historical fact, Congress could not have meant what the commenter asserts it appears to have said, and that as a matter of logic and statutory structure,

it almost surely could not have meant it. ( See, e. g.,

Engine Mfrs. Ass'n v. EPA, 88 F. 3d 1075, 1089 ( D. C. Cir.

1996).)

Our interpretation of the CAA is a reasonable accommodation of the statutory language and Congressional intent regarding the relationship of the statutory categorization and subcategorization, delisting, MACT and residual risk provisions that apply to the PCWP category.

This becomes clear in light of the issue addressed by 268 commenters, which is whether we may delist a subcategory of low risk PCWP facilities only if such a group of sources is defined by criteria we have traditionally used to define categories and subcategories for regulatory, rather than delisting purposes. Our approach implements Congressional intent to avoid the over regulatory result that flows from an overly rigid reading of the CAA. When the CAA is read as a whole, it is apparent that Congress   which in 1990 likely did not fully anticipate the policy considerations that come into play in regulating HAP emissions from PCWP facilities   has not spoken clearly on the precise issue. Our interpretation is necessary to fill this statutory gap and prevent the thwarting of Congressional intent not to unnecessarily burden low risk PCWP facilities by forcing them to meet stringent MACT controls when they already meet the risk based goals of section 112. Our interpretation thus lends symmetry and coherence to the statutory scheme.

While we do not feel that CAA section 112( c)( 1) actually restricts our authority to establish a low risk PCWP subcategory, even if the language is so restrictive, it must be read within the context of Congress' purpose in allowing us to delist categories and subcategories of low risk sources that are defined according to the traditional criteria under CAA section 111. It is beyond dispute that Congress determined that certain identifiable groups or sets 269 of sources may be delisted if, as a group and without a single constituent source's exception, they are below the enumerated eligibility criteria of CAA section 112( c)( 9).

There is no apparent reason why such a group or set of sources must be limited to those defined by traditional categorization or subcategorization criteria. This is because, first, Congress in section 112( c)( 1) clearly did not absolutely prohibit us from basing categories and subcategories on other criteria generally; and, second, the underlying characteristic of an eligible set or group of sources under section 112( c)( 9)   that no source in the set or group presents risks above the enumerated levels   can be applied under several approaches to defining categories and subcategories and is not dependent upon such set or group being traditionally defined in order to implement the purpose of section 112( c)( 9). Put another way, there is nothing apparent in the statute that precludes us from delisting a discernible set of low risk PCWP facilities just because that set cannot also be defined according to other traditional criteria that have nothing to do with the question of whether each of the constituent PCWP facilities is low risk. As a matter of logic and statutory structure,

Congress almost surely could not have meant to require that every identifiable group of low risk PCWP facilities, no matter how large in number or in percentage with respect to 270 higher risk facilities in the PCWP category, must remain subject to CAA section 112, simply because that group could not be subcategorized as separate from the higher risk PCWP facilities by application of traditional subcategorization criteria.

Where Congress squarely confronted the issue, it explicitly provided relief for categories and subcategories,

defined by traditional criteria, that also happen to present little risk. ( See CAA sections 112( d)( 4), 112( c)( 9), and 112( f)( 2).) These CAA provisions addressing risk based relief from, or thresholds for, HAP emissions regulation evidence Congressional concern that the effects of such pollution be taken into account, where appropriate, in determining whether regulation under CAA section 112 is necessary. At the time of the 1990 Amendments, Congress did not consider it necessary to provide express relief for additional groups such as low risk PCWP facilities, beyond those defined by traditional category and subcategory criteria, because it assumed we could implement a comprehensive regulatory scheme for air toxics that would both address situations where technology based standards were needed to reduce source HAP emissions to levels closer to the risk based goals of section 112, and avoid unnecessary imposition of technology based requirements on groups of sources that were already meeting those goals. 271 Congress enacted or revised various CAA air toxics provisions   including sections 112( c), ( d) and ( f)   to that end. Had events unfolded in that anticipated fashion,

in the case of each industrial category and subcategory,

there would have been a perfect correlation between the traditional criteria for defining categories and subcategories and the facts showing whether those groups are either high or low risk HAP sources.

This context turned out to be more complex than Congress anticipated, and in the case of PCWP facilities there is no clear differentiation between high versus low risk sources that corresponds to our traditional approach for identifying source categories and subcategories. Nevertheless, as in the case of a low risk source group defined by traditional category or subcategory criteria, for the PCWP industry, we are able to identify a significant group of sources whose HAP emissions pose little risk to public health and the environment, applying the same section 112( c)( 9) delisting criteria that would apply to any traditionally defined source group. We feel it is reasonable to conclude that Congress would not have intended to over regulate the lowrisk PCWP facilities due to the inability to define such a group by traditional criteria and thereby frustrate the coherent scheme Congress set forth of ensuring that HAP sources ultimately meet common risk based goals under 272 section 112.

The commenter's assertion that we are inappropriately altering our interpretation of the applicable statutory provisions and departing from the traditional categorization and subcategorization criteria in addressing low risk PCWP facilities is thus unfounded. As explained above, the complexity of the air toxics problem and the relationship between the traditional criteria and what might be groups of low risk sources, a context not fully understood by either Congress or EPA at the time of the 1990 Amendments, provides adequate justification for any unique applications of the our approach for low risk PCWP facilities.

Our approach does not equate to one that Congress considered and rejected that would have allowed source

bysource exemptions from MACT based on individualized demonstrations that such sources are low risk. This is because, contrary to that approach, we rely upon the application of specific eligibility criteria that are defined in advance of any source's application to be included in the low risk PCWP subcategory, in much the same way as any other applicability determination process works.

Moreover, in response to the assertion that our approach nevertheless conflicts with legislative history rejecting a similar ( but not identical) approach Congress considered under CAA section 112, this legislative history is not 273 substantive legislative history demonstrating that Congress voted against relief from MACT in this situation   there is no such history. The commenters point to a provision in the House bill that was not enacted but that would have provided in certain situations for case by case exemptions for lowrisk sources. There is no evidence that this provision was ever debated, considered, or voted upon, so its not being enacted is not probative of congressional intent concerning our ability to identify and delist a group of low risk PCWP facilities. Instead, it is reasonable to assume that, had Congress been aware in 1990 of the possibility that an identifiable group of PCWP facilities is low risk, while that group does not correspond to traditional criteria differentiating categories and subcategories, Congress would have expressly, rather than implicitly, authorized our action here.

Moreover, the commenters are unable to cite any provision in CAA section 112 that would prevent us from being able to add individual or additional groups of lowrisk PCWP facilities to the group we initially identify in our final delisting action, as those additional low risk PCWP facilities prove their eligibility for inclusion in the delisted group over time. In fact, the approach we are taking for identifying additional low risk PCWP facilities is fully consistent with the approach we have long taken in 274 identifying, on a case by case basis and subject to appropriate review, whether individual sources are members of a category or subcategory subject to standards adopted under CAA sections 111 and 112.

Regarding the comment that Congress did not expressly provide relief for carcinogen emitting low risk groups of sources within the PCWP category other than as an entire category, we construe the provisions of CAA section 112( c)( 9) to apply to listed subcategories as well as to categories. This construction is logical in the context of the general regulatory scheme established by the statute,

and it is the most reasonable one because section 112( c)( 9)( B)( ii) expressly refers to subcategories. Under a literal reading of section 112( c)( 9)( B), no subcategory could ever be delisted, notwithstanding the explicit reference to subcategories, since the introductory language of section 112( c)( 9)( B) provides explicit authority to only delist categories. Such a reading makes no sense, at the very least because Congress plainly assumed we might also delist another collection of sources besides either categories or subcategories, even in the case of sources of carcinogens. Both sections 112( c)( 9)( B)( i) and ( ii) refer additionally to groups of sources in the case of area sources as being eligible for delisting, even though only a category of sources is specifically identified as eligible 275 for delisting in the introductory language of section 112( c)( 9)( B). In light of the broader congressional purpose behind the delisting authority, we interpret the absence of explicit references to subcategories in this introductory language and in section 112( c)( 9)( B)( i) as representing nothing more than a drafting error.

Regarding the comments about establishing PCWP subcategories based on characteristics other than risk, the criteria for the low risk subcategory we are delisting are based solely on risk and not on technological differences in equipment or emissions. We performed an analysis to determine which major source PCWP facilities may be low risk facilities. Whether facilities are low risk or not depends on the facility HAP emissions; and facility HAP emissions are a function of the type and amount of product( s)

produced, the type of process units ( e. g., direct fired versus indirect fired dryers) used to produce the product,

and the emission control systems in place. Our analysis indicates that the facilities which show low risk could include facilities producing various products such as particleboard, molded particleboard, medium density fiberboard, softwood plywood, softwood veneer, fiberboard,

engineered wood products, hardboard, and oriented strandboard. However, there are also major sources that produce these products that are not low risk, and, 276 therefore, product type cannot be used to define the lowrisk subcategory. There is no correlation between production rate and low risk facilities ( e. g., when facilities are sorted by production rate for their product,

the low risk facilities are not always at the lower end of the production rate range), so production rate cannot be used as criteria for defining the low risk subcategory. The low risk facilities use a variety of process equipment ( e. g., veneer dryers at softwood plywood plants and tube dryer at MDF plants). This same equipment is used at PCWP plants that are not low risk, and, therefore, there is no process unit type distinction that can be used to define the low risk subcategory. The pollutant that drives the risk estimate can vary from facility to facility because of the different types of process units at each facility. There is no clear distinction among low risk and non low risk facilities when ranked by emissions of individual pollutants because of other factors that contribute to facility risk such as presence of a co located PCWP facility or variability in the pollutants emitted. Thus, there is no emissions distinction that can be used to define the lowrisk subcategory. There is no technological basis for creating a subcategory of PCWP facilities that are low risk.

The commonality between all of the low risk PCWP facilities is that they are low risk, and, therefore, we have 277 established the low risk subcategory based on risk.

We do not agree with the commenters' assertions that our approach for the low risk PCWP subcategory undermines our ability to identify the MACT floor for the larger PCWP category, either in today's final PCWP rule or in any future consideration of technological development under CAA section 112( d)( 6). This is because, while low risk PCWP facilities will literally be part of a separate subcategory, there is nothing in the CAA that prevents us from including them in any consideration of what represents the best controlled similar source in the new source MACT floor context, and because it is not unprecedented for us to look outside the relevant category or subcategory in identifying the average emission limitation achieved by the best controlled existing sources if doing so enables us to best estimate what the relevant existing sources have achieved. In fact, EPA has taken this very approach in the Industrial Boilers MACT rulemaking, in order to identify the MACT floor for mercury emissions. Moreover, the unique issues presented by the low risk PCWP subcategory show that it would be unreasonable to exclude any better performing low risk PCWP sources from the MACT floor pool for the larger PCWP category.

Traditionally, EPA has based categories and subcategories partly on determinations of what pollution control measures can be applied to the relevant groups of sources in order to 278 effectively and achievably reduce HAP. In other words, EPA has identified subcategories for purposes of identifying the MACT floor in a way that accounts for the differences of sources types in their abilities to control HAP emissions.

But whether a PCWP source is a low risk source does not necessarily turn on such a distinction   two sources might have identical abilities to control HAP emissions, but the unique circumstances of one source regarding the impacts of its HAP emissions will determine whether or not it is a lowrisk PCWP source. ( In fact, it is theoretically possible that between two sources the better performing source will be a high risk source, and the worse performing source will be a low risk source, based on circumstances that are unrelated to the question of what abilities the sources have to control HAP emissions through application of MACT, such as the sources' locations vis a vis exposed human populations.) Therefore, EPA feels that not only is it appropriate to include any better performing low risk PCWP sources in the MACT floor determinations for the larger PCWP category, but that excluding such sources simply based on the unique facts of the impacts of their emissions, with there being no difference in the abilities of high risk and low risk sources to apply HAP emission control measures,

would becould result in an unlawfulundesirable weakening of the MACT floor for the larger PCWP category. To that end, 279 the MACT floors established for PCWP process units today are in no way affected by our establishment of the low risk PCWP subcategory.

Finally, we disagree with the argument by one commenter that the low risk PCWP subcategory approach represents an impermissible cost based exemption from MACT or factor in determining MACT. Certainly it is true that costs may not be considered in setting the MACT floor. However, there is nothing in the CAA that prevents us from noting the cost impacts, beneficial or adverse, of our actions in setting MACT floors, assessing possible beyond the floor measures,

or conducting risk based actions under CAA section 112. In fact, we routinely evaluate the costs of our regulatory actions, even when cost factors may not be used to influence the regulatory decision itself, in order to comply with applicable Executive Order and statutory administrative review requirements. Simply because there is a cost benefit to some members of the PCWP category in our establishing a low risk PCWP subcategory does not make that action impermissible, provided that our subcategorization and delisting are otherwise properly based on the appropriate risk based criteria under CAA section 112( c)( 9). Section 112 by its own terms does not forbid the goal of achieving environmental protection in a less costly manner.

Similarly, it is appropriate for EPA to note the beneficial 280 air pollution related impacts of not requiring low risk PCWP sources to, for example, install criteria pollutant emission producing RTOs. While it is true that such air quality related impacts could not constitute non air quality health and environmental impacts that EPA must consider when setting MACT under CAA section 112( d)( 2), nothing in the CAA prevents EPA from taking account of such impacts in developing its policy regarding whether it is appropriate to delist a subcategory under section 112( c)( 9) when that subcategory otherwise meets the statutory criteria for delisting. Therefore, EPA does not agree with commenters who claim that its approach to delisting the low risk PCWP subcategory conflicts with how it has argued issues regarding either de minimis authority, cost based exemptions from MACT, or the treatment of non air quality impacts and the consideration of risk in setting the actual MACT standard before the U. S. Court of Appeals for the D. C.

Circuit. Nor does our approach contravene any of that Court's rulings on these issues.

3. Criteria for demonstrating low risk Dose response values Comment: Two commenters suggested that EPA incorporate into the PCWP rule the findings of the nationwide wood products risk assessment, which they claim demonstrates that the vast majority of wood products sources cause no 281 meaningful risk to human health or the environment at current emission levels. The commenters stated that the risk assessment used existing air dispersion modeling studies of 34 wood products facilities throughout the U. S.

to estimate the maximum annual off site HAP concentrations at wood products facilities nationwide. According to the commenters, the risk assessment indicates that large subgroups of facilities that are affected sources under the PCWP rule as proposed ( i. e., fiberboard, medium density fiberboard, and plywood facilities) generally are expected to pose insignificant risks to human health, based on a comparison of predicted off site concentrations with applicable health benchmarks. One of the commenters stated that many of the facilities with low off site concentrations will likely be smaller plants that would not be able to justify installation of ( additional) emission controls and may face closure without a risk based compliance option.

The other commenter stated that a comparison of off site concentrations of formaldehyde and acetaldehyde with benchmarks reflecting the latest toxicological evidence indicates that exposures to those HAP are well below levels of concern. Acrolein was the only HAP with potential exposures at some facilities ( i. e., subset of fiberboard,

medium density fiberboard and plywood facilities) that exceeded the health benchmark. However, the commenter 282 stated that the acrolein findings may not represent an actual risk to human health because exceedences of the benchmark may be attributable to EPA averaging a large number of non detects at one half the detection limit,

thereby artificially increasing predicted acrolein emissions. Based on these overall findings, the commenter concluded that the wood products risk assessment indicates that incinerator control is not warranted on the basis of human health concerns for a large number of facilities.

Response: We acknowledge receipt of the industrysponsored nationwide wood products MACT risk assessment submitted by the commenter. However, we conducted our own risk analysis to evaluate the merits of including and delisting a low risk subcategory in today's final PCWP rule.

The methodology used in our risk analysis differed widely from the methodology used in industry's risk assessment.

For example, industry's risk assessment was based on previously conducted air dispersion modeling studies for 34 PCWP facilities, while our analysis used emission estimates developed for each PCWP facility expected to be a major source of HAP. We used different ( generally more protective) human health benchmarks in our risk assessment than were used in industry's risk assessment. We also considered all HAP ( including metal HAP) in our risk analysis, whereas industry's risk assessment considered only 283 methanol, formaldehyde, acetaldehyde, acrolein, phenol, and propionaldehyde.

Based on our risk analysis, we conclude that HAP emissions from some PCWP facilities pose little risk to human health and the environment. Therefore, we have included a subcategory of low risk PCWP facilities in today's final PCWP rule, and are delisting that subcatgeorysubcategory. Appendix B to subpart DDDD of 40 CFR part 63 includes procedures that facilities may use to demonstrate that they are part of the delisted low risk subcategory, and, therefore, are not subject to the compliance options included in today's final PCWP MACT rule.

To demonstrate eligibility for the low risk subcategory,

facilities must first conduct emissions testing for up to 13 HAP ( five organic HAP from all process units, seven metal HAP from direct fired process units, and MDI from presses processing product containing MDI resin). The rationale for selection of these 13 HAP is described elsewhere in this section and in the supporting documentation for the final rule. Facilities must use the results from emissions testing to preliminarily demonstrate, subject to EPA approval, that they are part of the low risk subcategory using either a look up table analysis ( based on the look up tables included in appendix B to subpart DDDD of 40 CFR part 63) or site specific risk assessment methodology ( described 284 in appendix B to subpart DDDD of 40 CFR part 63 and other analytical tools, such as the " Air Toxics Risk Assessment Reference Library" if appropriate for the specific source)

and risk benchmarks ( described in appendix B to subpart DDDD of 40 CFR part 63).

Regarding acrolein, the commenter is correct in that,

when developing AP 42 emission factors, we used a value of one half the detection limit for all non detect sample runs if acrolein was detected in any sample runs from the applicable source category. Acrolein has been detected in process unit emissions from all sectors of the PCWP industry, except for hardwood plywood manufacturing. When using emission factors to estimate emissions from PCWP facilities, we did not estimate emissions of a pollutant when all of the emissions test runs were non detect.

However, we did use emission factors that included a mixture of detectable values and values based on one half of the method detection limit ( MDL) when acrolein was detected at least once for a particular type of process unit. We maintain that this approach to handling non detects is appropriate for the purposes that we used the emissions data. Facilities will conduct emissions tests instead of using emission factors to demonstrate eligibility for the low risk subcategory. To prevent facilities from including HAP that are not detected in their low risk demonstrations, 285 appendix B to subpart DDDD of 40 CFR part 63 states that facilities may use zero for non detects when all of the emission test runs are below the MDL, provided that certain criteria are met to ensure that emissions testing and analysis procedures are adequate to detect low concentrations of HAP.

Comment: One commenter stated that CAA section 112( d)( 4) is particularly ill suited to the PCWP and industrial boiler source categories. The commenter stated that, even if EPA had authority to create individualized MACT exemptions based on health thresholds, it could not do so if there is insufficient evidence on the pollutants emitted to establish a NOEL. According to the commenter,

section 112( d)( 4) does not apply for chemicals that do not have a well defined threshold based on reliable science.

The commenter stated that available evidence does not establish a no effect threshold for acetaldehyde, acrolein,

benzene, carbon tetrachloride, chloroform, formaldehyde,

manganese, methylene chloride, and phenol. As rationale,

the commenter presented a summary of the available health effects data for each of these pollutants.

Response: As stated elsewhere in this preamble, we are not pursuing establishment of a threshold emission rate for the PCWP source category under CAA section 112( d)( 4) because PCWP facilities emit non threshold pollutants. Therefore, 286 this comment is irrelevant in the context of the PCWP rule.

Comments pertaining to the Industrial/ Commercial/ Institutional Boilers and Process Heaters NESHAP are addressed in the comment response document for that rule. ( See Docket ID No. OAR 2002 0058.)

Comment: Two commenters expressed concern about the health benchmark data sources that EPA used. The first commenter argued that the proposal inappropriately used draft guidelines and toxicity profiles that had not been subject to public review and/ or were not publicly available.

The commenter was particularly concerned with the use of non linear carcinogenic risk values and toxicity profiles ( for HAP) that have not been finalized and are not available for review by the public.

The second commenter argued that EPA should not rely solely on the health benchmarks in its Integrated Risk Information System ( IRIS) database. The commenter stated that IRIS, while useful for obtaining information about the health effects of chemicals, is far from definitive, as EPA resource constraints have resulted in many chemical summaries that are significantly outdated and do not reflect the most recent scientific developments. Moreover, the commenter stated that the IRIS database is a non statutory,

in house EPA activity, and IRIS entries are not subject to formal notice and comment. The commenter noted that EPA 287 management has repeatedly emphasized in directives that other information must be considered, in addition to the IRIS database, when evaluating the health effects of chemicals in a regulatory context. The commenter concluded that EPA must use a scientifically appropriate health benchmark based on a consideration of all relevant information to ensure that the health benchmark is up to date and scientifically credible, even if that means departing from the value in IRIS.

A third commenter agreed with EPA's choice to derive their data from IRIS, California EPA ( CalEPA), and Agency for Toxic Substances and Disease Registry ( ATSDR) for its documentation for establishing risk based threshold and nonthreshold values. The commenter added that almost all HAP are being reviewed and reevaluated on a regular basis, and it would be inappropriate to single out formaldehyde and acetaldehyde at this time. The commenter stated that EPA can only rely on what is currently published and has underdone either peer review or EPA review. According to the commenter, the issue of changing health based guideline values will always be a concern once health based regulations are promulgated.

Response: We agree with the first two commenters that we should use the best available sources of health effects information for risk or hazard determinations. As we have 288 1U. S. Environmental Protection Agency. 1999. Residual Risk Report to Congress. Office of Air Quality Planning and Standards, Research Triangle Park, NC 27711, March 1999, EPA 453/ R 99 001; available at http:// www. epa. gov/ ttn/ oarpg/ t3/ meta/ m8690. html. ( EPA 1999) stated previously, we will not be relying exclusively on IRIS values, but will be considering all credible and readily available assessments. 1 For air toxics risk assessments, we identify pertinent toxicity or dose response values using a default hierarchy of sources, with IRIS being the preferred source, to assist us in identifying the most scientifically appropriate benchmarks for our analyses and decisions. The IRIS process contains internal and external peer review steps and represent EPA consensus values. When adequate toxicity information is not available in IRIS, we consult other sources in a default hierarchy that recognizes the desirability of these qualities in ensuring that we have consistent and scientifically sound assessments.

Furthermore, where the IRIS assessment substantially lags the current scientific knowledge, we have committed to consider alternative credible and readily available assessments. For our use, these alternatives need to be grounded in publicly available, peer reviewed information.

Formaldehyde is an example of this situation. We are not using information that does not meet these requirements. We also agree with the third commenter that the issue of 289 changing health based guideline values is a general challenge in setting health based regulations. However, we are committed to setting such regulations that reflect current scientific understanding, to the extent feasible.

Facilities conducting low risk demonstrations should refer to appendix B to subpart DDDD of 40 CFR part 63 and other analytical tools, such as the " Air Toxics Risk Assessment Guidance Reference Library" ( if appropriate for the specific source) for guidance on choosing appropriate dose response values.

Comment: With the support of several others, one commenter pointed out that the science with respect to formaldehyde and acetaldehyde has changed since EPA's initial IRIS entries for those pollutants were completed.

Consequently, the commenter believed it would be inappropriate for EPA to rely on the unit risk factors for those pollutants in the IRIS database in establishing a property line concentration threshold in the PCWP rule as proposed. The commenter supported EPA's efforts in revising its formaldehyde and acetaldehyde IRIS assessment and noted that both revisions are expected to be finalized before the final PCWP rule is published in 2004. Regarding formaldehyde, the commenter noted that EPA plans on using the model from the Chemical Industry Institute of Technology ( CIIT) to revise its formaldehyde IRIS assessment and 290 encouraged this action. The commenter pointed out that the CIIT model has been recognized by several authoritative bodies ( e. g., Health Canada/ Environment Canada, Organization for Economic Coordination and Development, and World Health Organization) as providing the most scientifically defensible analysis of formaldehyde. ( Another commenter added that the IRIS risk criteria for formaldehyde clearly cause formaldehyde risk estimates to be overstated but argued that, even using the very conservative IRIS numbers,

risks are still low. A third commenter urged EPA not to use the formaldehyde values in ATSDR, stating that they are fundamentally flawed, as detailed in their comment.)

Regarding acetaldehyde, the commenter recommended that EPA use a health benchmark between 27 and 390 micrograms per cubic meter ( µ g/ m3) and included their rationale in an attachment to their comment. If EPA is unable to complete its reassessments before the PCWP rule is finalized, the commenter encouraged EPA not to revert to the original IRIS unit risk factors for formaldehyde and acetaldehyde.

Instead, the commenter recommended that EPA use the CIIT model ( or alternatively defer to Health Canada/ Environment Canada) for formaldehyde and, at a minimum, use the IRIS reference concentration ( RfC) of 9 µ g/ m3 for acetaldehyde.

Response: With the exception of formaldehyde, we are using the human health values currently used by EPA's air 291 toxics program and available at:

http:// www. epa. gov/ ttn/ atw/ toxsource/ summary. html. These dose response values come from several sources including EPA's IRIS, the Centers for Disease Control's ATSDR, and California EPA. See the supporting information for this rulemaking for a summary of the human health values we used in our assessment.

For formaldehyde, we do not use the dose response value reported in IRIS. The dose response value in IRIS is based on a 1987 study, and no longer represents the best available science in the peer reviewed literature. Since that time,

significant new data and analysis have become available. We based the dose response value we used for formaldehyde on work conducted by the CIIT Centers for Health Research ( formerly, the Chemical Industry Institute of Toxicology).

In 1999, the CIIT published a risk assessment which incorporated mechanistic and dosimetric information on formaldehyde that had been accumulated over the past decade.

The risk assessment analyzed carcinogenic risk from inhaled formaldehyde using approaches that are consistent with EPA's draft guidelines for carcinogenic risk assessment. The CIIT model is based on computational fluid dynamics ( CFD) models of airflow and formaldehyde delivery to the relevant parts of the rat and human respiratory tract, which are then coupled to a biologically motivated two staged clonal growth 292 model that allows for incorporation of different biological effects. These biological effects, such as interaction with DNA and cell proliferation, are processes by which formaldehyde may contribute to development of cancer at sites exposed at the portal of entry ( e. g., respiratory tract). The two staged model is a much more advanced approach for examining the relevance of tumors seen in animal models for human populations.

We believe that the CIIT modeling effort represents the best available application of the available mechanistic and dosimetric science on the dose response for portal of entry cancers due to formaldehyde exposures. We note here that other organizations, including Health Canada, have adopted this approach. Accordingly, we have used risk estimates based on the CIIT airflow model coupled to a two staged clonal growth model as the basis for the dose response values for this analysis. This model incorporates state

ofthe art analyses for species specific dosimetry, and encompasses more of the available biological data than any other currently available model. As with any model,

uncertainties exist, and this model is sensitive to the inputs, but we believe it represents the best available approach for assessing the risk of portal of entry cancers due to formaldehyde exposures.

We are aware of recently completed epidemiologic 293 analyses that update existing cohorts of workers exposed to formaldehyde. Not all these results have been published in peer reviewed journals at this point, and the new data are not fully available. These studies examine the potential for formaldehyde to cause cancer in other organs. These new results in combination with the previously available literature regarding the potential for systemic effects indicates the pollutant may be taken into the body and distributed to extra respiratory organs, potentially causing adverse effects that are not picked up by the CIIT model.

As these new studies are published in peer reviewed journals and data sets are made available for further analysis, we will evaluate them in the context of the full body of literature to determine the relevance to our conclusions with regard to risks from exposure to formaldehyde.

Currently, the CIIT information and other recent information, including recently published epidemiological studies, are being reviewed and considered in the reassessment of our formaldehyde unit risk estimate ( URE).

We plan to bring this reassessment to the Science Advisory Board in the summer of 2004. The feasibility of delisting a subgroup of facilities based on risk is not compromised by the existing formaldehyde dose response value because some facilities would qualify for delisting based on this current value. We are moving forward with the final PCWP rule at 294 this time because there is a court ordered deadline, and we are including the low risk PCWP subcategory delisting and basing our review of sources's eligibility on the CIIT model for formaldehyde because, at this point in time, that reflects the best available source of health effects information. We disagree with the statement by one of the commenters that risks are still low using the current IRIS number for formaldehyde. Our analysis has demonstrated that not all PCWP facilities can be considered low risk when either the current IRIS or CIIT URE for formaldehyde is employed.

While we recognize the similarities between acetaldehyde and formaldehyde with regard to suggested modes of action,

the reassessment of acetaldehyde is lagging behind that of formaldehyde. The formaldehyde reassessment is further along because of the preponderance of data specific to formaldehyde and the potentially greater impact of a change in potency to our regulatory decisions. Unlike for formaldehyde, an alternative, peer reviewed, publicly available assessment does not currently exist for acetaldehyde, leaving us with the current IRIS assessment.

We do not feel it is necessary to wait for our acetaldehyde reassessment to be completed, due to the court ordered deadline for the final PCWP MACT rule, and due to the fact that until otherwise concluded the IRIS values for 295 acetaldehyde reflect the best available source of health effects information. Therefore, we are relying on the IRIS values for acetaldehyde in both cancer and non cancer risk assessments for the final rule.

Facilities conducting low risk demonstrations should refer to appendix B to subpart DDDD of 40 CFR part 63 and other analytical tools, such as the " Air Toxics Risk Assessment Reference Library" ( if appropriate for the specific source) for guidance on choosing appropriate doseresponse values.

Comment: One commenter stated that EPA should consider formaldehyde and acetaldehyde as carcinogens unless a reassessment classifies them as threshold pollutants. A second commenter argued that formaldehyde and acetaldehyde are properly treated as threshold pollutants. This commenter contended that the legislative history of the CAA makes clear that Congress considered " threshold pollutants"

to be those for which a " no observed effect level" can be established. ( See, e. g., S. Rep. No. 228, 101st Cong., 1st Sess. 175 176 ( 1990)). By contrast, a non threshold pollutant is one for which a no observed effect level cannot be identified, i. e., a pollutant for which adverse effects may be seen at any dose level above zero. The commenter noted that EPA has historically assumed that all carcinogens are non threshold pollutants that may trigger a carcinogenic 296 effect at any exposure level, no matter how small. However,

as mechanistic data on the mode of action of carcinogenesis advances, that conservative assumption may prove not to be accurate for certain pollutants. The commenter stated that the available science strongly suggest that these pollutants act as threshold carcinogens. The commenter contended that there is a no observed effect level for formaldehyde below which the carcinogenic risk either does not exist or cannot be measured, as documented in an attachment to their comment. The commenter stated that acetaldehyde should be viewed similarly because acetaldehyde is similar to formaldehyde structurally and toxicologically, and is expected to behave similarly mechanistically. Because acetaldehyde is a less potent carcinogen than formaldehyde ( by an order of magnitude), non cancer health effects ( which clearly are threshold health effects) are the likely risk driver for that pollutant. Finally, the commenter noted that EPA's recently issued Draft Final Guidelines for Carcinogenic Risk Assessment provide that, for non linear carcinogens, EPA will calculate a reference dose ( RfD) or RfC, which are safe lifetime doses ( i. e., doses below which adverse effects will not occur). The commenter stated that this is exactly what a threshold pollutant is. Thus, EPA's revised guidelines support the conclusion that formaldehyde and acetaldehyde should be treated as threshold pollutants. 297 Response: We agree that we should consider formaldehyde and acetaldehyde as carcinogens unless a reassessment classifies them as threshold pollutants. Currently,

formaldehyde and acetaldehyde are considered probable human carcinogens. Both are under review, and their dose response values for carcinogenicity are likely to change. For the final rule, we are using an alternative dose response value for formaldehyde based on a peer reviewed, publicly available assessment. However, we do not have comparable quantitative information for acetaldehyde. Therefore, we will use the current IRIS value. Facilities conducting lowrisk demonstrations should refer to appendix B to subpart DDDD of 40 CFR part 63 ( and/ or the " Air Toxics Risk Assessment Reference Library") for guidance on choosing appropriate dose response values.

Comment: One commenter expressed concern about some of the health benchmarks that EPA plans to publish. The commenter reviewed various health studies for each pollutant and recommended several RfC values. The commenter noted that, because IRIS does not have an RfC for methanol, EPA has indicated it plans to determine a de minimis threshold for methanol using a value of 4.0 milloigrams per cubic meter ( mg/ m3) as an RfC. The commenter noted that this value is the noncancer chronic reference exposure level ( REL) derived by CalEPA. The commenter stated that CalEPA's 298 derivation of that REL contains some errors and inaccurate assumptions. According to the commenter, a more accurate estimate of a human safe level for chronic exposure to methanol by inhalation, derived from the same mouse study data, is 171 mg/ m3, which is discussed further in their comments. The commenter stated that their discussion presents new analyses not previously reviewed by EPA and a ground breaking new approach to a hazard assessment for methanol. The commenter noted that EPA is currently revising its assessment for acrolein and has provided for public information a draft toxicological review and draft IRIS summary for acrolein. The draft IRIS document states that the proposed new RfC of 0.03 µ g/ m3 replaces the previous RfC of 0.02 µ g/ m3, and that this new RfC is based on a more recent interpretation of the database. The commenter noted the basis for the revised acrolein RfC ( Feron et al, 1978) and argued that EPA's interpretation of this study is overly conservative. The commenter stated that EPA has used the maximum uncertainty factors that could reasonably be justifiable and thereby developed an RfC that almost certainly goes beyond what is needed to protect human health. The commenter suggested that EPA should instead use the more realistic reference exposure level developed by CalEPA, which is more conservative than the Health Canada Tolerable Concentration. 299 The commenter noted that EPA has not published a health benchmark for phenol. The commenter agreed with EPA's proposal to use the CalEPA REL of 200 µ g/ m3 for phenol in implementing the risk based approach for wood products facilities. According to the commenter, the REL is intended to serve the same goal as an RfC.

The commenter supported using a health benchmark of 110 µ g/ m3 for propionaldehyde and believed that this value would protect human health with an ample margin of safety. The commenter described how the 110 µ g/ m3 value was derived based on the threshold limit value ( TLV) for propionaldehyde identified by the American Conference of Governmental Industrial Hygienists ( ACGIH). The commenter explained that this benchmark is consistent with values developed by other organizations.

Response: We are currently developing an IRIS assessment for methanol, and any new information that exists that has undergone peer review will be considered in this re evaluation. We publish yearly in the Federal Register a list of all chemicals for which we are planning IRIS assessment activity. This action further requests submission of pertinent data for these chemicals. In lieu of the pending IRIS assessment, we will continue to draw on other sources identified by our established default hierarchy of data sources, which have as part of their 300 development processes external or peer review, in addition to extensive internal reviews.

A reassessment of acrolein was completed in June of 2003. The RfC resulting from that reassessment ( i. e., an RfC of 0.02

g/ m3, with an uncertainty factor of 1,000) is what is currently on IRIS. As with all announced IRIS reassessments, time was provided for new data or relevant information to be submitted. In addition, each assessment undergoes extensive internal review as well as external peer review to ensure that the data used are scientifically sound. We feel that we have developed the most scientifically sound RfC that will ensure that risk assessments using this number are health protective.

Facilities conducting low risk demonstrations should refer to appendix B to subpart DDDD of 40 CFR part 63 ( and/ or the " Air Toxics Risk Assessment Reference Library") for guidance on choosing appropriate dose response values.

We do not currently have plans to develop an IRIS assessment for phenol. We will continue to rely on our hierarchy of other sources when IRIS values are not available.

We do not have an IRIS file for proprionaldehyde, and an assessment is not available from the alternative sources in our default hierarchy. The hierarchy sources do not include ACGIH, as that organization develops reference values for 301 use in occupational exposure settings, as opposed to the ambient air exposures that are the focus of this action.

Development of an IRIS assessment for propionaldehyde is currently underway. Once available, it will be used in future risk analyses. In the meantime, this HAP was not included in the assessment conducted for PCWP facilities.

Comment: One commenter stated that comparison of modeled exposures to the RfC or similarly derived health benchmark is highly protective and meets the CAA's " ample margin of safety" requirement. Although the commenter claims the CAA does not explicitly define " ample margin of safety," in the Vinyl Chloride case, the D. C. Circuit Court of Appeals articulated the purpose of the ample margin of safety determination as obtaining a " reasonable degree of protection" in light of scientific uncertainties and information gaps. ( Natural Res. Def. Council v. EPA, 824 F. 2D 1146, 1152 53 ( D. C. Cir. 1987)). The commenter stated that, in regulatory practice, the ample margin of safety analysis consists of a consideration of the NOEL for a pollutant and the subsequent application of factors to account for scientific uncertainty surrounding that safe level of exposure. According to the commenter, this is the approach called for by the Senate Report accompanying the 1990 CAA Amendments ( S. Rep. No. 228, 101st Cong. Sess. 171 ( 1990)), and this is exactly what is done in deriving an RfC 302 or similar inhalation health benchmark. The commenter stated that EPA's derivation of the RfC contains multiple layers of conservatism to account for scientific uncertainty. The commenter believed that RfC values and similar inhalation health benchmarks already incorporate sufficient uncertainty factors to fulfill or exceed the ample margin of safety mandate of CAA sections 112( d)( 4) and 112( c)( 9).

Response: Today's final PCWP rule will utilize CAA section 112( c)( 9) rather than CAA section 112( d)( 4). We agree that the CAA does not define " ample margin of safety"

explicitly. The CAA does, however, in section 112( f)

explicitly recognize our Federal Register notice of September 14, 1989, which described our interpretation of ample margin of safety in the case of linear carcinogens,

and our approach to implementing that interpretation. While the first step identifies the presumptive limit on maximum individual risk, the second step of that 2 step approach describes the setting of the risk based standard at a level that provides an ample margin of safety, in consideration of a number of factors. As we noted in the 1989 notice, the objective in protecting public health with an ample margin of safety under CAA section 112 is to ensure an individual lifetime risk level no higher than one in a million to the greatest number of persons possible, and to limit to no 303 higher than one in ten thousand the estimated risk for a person living near a plant if they were exposed for 70 years.

In assessing risk or hazard of nonlinear effects, we use the RfC or comparable value. This value represents an estimate ( with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population ( including sensitive subgroups) that is likely to be without an appreciable risk of deleterious non cancer effects during a lifetime. The RfC values and comparable values are derived from assessments of pertinent toxicological information to identify the lowest point of departure ( in human equivalent terms) from the experimental data that is also representative of the threshold region ( the region where toxicity is apparent from the available data) for the array of toxicity data for that chemical. The objective is to select a prominent toxic effect that is pertinent to the chemical's key mechanism or mode of action.

This approach is based, in part, on the assumption that if the critical toxic effect is prevented, then all toxic effects are prevented. The RfC is derived from the point of departure ( POD) ( in terms of human equivalent exposure) for the critical effect by consistent application of uncertainty factors, which are to account for recognized uncertainties in the extrapolations from the experimental data conditions 304 2U. S. Environmental Protection Agency. 1994. Methods for Derivation of Inhalation Reference Concentrations and Application of Inhalation Dosimetry. Office of Research and Development. EPA/ 600/ 8 90/ 066F. ( EPA 1994) to an estimate appropriate to the assumed human scenario. 2 In considering the extrapolation of the ample margin of safety objective described for linear cancer risk to the management of risk for nonlinear effects under CAA section 112( c)( 9) ( i. e., in decisions to delist a subcategory from any further regulatory action), we consider exposures relative to the RfC or comparable values for all of the emitted HAP, with specific attention to those affecting a similar physiological target organ or system, and the contribution of co located sources of the same HAP.

Comment: One commenter stated that the uncertainty factors used in deriving the wood products HAP health benchmarks are particularly large. The unit risk factors for acetaldehyde and formaldehyde were calculated using the linear multi stage model, which assumes a linear relationship between cancer incidence and exposure to the pollutant at low doses. According to the commenter, the available data on acetaldehyde and formaldehyde strongly suggest that this assumption is incorrect and overly conservative.

The commenter pointed out that EPA's health assessment of acrolein is two to three times more conservative than 305 CalEPA's, even though both are intended to protect sensitive individuals from any adverse effects following a lifetime of exposure. The commenter stated that EPA has developed an extremely conservative RfC for acrolein. The commenter argued that adopting a HI of 0.2 would add another five fold safety factor to this already extremely conservative RfC.

The commenter noted that acrolein is the HAP of greatest importance in determining risk from PCWP facilities.

Response: The dose response values used to determine the criteria for defining the low risk subcategory are drawn from IRIS, as well as from certain alternative sources. The IRIS process contains internal and external peer review steps and represents EPA consensus values. When adequate toxicity information is not available in IRIS, we consult other sources in a default hierarchy that recognizes the desirability of these qualities in ensuring that we have consistent and scientifically sound assessments. In the case of acrolein, specifically mentioned by the commenter,

consultation of other sources was not necessary because the acrolein assessment was completed within the past 9 months and represents current scientific knowledge. In those cases ( e. g., formaldehyde), where the IRIS assessment substantially lags the current scientific knowledge, we consider alternative credible and readily available assessments. As pointed out elsewhere in this section, the 306 3Ibid. RfC values or comparable values have been derived with the incorporation of uncertainty factors. The uncertainty factors are to account for recognized uncertainties in the extrapolations from the experimental data conditions pertaining to the chemical's particular toxicological data set to an estimate appropriate to the assumed human scenario. 3 The size variation of the uncertainty factors across RfC values reflects the size variation of the uncertainties associated with that extrapolation.

Comment: One commenter stated that the combination of conservative air dispersion modeling techniques and a conservative human health benchmark ensure that, where a source meets the requirements for a risk based compliance option, human health will be protected with an ample margin of safety. The commenter pointed out that, for most individuals in the general population, actual exposures likely are one or more orders of magnitude below the maximum exposures predicted by the tiered modeling approach. The commenter noted that EPA's tiered modeling methodology is designed to identify the highest annual property line or off site concentrations that might occur around each facility ( as opposed to actual population exposure). The tiered approach models exposures of a maximally exposed individual ( MEI) and incorporates a number of conservative 307 assumptions. According to the commenter, actual average concentrations are likely to be much lower. The commenter argued that, even if the modeled concentrations were reflective of continuous average concentrations, it is highly unlikely that any individual would actually be exposed to such concentrations for a lifetime. The commenter noted that the Presidential/ Congressional Commission on Risk Assessment and Risk Management concluded that the conservatism inherent in use of the MEI was often so unrealistic that its use impaired the scientific credibility of health risk assessment.

Response: We discussed a tiered analytical approach in the preamble to the proposed rule, beginning with relatively simple lookup tables and followed by increasingly more sitespecific but more resource intensive tiers of analysis, with each tier being more refined and less conservative. In today's final rule, we are setting forth two options, as specified in Appendix B to subpart DDDD. In the first option, facilities can qualify for inclusion in the delisted subcategory by using site specific emissions test data and look up tables that were developed using health protective input parameters. As a second option, facilities may choose to use a more refined site specific risk assessment. A more refined analysis requires more effort, but produces results that are less likely to overestimate risk. 308 Comment: One commenter noted that the regulatory requirements in the proposed rule focused on six HAP that are emitted from PCWP facilities: acrolein, acetaldehyde,

formaldehyde, methanol, phenol, and propionaldehyde. Those HAP represent 96 percent of the emissions from PCWP facilities. The commenter believes that any risk based compliance mechanisms may reasonably be limited to consideration of the risks from these six HAP. The commenter noted that EPA's preliminary risk analysis conducted prior to proposal narrowed the list of HAP emitted from PCWP facilities to include the following: acrolein,

acetaldehyde, formaldehyde, methanol, phenol, benzene,

methylene chloride, and manganese. The commenter referred to the results of their sensitivity analysis, which was conducted based on the data used in EPA's pre proposal risk analysis. The analysis evaluated the impact of increasing or decreasing facility emissions by 30 percent, using different health benchmarks than those identified in EPA's analysis, and conducting the risk assessment with the six HAP targeted in the proposed rule versus the additional HAP identified by EPA. The commenter's sensitivity analysis showed that formaldehyde and acetaldehyde made up the bulk of the cancer risk, while benzene and methylene chloride had little or no influence on cancer risk, depending on the scenario considered. Under all scenarios, acrolein 309 contributed the most non cancer risk. The remainder of the non cancer risk was divided between acetaldehyde,

formaldehyde and manganese, with manganese contributing between 5.6 and 12.2 percent of the non cancer risk,

depending on the scenario. Under all scenarios, methanol,

benzene, methylene chloride and phenol did not contribute at all to the non cancer risk from wood products facilities ( with one exception, where the phenol risk contribution was 0.1 percent). Based on these results, the commenter stated that there appeared to be little reason to include evaluation of methylene chloride or benzene in the risk based compliance option. However, the commenter stated that it may be reasonable to take an extremely conservative approach and include evaluation of manganese in the risk based compliance mechanisms.

Response: We agree that it is appropriate to limit the number of HAP that must be included in PCWP facility lowrisk demonstrations to only those HAP that may possibly result in meaningful contributions to the facility risk.

However, we disagree that limiting the HAP included in the low risk demonstration to the six HAP defined as total HAP in subpart DDDD of 40 CFR part 63 ( acrolein, acetaldehyde,

formaldehyde, methanol, phenol, and propionaldehyde) is appropriate. We identified the most prevalent HAP based on mass emitted for purposes of developing MACT compliance 310 options because MACT is technology based ( i. e., the same technology that reduces emissions of the six HAP also reduces emissions of other organic HAP). As discussed earlier in this preamble, the six HAP defined as total HAP in subpart DDDD of 40 CFR part 63 are the HAP that are most often emitted in detectable amounts from the most PCWP process units, and these HAP make up 96 percent of the mass of nationwide HAP emissions from the PCWP industry.

However, the risk associated with emissions of HAP are dependent on the mass emitted and the relative toxicity of each HAP. Thus, the HAP emitted in the greatest mass may not result in the most risk because the HAP may not be as potent as other HAP emitted in lower mass. For example,

methanol is the HAP emitted from the PCWP industry in the greatest mass, but because methanol is not as toxic as other HAP emitted ( e. g, formaldehyde, certain HAP metals), it does not result in as much risk as do other HAP. To ensure protection of public health, all HAP must be considered when determining which facilities are low risk. Simply importing the surrogate pollutants that are reasonably used for MACT purposes into the risk assessment context is not appropriate, as surrogacy for MACT is based on factors and considerations relating to technological control capabilities and not on how surrogate pollutants might indicate how non surrogates affect risks to human health and 311 the environment. For example, just because in many cases particulate matter is a useful surrogate for measuring the control efficiency of devices used to capture non mercury HAP metals, that fact is unrelated to what risks the HAP metals may present individually or collectively, as HAP metals apart from the risks they pose as being particulates.

The commenter is correct in that our preliminary risk analysis conducted prior to proposal narrowed the list of HAP emitted from PCWP facilities. We acknowledge receipt of the commenter's sensitivity analysis based on the data used in our pre proposal risk analysis. Following proposal, we conducted a more detailed risk analysis to evaluate the merits of including a low risk subcategory in the final PCWP rule. This post proposal analysis considered emissions of more than 30 HAP emitted from the PCWP source category.

Many of these HAP are only emitted in minute amounts that have been detected from a small number of PCWP process units. Nevertheless, we included them in our risk analysis to determine their contribution to PCWP facility risk. We reviewed the toxicity values for each HAP and the mass of each emitted from PCWP facilities to determine if it would be appropriate to narrow the list of HAP that PCWP facilities must consider in their low risk demonstrations.

Based on our review, we determined that 95 percent of the cancer risk at PCWP facilities is accounted for by the 312 following HAP: acetaldehyde, benzene, arsenic, beryllium,

cadmium, hexavalent chromium, lead, nickel subsulfide, and formaldehyde. We also determined that 95 percent of the non cancer risk at PCWP facilities is accounted for by the following HAP: acetaldehyde, acrolein, formaldehyde,

phenol, MDI, arsenic, cadmium, and manganese. We feel that inclusion of these HAP in a demonstration of eligibility of the low risk PCWP subcategory is appropriate. Limiting the list of HAP that must be included in the low risk demonstration to 13 HAP minimizes emissions testing costs,

while ensuring that the HAP that drive the risk at PCWP facilities are accounted for on a site specific basis.

Background, multipathway, and ecological exposures Comment: Two commenters argued that multipathway exposures should not be considered for PCWP facilities. One commenter stated that, because the HAP emitted from the PCWP source category are not bioaccumulative, it is unnecessary to consider multipathway exposures. The other commenter stated that there is no policy basis for considering multipathway exposures because U. S. Government surveys and regulatory actions demonstrate that non inhalation exposure to the six HAP emitted by wood products facilities is insignificant. The commenter provided rationale for the conclusion that dietary and drinking water exposures to the six HAP are not significant. Because the six HAP primarily 313 emitted from the PCWP source category ( acetaldehyde,

acrolein, and formaldehyde, methanol, phenol, and propionaldehyde) do not exhibit bioaccumulative characteristics, the commenter considered it unnecessary to consider multipathway exposures.

Three commenters argued that multipathway exposures should be considered for PCWP facilities. One commenter stated that, when persistent biological toxicant or metal emissions are significant, ingestion and other pathways should be considered in the risk screening. Another commenter stated that the concentration based applicability threshold approach in the proposed PCWP rule does not address non inhalation exposures or adverse effects on the environment. The third commenter stated that CAA section 112( d)( 4) requires EPA to consider all possible ways that a pollutant could affect human health or the environment because it refers to pollutants " for which a health threshold has been established," i. e., pollutants that have no adverse health or environmental effects. ( See 5 Legislative History at 8511.) According to the commenter,

EPA has recognized repeatedly in the past that many of the pollutants emitted by the source category are re deposited from the atmosphere and then contaminate soil and water for long periods of time. The commenter added that these pollutants bioaccumulate in wildlife and food sources, 314 poisoning people and animals alike. ( See, e. g., 64 FR 52828, 53014 ( September 30, 1999); 64 FR 31898, 31908 31909 ( June 14, 1999); 63 FR 14182, 14193 ( March 28, 1998); 61 FR 17358, 17478 ( April 19, 1996) ( due to bioaccumulation,

mercury levels may be 10,000,000 times higher in fish than in water those fish inhabit).) The commenter concluded that, to evaluate whether a pollutant is a threshold pollutant and what its health threshold and ample margin of safety must be, EPA must consider all the potential health and environmental effects of deposition, persistence and bioaccumulation of that pollutant. The commenter argued that EPA would contravene section 112( d)( 4) by considering only health effects caused by inhalation.

Response: We agree with the last commenter that CAA section 112( d)( 4) requires us to evaluate ecological and multimedia human exposures when determining the applicability of section 112( d)( 4). This rule is relying not on CAA section 112( d)( 4), but on section 112( c)( 9),

which also hasstates that as similar requirements for determining risk applicability. Although the commenter refers to CAA section 112( d)( 4), their concern is addressed in section 112( c)( 9) also. We agree that potential ecological effects and multimedia human exposures need to be considered. We have conducted an ecological assessment and a multipathway exposure assessment on those HAP emitted from 315 PCWP facilities ( including HAP not among the six mentioned by one commenter) that we have identified as having the potential for persisting and bioaccumulating in the environment. From this analysis we determined that adverse ecological effects and/ or multimedia health effects are unlikely from PCWP facilities. Therefore, PCWP facilities attempting to demonstrate their low risk status will not be required to include an ecological assessment or a multimedia assessment.

Comment: Several commenters stated that there is no legal or policy basis for EPA to consider background or multipathway ( non inhalation) exposures. The commenters claimed that CAA section 112( d) requires that MACT standards be based only on emissions from the MACT regulated portion of the facility; it does not give EPA the authority to consider existing background levels. One commenter asserted that CAA section 112 can be distinguished from other statutory provisions, both in the CAA and in other environmental legislation, where EPA has clearly been given authority to consider background sources.

Another commenter argued that the CAA's legislative history does not support a requirement to consider other exposures. The commenter also claimed that the statutory provisions on which EPA would rely to implement the risk based mechanisms ( i. e., CAA section 112( d)( 4), CAA section 112( c)( 9)( B), or EPA's de minimis authority) 316 exclusively focus on the emissions from the source in making regulatory decisions. According to the commenter, EPA has existing regulatory programs ( e. g., for mobile and area sources ( Urban Air Toxics Strategy)) in place to address HAP emissions from other sources.

The commenter argued that over control of PCWP facilities is unjustified because PCWP facilities account for very small proportions of HAP emissions nationwideBless than 1.75 percent of acetaldehyde, 1.7 percent of acrolein,

and 1 percent of formaldehyde emissions, according to their industry sponsored risk assessment. Given these results,

the commenter concluded that PCWP facilities cannot reasonably be considered to contribute meaningfully to background concentrations.

The commenter stated that CAA section 112( d)( 4) does not require the complete absence of a hazard and noted that all substances have adverse effects on health and the environment at sufficiently high doses. Accordingly,

section 112( d)( 4) incorporates the concept of exposure, and the Senate Report made clear that the provision was applicable in instances where " it is possible to establish a NOEL below which human exposure is presumably safe." In addition, delisting criteria and the so called trigger component of the residual risk provision focus exclusively on emissions and whether the risk posed by any source in the category, by itself, exceeds one in a million cancer risk, 317 the commenter claimed.

Two commenters opposed the use of available data on background concentrations and facility specific measurement of background concentrations to determine the extent of exposures from other sources, arguing that the CAA and sound public policy warrant a focus exclusively on the emissions from the source category at hand when evaluating the applicability of a risk based compliance option. Because a HI of 1.0 ( or higher) is amply protective of public health and is warranted under EPA's statutory mandate, the commenters stated that consideration of background concentration is not appropriate.

Response: We disagree that we do not have the authority to consider background andare not considering background emissions in this section 112( c)( 9) delisting of the lowrisk PCWP subcategory. Rather, as we indicated in the Residual Risk Report to Congress, the Agency intends to assess such facility wide emissions of HAP in future residual risk actions under section 112( f)( 2), as appropriate and to the extent it is reasonable and appropriate to do so. See also, 54 Fed. Reg. at 38,059 ( Sept. 14, 1989) ( benzene NESHAP).

Regarding multipathway exposures. Section 112( f)( 1)( C) of the CAA explicitly directed EPA to investigate and report on risks presented by background concentrations of HAP in its residual risk report to 318 Congress, which presumably it would not have done if it assumed EPA was to ignore such concentrations in the subsequent risk based regulatory process. In fact, EPA explained in the Report that we generally would consider background concentrations as part of residual risk decisionmaking ( see Report at pp. 92 93). In our 1989 benzene NESHAP rulemaking, which Congress explicitly endorsed in CAA section 112( f)( 2)( B), we stated that we agree that exposures to background concentrations and multiple sources of a pollutant may be considered to the extent it is practical and reasonable to do so. See 54 FR at 38059 ( Sept. 14,

1989). Moreover, the legislative history to the 1990 CAA Amendments explicitly states that all risk from colocated sources at major sources subject to MACT must be considered,

albeit not necessarily at the same time. See 1 Legis. Hist.

at 868 69 ( statement of Sen. Durenberger, floor manager of air toxics provisions, during the debate on the Conference Report). Finally, we disagree with assertions that background levels are already accounted for by uncertainty factors built into RfC/ RfDs, as having no factual basis.

Commenters presumed the use of CAA section 112( d)( 4) for the PCWP delisting. However, we are using CAA section 112( c)( 9)

and not CAA section 112( d)( 4). For delisting purposes, we are required to make not only a determination of a level which is safe to the individual exposed to the maximum concentration but also a determination whether that 319 individual and the general population are protected to a level that provides a clear ample margin of safety in order to have the confidence that a decision to exclude these facilities from regulation ( including residual risk) may be made. These are the same requirements that are found as the trigger in the residual risk program because all decisions of this type under section 112 of the CAA ( sections 112( c)( 9), d( 4), and f( 2)) should be consistent and based on the same risk approaches.

We agree that imposition of costly regulatory requirements on sources that do not contribute meaningfully to background exposures is not beneficial. However,

regarding the results of the industry sponsored wood products MACT risk assessment, we note that CAA section 112( c)( 9) requires the evaluation of all facility level emission points, all HAP, and all pathways in order to satisfy that the emissions from these facilities provide protection of the population with an ample margin of safety.

The, the industry's wood products MACT risk assessment does not address HAP emitted from PCWP facilitiesaffected sources that have the potential to bioaccumulate and persist in the environment ( e. g., lead, cadmium, and mercury). We conducted an exposure assessment for these HAP to determine exposure from ingestion as well as inhalation. The maximum multipathway risks were considerably lower than the predicted maximum inhalation risks from the PCWP source 320 category. Therefore, PCWP facilitiesaffected sources are not required to conduct site specific multipathway risk assessments as part of their low risklow risk demonstrations. The look up tables included in appendix B to subpart DDDD were developed using conservative input parameters to ensure that facilitiesaffected sources qualifying for the low risk subcategory based on the look up tables would not pose a risk to multimedia endpointsvia multipathway exposures.

As discussed elsewhere in this preamble, for today's final PCWP rule, we consider that an HI limit of 1.0 provides an ample margin of safety for protecting public health under CAA section 112( c)( 9) for this delisting of low risk PCWP affected sources. The RfCs that are used to calculate the HI are developed to protect sensitive subgroups and to account for scientific uncertainties and their inclusion helps ensure, ensuring that usingthe use of an HI limit of 1.0 provides an ample margin of safety. We believeconclude that an HI limit of 1.0 is appropriate for the section 112( c)( 9) demonstrations for the PCWP source category that are described in today'' s action. In future risk based actions for this and other source categories ( e. g., residual risk rulemakings under CAA section 112( f))

we may identify factors on a case by casecase by case basis that would lead us to conclude that HI limits other than 1.0 would be more appropriate for those other actions. 321 The look up tables included in appendix B to subpart DDDD of 40 CFR part 63 were developed based on an HI of 1.0.

For site specific chronic inhalation risk assessments,

facilitiesaffected sources are required to ensure that the TOSHI is less thantheir TOSHI ( or, alternately, a sitespecific set of hazard indices based on mechanistic data or dose response data for their HAP mixture) are less than or equal to a value of 1.0. Facilities are required to determine TOSHI forThese assessments focus on respiratory effects and CNS effects, because based on our analysis,

noncancer impacts were dominated primarily by impacts on these systems. Other target organs or systems were found to be negligibly impacted.

Comment: One commenter stated that EPA had provided inadequate discussion of how environmental risks would be evaluated. The commenter added that the CAA requires EPA consider the environment as well as public health, and that,

at a minimum, a facility would be required to conduct an assessment based on EPA's 1998 Guidelines for Ecosystem Assessment. The commenter referred EPA to appendix A of " Generic Assessment for Endpoints for Ecological Risk Assessment" for a detailed discussion on the legal basis from " such statutes as the CAA... that require EPA to consider and protect organism level attributes or various taxa including fish, birds, and plants and more generally, 322 animals, wildlife, aquatic life, and living things."

Another commenter cited an analysis they commissioned that showed it to be highly unlikely that emissions from PCWP facilities would pose a hazard to ecological receptors at levels that are protective of human health. Thus,

concern over ecological receptors would not provide a valid basis for reducing the HI below 1.0.

Response: An ecological assessment is required under sections 112( d)( 4), ( c)( 9), and ( f)( 2) of the CAA regarding the presence or absence of " adverse environmental effects"

as that term is defined in CAA section 112( a)( 7).

Therefore, delisting under section 112( c)( 9) requires consideration of ecological effects. The look up tables developed for today's final PCWP rule are intended to accommodate enough conservatism that any facility qualifying for inclusion in the delisted subcategory using them will qualify based on all endpoints, including ecological endpoints. Based on our analysis of ecological effects ( in the supporting information for the final rule), we feel it is unlikely that PCWP facilities would pose any significant ecological risks to any actual ecosystem or ecosystems nearby. We also conclude, given the low impacts from the hypothetical worst case scenario investigated, that it is unlikely that any potentially exposed threatened or endangered species would be adversely affected by HAP 323 emissions from these facilities. Therefore, PCWP facilities are not required to conduct site specific ecological risk assessments as part of their low risk demonstration.

Assuming the assessment referenced by the first commenter included only the six HAP listed in subpart DDDD of 40 CFR part 63, we disagree that these six HAP should be the sole focus of an ecological assessment. It is not clear from the comment whether the commenter is suggesting that we might consider lowering the human health HI values to below 1.0 in order to reflect ecological concerns or whether they are suggesting that an ecological HI value should not be reduced below 1.0. In the former case, that is not done.

Human health and ecological assessments are independent assessments with their own risk management criteria.

Hazard index Comment: Two commenters stated that hazard quotients ( HQ) for chemical mixes should not be summed to determine the HI unless the primary effects are on the same organ by the same mechanism; otherwise the risk would be overestimated. One commenter stated that CAA section 112( d)( 4) refers to threshold pollutants, with each health threshold augmented by an ample margin of safety. These ample margin of safety values are already incorporated into RfC values. The risk criteria applied are confined to the effects upon which the RfC is based, which reflect the most 324 sensitive target organ. According to the commenter, a decision to add risk posed by chemicals that affect the same target organ but have unknown mechanisms of action represents an unnecessarily conservative assumption that would tend to inflate the final risk estimate.

The commenters also noted that, according to the National Research Council and the Presidential/ Congressional Commission on Risk Assessment and Risk Management,

additivity at low doses is more likely to overestimate than to underestimate total risk. As stated in the Commission's 1997 Final Report: " When the individual components of a chemical mixture exhibit different kinds of toxicity or have different biological mechanisms of toxicity, they do not interact   they act independently at low doses. In that case,

the dose response relationships for each chemical should be considered independently... [ By contrast] studies in which similar chemicals with similar mechanisms and target were administered simultaneously indicate that antagonism is the usual outcome..."( Falk and Kotin 1964, Schmal et al. 1977)

Response: Our recommended approach for assessing risks from exposure to a mixture of pollutants is to utilize a 325 4U. S. Environmental Protection Agency. 1986. Guidance for Conducting Health Risk Assessment of Chemical Mixtures. Risk Assessment Forum, Washington, DC. EPA/ 630/ R 98/ 002; available at http:// cfpub. epa. gov/ ncea/ raf/ recordisplay. cfm? deid= 20533. ( EPA 1986)

5U. S. Environmental Protection Agency. 2000. Supplementary Guidance for Conducting Health Risk Assessment of Chemical Mixtures. Office of Research and Development. EPA/ 630/ R 00/ 002 ( EPA 2000)

6Ibid. dose response assessment developed for that mixture. 4,5 There are few mixtures ( e. g., coke oven emissions), however,

for which such assessments are available. When mixturespecific dose response assessments are not available, a component by component approach is recommended. The method for component data depends on a judgment of toxicologic similarity among components. The specific term toxicologic similarity represents a general knowledge about the action of a chemical or a mixture and can be expressed in broad terms such as at the target organ level in the body. In our guidance, assumptions about toxicologic similarity are made in order to choose among risk assessment methods. In general, we assume a similar mode of action across mixtures or mixture components and, in some cases, this requirement may be relaxed to require that these chemicals act only on the same target organ. 6 The primary method for component based risk assessment of toxicologically similar chemicals is the HI, which is derived from dose addition. In our guidance, dose addition 326 7Ibid. is interpreted as simple similar action, where the component chemicals act as if they are dilutions or concentrations of each other differing only in relative toxicity. Dose additivity may not hold for all toxic effects. Furthermore,

the relative toxic potency between chemicals may differ from different types of toxicity or toxicity by different routes.

To reflect these differences, the HI is then usually developed for each exposure route of interest, and for a single specific toxic effect of toxicity to a single target organ. A mixture may then be assessed by several HI, each representing one route and one toxic effect or target organ. 7 To assess the cumulative risk or hazard associated with nonlinear effects of HAP in our analysis of PCWP facilities,

HAP hazard quotients pertaining to the same target organs or systems are summed to generate TOSHI. While it may be preferable to focus on the addition of HAP HQ that involve the same mechanism or mode of action, that level of information is not generally available for all of the HAP on which we are focusing. Pending the availability of such data for the HAP components of the mixture being assessed,

the method employed under CAA section 112( c)( 9) is to aggregate HAP HQ by target organ to generate a TOSHI.

Comment: Two commenters supported a HI of 1.0 ( or 327 greater) as an appropriate benchmark for comparing exposures attributable to facility emissions, which should fully provide for the statutory mandate of an ample margin of safety. The commenters referred to the 1997 Final Report of the Presidential/ Congressional Commission on Risk Assessment and Risk Management in Regulatory Decision Making as support for their position. Specifically, the Commission supported a noncancer HI of 10.0, stating that there are few HAP with RfC values within a factor of 10 of their no observable adverse effects level ( NOAEL). Because RfC values are typically one thousandth of a NOAEL, a noncancer HI of 10.0 in those cases would still leave a margin of exposure of 100. The Commission recommended that EPA should, on the basis of screening assessments of source categories, do further risk assessment and analysis of categories where the noncancer HI exceeds 10.0. Where more detailed risk assessments yield noncancer hazard indices less than 1.0,

the Commission recommended that no further action be required. The commenters agreed that sources should not be required to go below that level ( e. g., to an arbitrary level such as 0.2), arguing that EPA has neither a legal mandate nor a rational basis for limiting the HI to less than 1.0.

One of the commenters stated that the comparison of RfC or similarly derived health benchmarks to modeled maximum annual average concentrations is extremely health protective 328 and meets the ample margin of safety requirement of the statute. Given this high degree of conservatism, the commenter stated that neither the CAA nor sound policy requires that background and multipathway exposures be incorporated into an evaluation of the degree of risk posed by affected sources. Under these circumstances, the commenter argued, the mere possibility of exposure from multiple sources, or multiple HAP from a single source, does not justify a uniform adjustment to all RfC values or similarly derived health benchmarks for all facilities.

Similarly, the commenter believed that EPA should not mandate modeling risks from the entire facility, but rather only from the portions of the facility that are within the source category.

Two other commenters objected to a noncancer HI of 1.0 ( or greater). The first commenter stated that, while the HI is useful in evaluating site specific impacts, choosing a generic HI ( some multiple of 1.0) for application to a wide range of sites is inappropriate. The commenter added that selection of an arbitrary multiple of 1.0 is not science,

does not conform with CAA section 112( d)( 4), and does not protect public health. The commenter stated that the selection of a HI of 0.2 as a rough screening tool seemed reasonable, although it was unsupported by any analysis.

The commenter added that if a default HI is used, then EPA 329 should include a provision that would disallow its use to exclude a facility from MACT if better background information is available suggesting the default HI does not protect public health. However, the commenter believed that the CAA does not support an interpretation that includes the use of such a default to allow exemptions for individual sources. The commenter believed that the expansion of the interpretation to include non threshold pollutants is in direct conflict with section 112( d)( 4).

The second commenter evaluated the four potential options that EPA proposed to ensure that a risk analysis under CAA section 112( d)( 4) considered the total ambient air concentrations of all the HAP to which the public is exposed. Option 1, which requires that the HI for all pollutants be no greater than 1.0, does not consider additional sources or background and is unacceptable,

according to the commenter. Option 3, which uses existing data such as NATA to determine background and requires that the HI be no greater than 1.0, is also unacceptable,

according to the commenter. The commenter pointed out that EPA has clearly stated at public meetings that the NATA is not to be used to make regulatory decisions. ( As the first commenter noted, NATA information includes warnings that the information is useful for large scale planning purposes and not for local area assessment.) The commenter added that 330 NATA relies on data submitted to EPA voluntarily and has been reported to consistently underestimate measured concentrations. Until EPA requires that HAP inventories be submitted as proposed in the Consolidated Emissions Reporting Rule ( CERR), and the NATA conducts refined modeling around stationary sources, the commenter argued that NATA should not be considered for estimating background concentrations. Option 4, which allows individual facilities to monitor the HAP backgrounds for use in their own analysis, requires oversight and evaluation by the States to ensure proper site selections and analytical methods and should not be considered, according to the commenter. The commenter believed Option 2, which requires that the HI be no greater than 0.2, would be the only viable option at this time using a conservative risk screening analysis. However, the commenter did not endorse using any of the proposed threshold limit applicability methods to exempt process sources from NESHAP requirements.

Two other commenters raised additional objections to EPA's proposed methodologies for determining the contribution of other sources to the overall hazard. The first commenter stated that EPA had not discussed the need to assess cumulative risks, aggregate exposures, and health impacts associated with exposure to chemical mixtures emitted from facilities within the source categories. The 331 commenter referred EPA to the extensive progress that has been made in more completely addressing risks from exposure to air pollution and integrated decisionmaking in such areas as children's risk issues, cumulative exposure, and chemical mixtures. The commenter requested that the recent advancements be incorporated into the risk assessment methods and overall cost estimates associated with riskbased exemptions in the proposed rules.

The second commenter stated that EPA's proposed alternative methodologies for determining the contribution of other sources to cumulative risk are untenable and deeply flawed. According to the commenter, the first and second approaches ( HI of 1.0 and HI of 0.2) would allow exemptions based on blanket assumptions about exposure, but EPA provided no basis for making any assumption. The commenter noted that the third option suggests relying on existing estimates of background levels of certain HAP, but argued that these information sources ( e. g., NATA, ATSDR) are neither designed nor adequately precise to be used as the basis of regulatory applicability determinations. According to the commenter, EPA has cautioned that NATA emission estimates " cannot be used to identify exposures and risks for specific individuals, or even to identify exposures and risks in small geographic regions such as a specific census tract." ( U. S. EPA, Limitations in the 1996 National Scale 332 Air Toxics Assessment) The commenter pointed out that NATA does not estimate exposure to a number of HAP, ( e. g.,

hydrogen fluoride ( HF), HCl), and the ATSDR profiles offer generalized assessments, but are not specific enough to establish as baseline for a given facility.

Response: For today's final PCWP rule, we are considering an HI limit of 1.0 to provide an ample margin of safety for protecting public health under CAA section 112( c)( 9). However, we do not feel that increasing the HI limit above 1.0 is justified by currently available science.

Safety factors are included in the dose response values used to calculate the HI to account for scientific uncertainties,

and their inclusion helps ensure that using a HI limit of 1.0 provides an ample margin of safety. The TOSHI approach required for site specific risk assessment in today's final PCWP rule assumes additivity in mixtures of chemicals that target the same organ system. For their site specific risk assessments, facilities are requiredencouraged to determine TOSHI for respiratory and CNS effects to simplify analysis.

More detailed analysis of mixture additivity, incorporating mechanistic data and uncertainty and including dose response data for specific mixtures, where available, may also be included in site specific analyses using scientificallyaccepted peer reviewed methodologies. Based on our analysis, noncancer impacts were dominated primarily by 333 impacts on these systems and other target organ systems were found to be negligibly impacted. We are not using background concentrations from NATA in today's final PCWP rule. Several commenters presumed the use of CAA section 112( d)( 4) for the PCWP rule as proposed. However, we are using CAA section 112( c)( 9) and not section 112( d)( 4).

Discussion of our authority to consider background and multipathway exposures is provided elsewhere in this section.

Tiered approach Comment: Several commenters supported EPA's proposed tiered modeling approach, which begins with simple look up tables and progresses to more refined facility specific risk assessments. One commenter noted that the State of Wisconsin uses a tiered approach similar to the approach proposed by EPA, and in general, this approach has worked well. The approach first allows sources to demonstrate compliance if their potential emissions, stack height, and exhaust direction are within the ranges provided in conservative look up tables. The second tier allows facilities to provide site specific modeling to demonstrate compliance with ambient air standards at the property line.

Another commenter added that EPA should be flexible in accepting evolving improvements in exposure assessment and risk modeling, and should take into account the inherent 334 strengths and weaknesses of the types of modeling used. A third commenter noted that most sources would use the tiered modeling approach but believed that facilities should be allowed to use any EPA approved modeling technique to demonstrate that their emissions are below the applicable health benchmark. The commenter also recommended that, for the final PCWP rule, EPA adopt the model regulatory text that they provided for the risk based framework.

One commenter opposed EPA's proposed tiered modeling approach, stating that if EPA decided to pursue a generic risk screening approach under section 112( d)( 4), it would need to be conservative. According to the commenter, the use of a ( non tiered) conservative approach would represent the least cost to the regulated community and would be the least time consuming for States reviewing the facility's application.

Response: We acknowledge the model regulatory text submitted by one of the commenters. However, as discussed elsewhere, we developed our own regulatory text to specify how facilities must demonstrate that they are part of the low risk subcategory. We discussed a tiered analytical approach in the preamble to the proposed rule, beginning with relatively simple lookup tables and followed by increasingly more site specific but more resource intensive tiers of analysis, with each tier being more refined and 335 less conservative. We are specifying a similar approach for today's final rule in Appendix B to subpart DDDD.

Facilities can qualify for inclusion in the delisted subcategory by using site specific emissions test data and look up tables that were developed using conservative input parameters. These look up tables are included in Appendix B to subpart DDDD. Facilities that cannot demonstrate eligibility for the low risk subcategory based on the conservative screening assessment ( i. e., look up tables) may used more refined site specific risk assessments as described in appendix B to subpart DDDD of 40 CFR part 63 and in other analytical tools such as the " Air Toxics Risk Assessment Reference Library," ( which may be appropriate for specific sources). A more refined analysis requires more effort, but produces results that are less uncertain and less conservative ( i. e., less likely to overestimate risk).

Appendix B to subpart DDDD of 40 CFR part 63 describes a process based on this air toxics approach that must be used by facilities attempting to demonstrate eligibility for the low risk subcategory of PCWP facilities. Facilities may demonstrate eligibility for the low risk subcategory using either the look up table analysis ( described in appendix B to subpart DDDD of 40 CFR part 63) or a tiered site specific analysis ( described in appendix B to subpart DDDD of 40 CFR part 63 and other analytical tools which may be appropriate 336 for a specific source, such as the " Air Toxics Risk Assessment Reference Library.") As discussed elsewhere in this section, through low risk demonstrations. Also, we will be reviewing the low risk demonstrations submitted by PCWP facilities to remove the burden of reviewing risk assessments from States.

We will review all risk assessments performed in support of a demonstration of eligibility for the low risk subcategory with regard to a variety of aspects, including the consistency of the methodology and modeling techniques with those currently accepted by the scientific community and EPA. However, we will consider assessments that use risk methodology and modeling techniques in addition to or in lieu of those described in EPA's " Air Toxics Risk Assessment Reference Library," as appropriate, provided they have undergone scientific peer review pertinent to their use in the submitted assessment.

Comment: One commenter stated that, for EPA to conduct an up front risk analysis, the procedure would need to be conducted using the most conservative stack parameters, with a hypothetical facility fence line to satisfy the many impact scenarios that could occur.

Response: We conducted a rough risk assessment to estimate the number of PCWP facilities that might qualify for the delisted low risk subcategory. The data used in our 337 rough risk assessment were a combination of facilityspecific data ( e. g., process unit throughput) and industry average data ( e. g., industry average stack parameters,

average emission factors for estimating emissions).

Facilities do not qualify for the low risk subcategory based on our rough risk assessment, with the exception of eight facilities who were determined to pose very low risk based on our analysis ( i. e. g., with TOSHI less than 0.1, and a cancer risk of less than 0.1 in 1 million). However,

facilities can qualify for inclusion in the delisted subcategory by using site specific emissions test data and the look up tables or by conducting a low risk demonstration, as described in appendix B to subpart DDDD of 40 CFR part 63 and in other analytical tools such as the " Air Toxics Risk Assessment Reference Library," ( which may be appropriate for specific sources). Look up tables were developed using the conservativehealth protective air dispersion model SCREEN3. Stack height and fenceline distance vary in the tables, so facilities will choose the most appropriate combination of these parameters. Invariant facility parameters built into the look up tables are either average values or biased towards conservativehealthprotective values, based on available data. Thus, we believe the look up tables are appropriately conservativehealth protective to accommodate the many impact 338 scenarios that could occur.

Risk assessment guidance Comment: Several commenters stated that EPA neglected to follow its own guidelines and science policies in its proposal for risk based exemptions. One commenter argued that EPA had proposed a disorganized and cursory approach to implement risk based exemptions that fell far below the quality of risk analysis typically required by EPA across its other programs. According to the commenter, the proposal did not adhere to EPA's established guidelines for characterizing human health and ecological risks, did not incorporate risk assessment guidelines for conducting multipathway risk assessments, and did not reference EPA guidelines for cumulative risk assessment that specifically require consideration of non inhalation pathways. The commenter noted that EPA's March 1995 Risk Characterization Policy set goals of transparency, clarity, consistency, and reasonableness which apply to risk assessment practices across EPA. The commenter argued that the inconsistencies between EPA's proposal to provide risk based exemptions in the MACT standard process and its risk assessment guidelines would undermine many regulatory programs throughout EPA.

The commenter stated that the risk based scheme was based on a fundamental misunderstanding of the use of public health and ecological risk assessments in the regulatory 339 process. The commenter added that the Federal risk assessment guidelines require EPA to conduct risk assessments consistently across all Federal environmental programs. According to the commenter, the approaches outlined by industry's white papers neglected to include risk characterization, which provides needed and appropriate information to decision makers. The approaches also did not incorporate the critical recommendation of the Commission of Risk Assessment and Risk Management to establish a framework for stakeholder based risk management decision making. The commenter stated that these omissions in the proposal would prevent regulatory agencies from demonstrating to the public that public health and the environment are adequately protected.

Several commenters stated that EPA also needed to be consistent with residual risk guidelines currently under development. One commenter stated that the tools needed to identify sources eligible for the risk based exemption would be the same tools necessary for a CAA section 112( f)

residual risk assessment, which the commenter understood were not yet ready for general use. Another commenter noted that the cancer risk guidelines are currently undergoing public review.

A third commenter stated they had serious reservations about EPA's apparent attempt to conduct an ad hoc risk 340 analysis for specific source categories by seeking comments on the specific elements to be included in the risk analysis. The commenter did not believe these rulemakings were an adequate forum to develop this risk analysis process. The commenter indicated that any risk analysis conducted by the EPA must adhere to the risk assessment principles outlined in the Residual Risk Report to Congress.

One commenter argued that the proposal is consistent with EPA risk assessment guidelines and policies and believed that others' technical objections were without merit. The commenter added that the contemplated risk based applicability criteria were not in conflict with the classification of carcinogens and noncarcinogens.