coherence to the statutory scheme.

While we do not believe 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 lowrisk sources that are defined according to the traditional criteria under section 111. It is beyond dispute that Congress determined that certain identifiable groups or sets 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 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 higher risk facilities in the PCWP category, must remain subject to 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 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. 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 believe it is reasonable to conclude that Congress would not have intended to over regulate the low risk 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 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 section 112, this legislative history is not 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 low risk 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 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 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 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 proposing to delist 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 include facilities producing particleboard,

molded particleboard, medium density fiberboard, softwood plywood, softwood veneer, fiberboard, engineered wood products, hardboard, and oriented strandboard. ( Need to determine who we say is in the subcategory and revise accordingly.) However, there are also major sources that produce these products that are not low risk, and therefore, product type cannot be used to define the low risk 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 low risk 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 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 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 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 believes 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 be an unlawful weakening of the MACT floor for the larger PCWP category. To that end, 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 beyondthe floor measures, or conducting risk based actions under 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 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 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 de list a subcategory under section 112( c)( 9) when that subcategory otherwise meets the statutory criteria for de listing.

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 a. Dose response values Comment: Two commenters suggested that EPA incorporate into the PCWP rule the findings of the nationwide wood products risk assessment, which demonstrates that the vast majority of wood products sources cause no 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 offsite 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 ( 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 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 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 believed 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 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. Appendix B to Subpart DDDD includes procedures that facilities may use to demonstrate that they are part of the low risk subcategory, and,

therefore, are not subject to the compliance options included in today's final PCWP rule. To demonstrate eligibility for the low risk subcategory, facilities must first conduct emissions testing for up to 13 pollutants ( 5 organic HAP from all process units, 7 metal HAP from direct fired process units, and MDI from presses processing product containing MDI resin). The rationale for selection of these 13 pollutants is described elsewhere in this section. Facilities must use the results from emissions testing to determine if 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) or site specific risk assessment methodology ( described in Appendix B to Subpart DDDD and the " Air Toxics Risk Assessment Reference Library") and risk benchmarks ( described in Appendix B to Subpart DDDD).

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 nondetects 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, Appendix B to Subpart DDDD states that facilities may use zero for non detects when all of the emission test runs are below the MDL, provided that certain criteria to ensure that emissions testing and analysis procedures are adequate to detect low concentrations of HAP.

Comment: One commenter stated that 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 section 112( d)( 4)

because PCWP facilities emit non threshold pollutants.

Therefore, 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 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 non threshold 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 healthbased 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 stated previously, we will not be relying exclusively on IRIS values, but will be considering all credible and readily available assessments. 1 For air 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 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 changing healthbased guideline values is a general challenge in setting health based regulations. However, we are committed to http:// www. epa. gov/ ttn/ oarpg/ t3/ meta/ m8690. html. ( EPA 1999) 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 ( and/ or the " Air Toxics Risk Assessment Reference Library") 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.

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 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 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 (

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

g/ m3 for acetaldehyde.

Response: We are re evaluating the dose response value for 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 this time because there is a courtordered deadline. 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 cannot wait for our acetaldehyde reassessment to be completed due to the court ordered deadline for the final PCWP rule. Therefore, we are relying on the IRIS values for acetaldehyde in both cancer and non cancer risk assessments for this rule.

Facilities conducting low risk demonstrations should refer to Appendix B to Subpart DDDD ( and/ or the " Air Toxics Risk Assessment Reference Library") for guidance on choosing appropriate dose response 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 nonthreshold pollutants that may trigger a carcinogenic 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 an 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.

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 this rule, we are using an alternative dose response value for formaldehyde based on a peerreviewed publicly available assessment. However, we do not have comparable quantitative information for acetaldehyde. Therefore, we will use the current IRIS value. Facilities conducting low risk demonstrations should refer to Appendix B to Subpart DDDD ( 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 mg/ m3 as an RfC. The commenter noted that this value is the Noncancer Chronic REL derived by CalEPA. The commenter stated that CalEPA's 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

g/ m3 replaces the previous RfC of 0.02

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.

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

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

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

g/ m3 value was derived based on the 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 notice 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 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 believe 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 ( 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 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 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 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 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 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 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 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) the management of risk for nonlinear effects under 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 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 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 3Ibid. number of conservative 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: For air toxics risk assessments, we are implementing a tiered approach. The screening level, or tier of analysis, is designed, through the use of conservative inputs, to identify situations for risks that are unlikely to be of concern. Screening tier analyses are designed to be relatively simple, inexpensive, and quick, using existing data, defined decision criteria, and models with simplifying conservative assumptions as inputs. More refined levels of analysis include the refinement of aspects of the analysis that are thought to influence risk most or may contain the greatest uncertainty. At the refined tier, each analysis requires more effort, but produces results that are less uncertain and less conservative ( i. e., less likely to overestimate risk). 4 Appendix B to Subpart DDDD describes a process based on this air toxics approach particular to risk assessment for CAA section 112( c)( 9).

b. HAP to include in risk assessments 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 preproposal risk analysis. The analysis evaluated the impact 4EPA 1999 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 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 riskbased 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 low risk demonstrations to only those HAP that 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 ( 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 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 in section IV. A of this preamble, the six HAP defined as " total HAP" in Subpart DDDD 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.

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 following HAP:

acetaldehyde, benzene, arsenic, beryllium, cadmium,

hexavalent chromium, lead, nickel subsulfide,

formaldehyde, and arsenic. 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 believe 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.

c. 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 noninhalation 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 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 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, 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 include oral exposures as well as inhalation 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 has 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 exposures via all relevant routes need to be considered, and we have conducted a multipathway exposure assessment on those HAP emitted from PCWP facilities ( including HAP not among the six mentioned by one commenter) that we have identified as having the potential health concerns by pathways other than inhalation of ambient air. The three HAP identified, and which were analyzed for risk associated with projected oral exposures, are mercury, lead, and cadmium. Note:

Place holder for multipathway discussion.

Comment: Several commenters stated that there is no legal or policy basis for EPA to consider background or multipathway ( non inhalation) exposures. The commenters noted 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 noted that 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 noted that the CAA's legislative history does not support a requirement to consider other exposures. The commenter also pointed out that the statutory provisions on which EPA would rely to implement the risk based mechanisms ( i. e., section 112( d)( 4),

section 112( c)( 9)( B), or EPA's de minimis authority)

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 nationwide   less 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 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 ` no observable effects level' ( 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.

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 and multipathway exposures. ( Need OGC help to explain our authority to consider background and multipathway exposures.)

Commenters presumed the use of CAA section 112( d)( 4) for the PCWP delisting. However, we are using section 112( c)( 9) and not 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 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 Title III 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 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 wood products MACT risk assessment does not address HAP emitted from PCWP facilities 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. Note: Need to present conclusion of EPA's multipathway analysis. The look up tables included in Appendix B to subpart DDDD were developed using conservative input parameters to ensure that facilities qualifying for the low risk subcategory based on the look up tables would not pose a risk to multimedia endpoints.

As discussed elsewhere, for today's final PCWP rule,

we are considering a HI limit of 1.0 to provide an ample margin of safety for protecting public health under section 112( c)( 9). Safety factors are included 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 look up tables included in Appendix B to Subpart DDDD were developed based on a HI of 1.0.

For site specific chronic inhalation risk assessments,

facilities are required to ensure that the target organ specific hazard index ( TOSHI) for exceeds a value of 1.0.

Note: Need to explain why limiting to CNS and respiratory effects.

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, 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 Title III of the CAA, including sections 112( d)( 4),

( c)( 9), and ( f)( 2). 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. Note: Hold for ecological risk discussion.

Assuming the assessment referenced by the first commenter included only the six HAP listed in subpart DDDD, 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.

d. Hazard index ( HI)

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 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 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 dose response assessment developed for that mixture. 5,6 There are few mixtures ( e. g., coke oven emissions), however, for which such assessments are available. When mixture specific dose response 5U. 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)

6U. 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) 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. 7 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 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 7Ibid. 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. 8 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 greater) as an appropriate benchmark for comparing exposures attributable to facility emissions, which should 8Ibid. 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 healthprotective 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 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 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 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 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 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 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 risk based 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 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 section 112( c)( 9). However, we do not believe 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. Note: May want to repeat exmplanation here of why we selected TOSHI for only CNS and respiratory effects. 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 delisting. 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.

e. Tiered approach Comment: Several commenters supported EPA's proposed tiered modeling approach, which begins with simple "

lookup tables" and progresses to more refined facilityspecific 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 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 riskbased 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, we developed our own regulatory text to specify how facilities must demonstrate that they are part of the low risk subcategory. For air toxics risk assessments, we are implementing a tiered approach. The screening level or tier of analysis is designed, through the use of conservative inputs, to identify situations for risks are unlikely to be of concern. Screening tier analyses are designed to be relatively simple, inexpensive, and quick,

using existing data, defined decision criteria, and models with simplifying conservative assumptions as inputs. More refined levels of analysis include the refinement of aspects of the analysis that are thought to influence risk most or may contain the greatest uncertainty. At the refined tier, each analysis requires more effort, but produces results that are less uncertain and less conservative ( i. e., less likely to overestimate risk). 9 Appendix B to Subpart DDDD 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) or a tiered site specific analysis ( described in Appendix B to Subpart DDDD and the " Air Toxics Risk Assessment Reference Library.") As discussed 9EPA 1999 elsewhere in this section, we will be reviewing the lowrisk demonstrations submitted by PCWP facilities to remove the burden of reviewing tiered 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 other than those described in EPA's " Air Toxics Risk Assessment Reference Library," 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 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 __ facilities who were determined to pose very low risk based on our rough risk assessment ( e. g., with HI less than ___, and a cancer risk of less than one in _____).

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 and/ or in the " Air Toxics Risk Assessment Reference Library." Look up tables were developed using the conservative 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 conservative values, based on available data. Thus, we believe the look up tables are appropriately conservative.

f. 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 multi pathway risk assessments,

and did not reference EPA guidelines for cumulative risk assessment that specifically require consideration of noninhalation 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 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 stakeholderbased 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 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 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 riskbased applicability criteria were not in conflict with the classification of carcinogens and non carcinogens.

Response: The tiered approach to risk assessment for risk management decision making under CAA section 112( c)( 9) being implemented with this rule is consistent with that described in the Residual Risk Report to Congress. For air toxics risk assessments in general, we are implementing a tiered approach. The screening level or tier of analysis is designed, through the use of conservative inputs, to identify situations for risks are unlikely to be of concern. Screening tier analyses are designed to be relatively simple, inexpensive, and quick, using existing data, defined decision criteria, and models with simplifying conservative assumptions as inputs. More refined levels of analysis include the refinement of aspects of the analysis that are thought to influence risk most or may contain the greatest uncertainty. At the refined tier, each analysis requires more effort, but produces results that are less uncertain and less conservative ( i. e., less likely to overestimate risk).

While Appendix B to Subpart DDDD describes a process based on this air toxics approach particular to risk assessment for 112( c)( 9), the basis for this process stems from the general air toxics tiered approach presented in the Residual Risk Report to Congress, which was developed with full consideration of EPA risk assessment policy,

guidance, and methodology.

Section 112( c)( 9) requires us to determine whether the public and the environment are protected. Any analyses we did to establish the feasibility of the riskbased approach or to develop health protective look up tables included consideration of human health as well as ecological criteria. The supporting information to this rule details the assessment we conducted to determine the feasibility of delisting a low risk subcategory and the look up tables we developed to be used by facilities in their demonstrations, thereby providing a public demonstration of the method employed to ensure protection of the public health and environment in decisions associated with this rule. Additionally, potential for exposures via non inhalation pathways ( e. g., ingestion) is also inherent in the values for persistent,

bioaccumulative HAP in the look up tables. ( Note: Need to explain how eco and multipathway are already addressed.

Perhaps OGC can address the role of public comments in regards to " stakeholder based risk management decision making").

We agree that the tools needed to identify sources eligible for the delisted low risk subcategory of PCWP facilities are the same tools necessary for a section 112( f) residual risk assessment. And, as stated in the Residual Risk Report to Congress, we intend to rely on the general methodology and process illustrated by the framework presented in that report in our risk assessment activities throughout the air toxics program. Facilities must determine eligibility for the delisted low risk subcategory using either a look up table analysis ( based on the look up tables included in Appendix B to Subpart DDDD) or using the suggested site specific methodology described together with the criteria in Appendix B to Subpart DDDD. The " Air Toxics Risk Assessment Guidance Library" is the same document that could be used for facility specific residual risk assessments required under CAA section 112( f). This document has been peer reviewed and was developed according to the principles, tools and methods outlined in the Residual Risk Report to Congress.

The comment that the new cancer guidelines are still under review is correct but, as stated in the November 29,

2001 Federal Register notice ( 66 FR 59593), these 1999 draft guidelines are to be considered the interim guidance. 10 4. Implementation a. State and local resources Comment: Several commenters contended that the proposal would place a very intensive resource demand on State and local agencies ( e. g., permitting authorities) to review sources' risk assessments. State and local agencies may not have expertise in risk assessment methodology or the resources needed to verify information 10U. S. EPA. 1999. Guidelines for Carcinogen Risk Assessment.

NCEA F 0644. Risk Assessment Forum, Washington, DC submitted with each risk assessment. The commenters argued that, if EPA intends to have the affected industries conduct the analysis, then EPA must consider the cost incurred by States, which may lack the necessary expertise to evaluate and review these analyses.

One commenter pointed out that the proposal only considered cost for the regulated source category, and not for regulatory agencies. According to the commenter, EPA did not consider the cost and resources associated with the following: ( 1) the public process required in reviewing and approving the proposed approaches and, if approved, making substantial changes to existing regulations; ( 2) the development of methods and guidance for human health and ecological risk assessments of affected sources; ( 3) the review by already budgetarily constrained State agencies of the assessments and assurance of adequate public participation in the process;

and ( 4) the collection and verification of source specific data needed for conducting risk assessments ( e. g.,

emissions data and stack parameters). The commenter added that the proposal did not address the critical need for qualified risk assessors to evaluate the scientific and technical basis for exempting facilities from regulation on a case by case basis. The commenter estimated that if one additional full time employee ( FTE) were required per State to review risk based exemptions, then the cost would be an additional $ 7.5 million annually.

Another commenter pointed out that the ongoing assurance that low risk facilities remain low risk would also increase the burden for the State and local agencies.

The commenter also stated that diverting State and local resources to focus on presumably insignificant sources would detract from efforts associated with significant sources.

A third commenter stated that, since States generally do not have the right staff or resources to hire additional staff to review lengthy and complex risk analyses, they may refuse delegation of the PCWP rule,

which would shift the burden to EPA in a time of tight budgets. According to the commenter, large expenditures are not justified when only a small number of facilities may end up qualifying for an exemption.

By contrast, several commenters believe that a riskbased program approaches could be structured and implemented in a manner that would not impose a substantial cost or resource burden on States. One commenter stated that assuring compliance with risk based applicability criteria would be straightforward and would not entail an added resource burden. Another commenter suggested that EPA work closely with States and industry to implement the risk based approach in a non burdensome manner. Two commenters stated that the risk based approaches, like other MACT standards, would simply be incorporated into each State's existing title V program.

Because the title V framework already exists, the addition of a risk based MACT standard would not require States to overhaul existing permitting programs. One commenter stated that the risk based approach would not increase the number of sources regulated by each State. The commenter believed that the final MACT rule itself should set forth the applicability criteria, including the threshold levels of exposure, that sources must meet to qualify for a riskbased determination. Each source would have the burden of demonstrating that its exposures are below this limit,

and, therefore, the States would not be required to develop their own risk assessment guidance or to conduct source specific risk assessments. One commenter stated that the risk assessment guidance to be issued by EPA within the next several months would streamline the riskbased approach and further reduce any burden on the States. Three commenters supported having States charge reasonable increased fees ( as a component of annual operating permit fees or other fees) to cover any significant additional workload demands associated with reviewing more detailed tier 2/ 3 modeling.

Response: We acknowledge that review of the eligibility demonstrations for the delisted low risk subcategory will require resources for verification of information and may require expertise in risk assessment methodology that is not yet available is some States. We also acknowledge that States may reject delegation of the PCWP rule. To alleviate these concerns and to ensure consistency in the applicability determinations for the delisted low risk subcategory from State to State, we will review and approve/ disapprove the low risk subcategory eligibility demonstrations submitted by PCWP facilities.

We have considered the above comments in developing the criteria defining the delisted low risk subcategory of PCWP facilities, and we believe that the approach that is included in today's final PCWP rule provides clear,

flexible requirements and enforceable compliance parameters. Today's final PCWP rule provides two ways that a facility may demonstrate that they are part of the delisted low risk subcategory of PCWP facilities. First,

look up tables, which are included in Appendix B to Subpart DDDD, allow facilities to determine, using a limited number of site specific input parameters, whether emissions from their sources might cause an HI limit to be exceeded. Finally, a tiered modeling approach ( each tier less conservative and more complex than the previous) can be used by those facilities that cannot demonstrate that they are part of the delisted low risk subcategory using the look up tables. With respect to guidance for performing low risk demonstrations, all of the procedures for performing risk assessments are provided in EPA's " Air Toxics Risk Assessment Reference Library," and, therefore,

no additional guidance needs to be developed.

Only a portion of the 223 PCWP major sources will submit eligibility demonstrations for low risk subcategory. Of this portion of major sources, we believe that most will find themselves in the low risk subcategory based on screening analyses ( e. g., look up table or tier 1 site specific risk assessment). However, it is likely that some facilities will submit more detailed tiered risk modeling results. We are experienced in reviewing emission test results and site specific risk assessments and will allocate resources for completion of these tasks.

We will review and approve/ disapprove low risk subcategory eligibility demonstrations based on look up table analyses and low risk demonstrations. Following review of each low risk subcategory eligibility demonstration for a facility, we will issue a letter of approval/ disapproval to the facility and will send a carbon copy to the facility's title V permitting authority. The criteria for low risk subcategory delisting are clearly spelled out in today` s final PCWP rule, along with criteria needed to ensure that facilities in the low risk subcategory remain low risk. Because these requirements are clearly spelled out in today's final PCWP rule and because any standards or requirements created under section 112 are considered applicable requirements under 40 CFR Part 70, the low risk subcategory delisting would be incorporated into title V programs, and States would not have to overhaul existing permitting programs.

With respect to the burden associated with ongoing assurance that facilities remain low risk, the burden to States of assuring that facilities continue to be low risk will be no more than the burden associated with ongoing title V enforcement because the parameters that rendered a facility low risk will be incorporated into the title V permit. We have developed continuous compliance requirements for facilities that initially qualify as lowrisk and the facilities will be responsible for demonstrating that they continue to be low risk if changes are made to the facilities' operations that would affect the risk that the facilities pose to human health and the environment. We will review and approve/ disapprove revised low risk demonstrations.

With respect to our consideration of the public process required in reviewing/ approving the proposed approaches and making substantial changes to existing regulations, our inclusion of a risk based compliance option in today's final PCWP rule applies only to the PCWP rule and does not directly impact other regulations.

Furthermore, the PCWP proposal provided the public with the opportunity to comment on the consideration of risk in the PCWP rule.

Regarding the costs and resources associated with assuring adequate public participation in the process of reviewing the risk analyses, draft permits and permit applications must be made available from the state or local agency responsible for issuing the permit, or in the case where EPA is issuing the permit, from the EPA regional office. Members of the public may request that the state or local agency include them on their public notice mailing list, thus providing the public the opportunity to review the risk analysis. Therefore, most of the burden of assuring public participation falls on the public to notify the permitting authority of their interest in the permitting process. Regarding the opportunity for public participation, every proposed Title V permit has a 30 day public comment period and a 45 day EPA review period. If EPA does not object to the permit,

any member of the public may petition EPA to object to the permit within 60 days of the end of the EPA review period.

Comment: One commenter stated that if EPA intends to have the affected industries conduct the analysis, then EPA must consider the additional cost incurred by smaller sources to do the analysis.

Response: As mentioned previously, there are two ways that a PCWP facility may demonstrate eligibility for the delisted low risk subcategory: ( 1) look up tables, and ( 2) a tiered modeling approach ( each tier less conservative and more complex than the previous) that can be used by facilities that cannot demonstrate eligibility for the delisted low risk subcategory using the look up tables. The look up tables included in Appendix B to Subpart DDDD allow facilities to determine, using a limited number of site specific input parameters, whether they are eligible for the low risk subcategory.

Attempting to demonstrate eligibility for the delisted low risk subcategory is completely voluntary. Facilities that are not eligible for the delisted low risk subcategory based on look up tables are not required to pursue any of the tiered analyses ( which are increasingly complex and expensive for each tier). Each facility must weigh the costs of making a low risk demonstration with the costs of MACT compliance. We believe the costs associated with demonstrating eligibility for the low risk subcategory will be lower than the costs associated with complying with MACT for many facilities, particularly smaller facilities. The majority of the cost associated with demonstrating eligibility for the delisted low risk subcategory will be emissions testing costs. Smaller facilities have fewer process units to be tested, and,

because of their lower production rates relative to larger facilities, they will also likely have lower emissions.

Thus, smaller PCWP facilities may be more likely than their larger counterparts to fall into the delisted lowrisk subcategory. Successfully demonstrating eligibility for the low risk provisions will result in cost savings for smaller facilities because these facilities will not have to expend the costs ( e. g., the costs of installing operating, and maintaining emission controls) for MACT compliance.

The cost and economic analyses developed as part of the MACT rulemaking were based on the costs to install controls and comply with the MACT requirements. The costs associated with voluntarily conducting risk analyses were not estimated. Therefore, our estimate of costs associated with today's final PCWP rule are conservative,

because the control costs are significantly higher than the costs of conducting emissions tests and risk analyses.

b. Title V Comment: Two commenters opposed implementing the risk based approaches through the States' existing title V programs. One commenter stated that risk based exemptions are such an implausible interpretation of the CAA that States do not even have the authority to grant them under their title V permit programs. The commenter was not aware of any approach to ensure that emissions remain below specified levels. According to the commenter, MACT standard applicability is the gate keeper for being subject to a title V operating permit. Once a source is exempt from a MACT standard, it would be exempt from the monitoring, reporting and recordkeeping requirements needed to demonstrate compliance.

The other commenter stated that implementing the section 112( d)( 4) exemption interpretation through title V would be unlawful and unworkable. The commenter stated that Congress knew how to authorize States to establish case by case emission standards and implement them using post rulemaking title V permits because it did so in section 112( j). However, it did not do so in section 112( d)( 4). The commenter argued that EPA lacks the authority to delegate section 112( d)( 4) to the States and may not implement any section 112( d)( 4) applicability cutoff through a post rulemaking mechanism such as a title V permit. With the exception of carefully delineated compliance monitoring, reporting, and certification provisions in the statute, title V permits may not create applicable requirements or exemptions from applicable requirements. The commenter added that, even if this approach is legal, it is still unworkable because of the resource challenges faced by States and the widespread delays in issuing title V permits. The commenter noted that State permit engineers and officials that prepare and issue title V permits generally are not experts in risk assessment or air dispersion modeling. According to the commenter, States and the public would be confronted with more self serving facility arguments and data than could be adequately scrutinized, which could cause important health and risk determinations to be rubber stamped or cause the permit process to grind to a halt. The commenter added that most State title V permit programs are already behind the statute's permit issuance deadlines, and implementation of EPA's risk based approach would exacerbate this unlawful situation further.

Several commenters supported implementing the riskbased approaches in the PCWP rule through the States'

existing title V programs. One commenter suggested that States which qualify and choose to do so should be delegated the authority to implement the risk based alternatives. The commenter added that this would allow States to coordinate between the MACT alternatives and State air toxics requirements.

A second commenter stated that implementing the section 112( d)( 4) risk based approach though title V would be lawful and workable. According to the commenter, no facility specific post rulemaking mechanisms nor expansion of the scope of title V permit process would be necessary,

just the incorporation of the NESHAP's risk based compliance option, which would contain the criteria for showing what the source would have to meet to qualify for the risk based approach. The commenter believed that the objections from other commenters to the risk based criteria were invalid, arguing that their objections were in tension with the conclusions of a CAAAC Workgroup on State/ Local/ Tribal air toxics issues and that their comments provided no basis for concluding that States lack the legal authority to implement the risk based approach.

A third commenter noted that title V permits could provide enforceable limitations, appropriate recordkeeping requirements, and periodic review upon renewal. The commenter added that, since the PCWP rule would apply only to major sources, title V permits already are required and would not be an added burden; title V could also be used to implement applicability cutoffs. However, the workload involved with the options requiring modeling, ambient monitoring, or other means to establish background concentrations would be a hindrance to any implementation mechanism. The commenter stated that, with respect to potential risk based provisions, monitoring is more useful for demonstrating non compliance than compliance because the regulation would apply to potential emissions under any weather conditions, whereas monitoring reflects current weather and emission conditions.

A fourth commenter suggested changes to the § 63.2240 of the proposed rule that would incorporate permitting procedures similar to those under 40 CFR Part 70, which would allow facilities that pose little risk in their respective airsheds to apply for a risk determination to be incorporated into their title V permits. Each source applying to be permitted as a subcategorized toxic emitter with an acceptable risk determination would be required to perform detailed risk analyses for review by the public at large, local citizens, State agencies, and Federal authorities. This permitting exercise would allow managers of the airshed to develop custom fit compliance plans that address source specific risks and would allow the most flexibility for forest producers to reduce their identified risks.

Response: As discussed previously, we have determined that a section 112( d)( 4) risk based exemption would not be appropriate for the PCWP source category.

Instead, using our discretion in establishing subcategories of sources based on size, type, or class, we have established and delisted a low risk subcategory of PCWP facilities. The requirements for qualifying for and remaining in the delisted low risk subcategory are clearly spelled out in Appendix B to Subpart DDDD, and any standards or requirements created under section 112 are considered applicable requirements under 40 CFR Part 70.

Therefore, the parameters used to certify that facilities are part of the delisted low risk subcategory would be incorporated into title V permits, and States would not have to overhaul existing permitting programs.

Facilities that qualify as part of the delisted lowrisk subcategory will initially demonstrate that they are low risk using either the look up tables or site specific monitoring. They will demonstrate that risk does not increase by documenting that parameters that impact the risk analysis do not change in a way that increases risk.

Facilities will not be required to perform detailed risk analyses for public review.

We acknowledge the resource challenges faced by States, and, therefore, we will retain the authority to review and approve/ disapprove the low risk subcategory eligibility demonstrations submitted by PCWP facilities.

With regard to the title V permit programs being behind the statute's permit issuance deadlines, the incorporation of the NESHAP requirements is a necessary step that will require some resources. Inclusion of the low risk subcategory delisting should be a straightforward part of the process and should not cause significant delay. May want input from OGC.