Finally, the D. C. Circuit upheld EPA's adoption of de minimis exemptions under Comprehensive Environmental Response, Compensation, and Liability Act ( CERCLA,) even where the plain language of the statute did not authorize such exemptions, reasoning that "[ t] he literal meaning of the statute need not be followed where the precise terms lead to ... futile results, or where failure to follow a de minimis exception is contrary to the primary legislative goal.... EPA's interpretation ... squares with the health protective purpose of the statute." Ohio v. EPA, 997 F. 2D 1520, 1535 ( D. C. Cir. 1993). Commenter IV D 175 contended that these cases clearly establish EPA's inherent authority to 219 promulgate de minimis exemptions under regulatory schemes, even where the statute itself does not specifically authorize such exemptions. The commenter also pointed out that EPA has included de minimis, concentration based thresholds in a number of NESHAPs ( e. g., 40 CFR part 63, subparts F, G, H, U, CC, GG, TT, GGG, etc.), without reference to explicit statutory authority, and without ensuing legal challenges. Similarly, Commenter IV D 72 referred to AF& PA's white paper on concentration based de minimis exemptions ( the white paper not discussed in the preamble) and stated that they believe EPA has broad de minimis authority under the CAA for purposes of determining applicability of standards. The commenter stated that EPA has general authority to exempt de minimis emissions based on concentration and risk. Emissions could be de minimis based on either the quantity and/ or ambient impact of the emissions or by establishing a risk based approach to establishing de minimis emissions. On the other hand, two commenters ( IV D 14 and IV D 96) opposed risk based approaches under de minimis authority. Commenters IV D 96 and IV D 14 argued that EPA does not have de minimis authority for risk based exemptions. Commenter IV D 96 stated that the de minimis theories proposed by AF& PA in EPA's docket violate the CAA. EPA may not rely on its narrow de minimis exemption authority to escape the highly prescriptive provisions in section 112. Commenter IV D 96 noted that de minimis authority does not exist to create MACT exemptions on a facility by facility or category wide basis. According to the commenter, subsection ( d)( 4) is clearly limited to threshold pollutants, but AF& PA suggests that EPA has de minimis exemption authority for non threshold pollutants under 112( f)( 2)( A). The commenter stated that this is not the purpose of 112( f)( 2)( A), rather, § 112( f)( 2)( A) demands that EPA reduce significant residual risks remaining after a MACT standard is applied. The section applies to risks from carcinogens in order to provide a safety net if the MACT standard set under § 112( d) does not sufficiently reduce cancer risk. Commenter IV D 96 stated that EPA rejected the idea that de minimis exceptions are available in the way that AF& PA suggests in the response to comments for the Proposed NESHAP for Chemical Recovery Combustion Sources at Kraft, Soda, Sulfite, and Stand Alone Semichemical Pulp Mills ( p. 23, attached to the comment). Allowing facilities to conduct a risk assessment in an attempt to prove that they are de minimis sources of carcinogens is to convert MACT's technology based standard setting approach back into a health based standard setting process  a result explicitly rejected by Congress. The commenter stated that invoking § 112( f)( 2)( A) as a de minimis granting authority for individual assessment of non threshold pollutants converts a back end safety net into a front end excuse for not establishing a technology based standard. The commenter referenced a floor statement made by Senator Durenberger that emphasizes the fact that no de minimis regulatory authority exists for nonthreshold pollutants ( Legislative History at 876). Courts have held that EPA may not create individual de minimis exemptions in the absence of clear statutory authority to do so ( Alabama Power Co. v. Costle, 636 F. 2D ( D. C. Cir. 1979)). Commenter IV D 96 stated that no logical reading of section112( f)( 2)( A) or the CAA's legislative history would lead a reasonable person to believe that Congress granted EPA authority to reduce or avoid a MACT standard because it was too protective of cancer risks. Commenter IV D 96 also asserted that EPA lacks de minimis authority to delist subcategories based on risk According to the commenter, the CAA's text and purpose prohibit the establishment of a separate de minimis subcategory to delist sources. Section 112( c)( 9)( B)' s 220 text precludes development of a de minimis exemption that exceeds the section's narrow authority. The commenter stated that this provision is highly prescriptive in addressing the role of risk assessment in avoiding MACT. The Administrator may delist a category only if no source emits more than a specified amount of HAP. Congress did not intend to include de minimis authority in § 112( c)( 9)( B). Commenter IV D 96 stated that a de minimis exception in § 112( c)( 9)( B) would not further the CAA's purpose of decreasing HAP emissions. Commenter IV D 96 charged that AF& PA developed a series of " white papers" exploring multiple iterations of de minimis theories that ended up serving as an industry " how to" manual for the illegal exemptions and deregulation promoted by the White House OMB and reflected by EPA in section IV. G of its proposal. EPA has repeatedly and explicitly recognized, however, that a de minimis theory is not available to authorize the exemption of any source from a MACT limitation that has been promulgated for the category to which that source belongs. See, e. g., EPA's Brief as Respondent in Cement Kiln Recycling Coalition v. Browner, U. S. Court of Appeals for the D. C. Circuit, Case No. 99 1457, January 18, 2001, at 53 ( Attachment 1 to IV D 96) (" EPA rightly concluded that `[ s] ection 112 of the Act does not provide for exceptions from emission standards based on de minimis principles where a MACT floor exists.' RTC at 211 ( JA 1542)."). EPA may not rely on its narrow de minimis exemption authority to escape the highly prescriptive provisions in § 112. Commenter IV D 96 stated that first, the basic legal purpose justifying invocation of the de minimis doctrine  " spar[ ing] agency resources for more important matters"  is not relevant here. See Public Citizen v. Young, 831 F. 2D 1108, 1112 ( D. C. Cir. 1987). EPA can and must issue MACT standards to regulate every HAP emitted by the source category, and the agency resources devoted to this task will not be reduced by relying upon AF& PA's unfounded de minimis theories. The commenter noted that EPA's own proposal discusses the complex issues associated with establishing these various de minimis exemptions, and EPA acknowledges in the final BSCP MACT standard that creation of these exemptions is complex and time consuming. Agency resources would be consumed to a far greater degree attempting to develop these approaches than not. Commenter IV D 96 added that these conclusions are doubly true with respect to the resources of state and local permitting authorities, as explained by these officials in comments on the BSCP MACT proposal and other proposals ( including this one) in which EPA has proposed saddling them with time consuming, complex risk assessments and cumbersome determinations in service of industry's deregulatory agenda. Commenter IV D 96 stated that second, any purported inconvenience to industry from complying with MACT is not a justification for invoking the de minimis doctrine so as to escape clear statutory provisions. It is hardly an " absurd or futile" result to require facilities to control their cancer causing emissions and other HAPs with MACT level controls, since this is the plain purpose of § 112' s technology based system of regulation. Alabama Power v. Costle, 636 F. 2D 323, 360, n. 89 ( D. C. Cir. 1979). Commenter IV D 96 stated moreover, that requiring these HAPs to be controlled with MACT emissions standards clearly " provide[ s] benefits, in the sense of furthering the regulatory objectives" of the Act's HAP program. Alabama Power, 636 F. 2D at 361. At any rate, EPA has not revealed any administrative record justifying a de minimis exemption, to demonstrate in any way that compliance with MACT would " yield a gain of trivial or no value." Id. Commenter IV D 96 stated further that Congress defined the terms, " modification" and " major source," such to ensure that the severe proscription in subsection 112( g)( 2) would not apply where only de minimis HAP emissions resulted from the construction of a new source or 221 the modification of an existing one. The commenter noted that in its white paper, AF& PA asserts that EPA's " presumptive de minimis authority" empowers the agency to promulgate a rule instituting the concentration based applicability threshold. Commenter IV D 96 submitted that this assertion is specious. First of all, to the extent Congress has authorized any de minimis exclusion from the MACT requirement, the exclusion is for de minimis amounts of a HAP emitted. A limit on the concentration of a HAP in a source's exhaust stream imposes no limit whatsoever on the amount of the HAP that is emitted from that source. The commenter stated that furthermore, Congress already set forth in the text of § 112 the extent of the allowance that it wished to make for de minimis amounts of HAP emissions. Commenter IV D 96 claimed that the rule that AF& PA promotes would purport to exempt from the MACT requirement changes that cause HAP emissions increases in excess of any de minimis thresholds that Congress has set, or that EPA has instituted pursuant to the limited discretion afforded the agency by subsection 112( a)( 5). AF& PA's misplaced invocation of the de minimis doctrine fails to hide the conflict between its proposed exemption and the dictates of the CAA. Commenter IV D 96 stated that the third exemption suggested by AF& PA and proposed by EPA, a so called " concentration based applicability threshold," is unlawful. Subsection 112( g)( 2) of the CAA declares that no person may construct, reconstruct, or modify " a major source of hazardous air pollutants" unless the permitting authority " determines that the maximum achievable control technology emission limitation under this section for existing sources will be met." As long as the source meets the " major source" threshold and the proposed modification would cause more than a de minimis increase in HAP emissions, the CAA requires a MACT determination. The CAA does not contain any provision authorizing EPA to waive this requirement where the post change HAP concentration in the source's exhaust stream will not exceed a certain level, or where the source owner commits to adopt just enough HAP control to bring the concentration down to that level. After all, whether a source's exhaust stream meets a certain HAP concentration limit has no bearing on whether the source meets the MACT emission limitation. As EPA itself acknowledges in the preamble of its final BSCP NESHAP: " Exhaust gas concentrations have no effect on mass emission rates, provided the concentrations are above the test method detection limit. The mass emission rate ( e. g., pounds of pollutant emitted per hour) for a source is unchanged regardless of how much dilution air is introduced." Because it purports to replace the MACT emission limitation with a requirement that places no limit on the mass emission rate, the rule that the AF& PA wants EPA to promulgate stands in direct conflict with the clear language of the CAA. Regarding the use of a concentration based applicability threshold, commenter IV D 14 stated that this is not what Congress intended in the CAAA of 1990. Congress mandated that the MACT floor be established as initial level of control. Response: We disagree with the commenters who claimed that the low risk approaches, especially the concentration based exemption, can be justified by de minimis principles. Our de minimis authority exists to help avoid excessive regulation of tiny amounts of pollutants, where regulation would yield a result contrary to a primary legislative goal. It is unavailable " where the regulatory function does provide benefits, in the sense of furthering the regulatory objectives, but the EPA concludes that the acknowledged benefits are exceeded by the costs." EDF v. EPA, 82 F. 3d 451, 466 ( D. C. Cir. 1996); Public Citizen v. Young, 831 F. 2d 1108, 1112 13 ( D. C. Cir. 1987); Alabama Power v. EPA, 636 F. 2d 323, 360 61 & n. 89 ( D. C. Cir. 1979). Accordingly, a de minimis exemption to § 112( d)( 3) is unavailable in this final rule, because it would frustrate a primary legislative goal by preventing application of the MACT floor to tons of HAP emissions 222 from boiler and process heater affected sources. The U. S. Court of Appeals for the District of Columbia Circuit has already addressed the de minimis concept in the MACT context, in National Lime Ass'n v. EPA, 233 F. 3d 625, 640 ( D. C. Cir. 2000) (" National Lime"), in which the court rejected the industry petitioner's claim that in light of both the high costs and low quantities of HAP at issue in that case, EPA should read a de minimis exception into the requirement that it regulate all HAP emitted by major sources. In that case, the Court found that " EPA reasonably rejected this argument on the ground that the statute ` does not provide for exceptions from emissions standards based on de minimis principles where a MACT floor exists.'" ( See National Lime at 640.) We recently re affirmed our position on the unavailability of de minimis exemptions from the MACT floor in the final rule regulating organic liquids distribution. ( See 69 FR 5038, 5048 49 ( February 3, 2004).) We see no reason to revisit this fundamental issue. Section 112 of the CAA is replete with careful definitions of mass or effect based limitation on regulation, indicating that Congress has already defined what amounts of HAP emissions are too small to warrant MACT standards or other controls under § 112. The requirement to adopt MACT emission limitations, for example, applies without exception to " each category or subcategory of major sources . . . of [ HAP]." ( see CAA § 112( d)( 1).) For sources below the major source threshold, however, we have discretion to require " generally available control technologies or management practices." ( see CAA § 112( d)( 5).) Congress has thus itself defined which sources' emissions are small enough not to warrant mandatory MACT standards. Congress likewise defined several MACT exceptions applicable where emissions have de minimis health effects. Section 112( d)( 4) of the CAA allows us to establish standards less stringent than MACT for HAP with an established health threshold, so long as we set a standard below the health threshold with " an ample margin of safety." Section 112( b)( 3)( C) of the CAA directs us to de list HAP   precluding § 112( d) MACT standards   if we determine that " there is adequate data on the health and environmental effects of the substance to determine that emissions, ambient concentrations, bioaccumulation or deposition of the substance may not reasonably be anticipated to cause any adverse effects to the [ sic] human health or adverse environmental effects." Section 112( c)( 9)( B)( i) of the CAA ( discussed further below) lets us delete source categories and subcategories from the category list   the consequence again being no MACT control   if we determine that, for emissions of carcinogenic HAP, " no source in the category . . . emits such [ HAP] in quantities which may cause a lifetime risk of cancer greater than one in one million to the individual in the population who is most exposed to emissions of such pollutant from the source." For noncarcinogens, we may delete source categories and subcategories if we determine that " emissions from no source in the category or subcategory . . . exceed a level which is adequate to protect public health with an ample margin of safety and no adverse environmental effect will result from emissions from any source." ( See CAA § 112( c)( 9)( B)( ii).) Moreover, in defining which source modifications trigger additional regulatory standards, CAA § 112( g)( 1)( A) mentions a " greater than de minimis increase in actual emission of a [ HAP]." This shows that Congress knew how to use the de minimis concept when it considered it appropriate in § 112, and the fact that Congress did not use it in § 112( d)( 3) supports EPA's   and the D. C. Circuit's   conclusion that it is unavailable to support an exception to a MACT floor that is not otherwise authorized under § 112. We do not find persuasive the proposition that the overall purpose of § 112 is protecting human health and the environment, and that, therefore, as long as this general purpose is met, we may fashion de minimis exceptions from MACT beyond those allowed under § 112. First, this 223 position appears to assume that the issue is to be drawn on a clean slate, while the D. C. Circuit has affirmed our view that § 112( d)( 3) provides no discretion to use a de minimis rationale to avoid MACT. Second, the commenter appears to give prominence to an over arching statutory goal over the specific language of the statutory provisions themselves, in assessing whether those provisions are " extraordinarily rigid" regarding EPA's otherwise inherent de minimis authority; the logical extension of such an approach would be to find that no single provision in the CAA could restrict our de minimis authority, in light of the CAA's over arching purpose " to promote the public health and welfare." ( see CAA § 101( b)( 1).) Third, the commenter does not present any persuasive statutory arguments to overcome those that we presented to the court   and which the court affirmed   in National Lime. Fourth, we are unable to discern the basis for the commenter's suggestion that we have in fact been relying on de minimis authority in the MACT program for several years in establishing applicability thresholds, and we are not aware of any instance in which we have explicitly justified an exception from an applicable MACT floor based on a de minimis rationale that would be, like the commenter's requested exemption, in contravention of the Court's ruling in National Lime. Fifth, notwithstanding any asserted overall risk based goal of § 112, § 112( d)( 2) repeatedly directs EPA to " eliminate emissions," where feasible, and § 112( d)( 6) imposes an ongoing obligation for EPA to review and revise standards as necessary to account for developments in technology, and neither of these specific goals is restricted to situations where health targets have not yet been reached. Finally, to the extent the commenters believe such a de minimis exemption is justified by the wish to reduce the costs of the final rule, we are not free to grant a de minimis exemption to account for costs: Congress already took cost into account in § 112( d), relying on prior business judgments by the best performing sources to substitute for the judgment of the rest of the Industrial/ Commercial/ Institutional Boilers and Process Heaters source category, therefore denying us the leeway to consider costs as a factor to modify the MACT floor. Only in considering more stringent " beyond floor" standards may we consider costs in the MACT context. Therefore, we do not believe it is appropriate or necessary to revisit our and the D. C. Circuit's prior conclusions regarding the availability of the de minimis principle in the final rule.

18.4 LEGAL ISSUES: SECTION 112( c)( 9) AUTHORITY 18.4.1 Section 112( c)( 9) Authority for Delisting Source Categories or Subcategories Comment: Several commenters ( IV D 72, IV D 73, IV D 75, IV D 123) supported the use of § 112( c)( 9) authority to delist categories or subcategories. Commenter IV D 72 stated that EPA has broad discretion to tailor source categories as appropriate to best reflect the source category. Specifically, § 112( c)( 9) provides that EPA establish categories and subcategories of sources, as appropriate, pursuant to CAA § 112( c)( 1), in order to facilitate the development of MACT standards consistent with § 112 of the CAA. The commenter further pointed out that § 112( c)( 9)( B) allows EPA to delete a category ( or subcategory) from the list of major sources for which MACT standards are to be developed when the following can be demonstrated: ( 1) in the case of carcinogenic pollutants, that "... no source in the category... emits ( carcinogenic) air pollutants in quantities which may cause a lifetime risk of cancer greater than 1 in 1 million to the individual in the population who is most exposed to emissions of such pollutants from the source..."; ( 2) in the case of pollutants that cause adverse non cancer health effects, that "... emissions from no source in the category or subcategory... exceed a level which is adequate to 224 protect public health with an ample margin of safety..."; and ( 3) in the case of pollutants that cause adverse environmental effects, that " no adverse environmental effect will result from emissions from any source..." Commenter IV D 72 stated that this level of discretion clearly grants EPA the authority to establish subcategor[ ies] of facilities within the larger source category that would meet the risk based criteria for delisting. Such criteria would likely include HI limits for threshold pollutants and a cancer risk level of 10 6 for non threshold pollutants. Commenter IV D 75 noted that EPA stated in its initial list of source categories that Congress appeared to use the terms category and subcategory interchangeably, and that either a category or subcategory, therefore, can be delisted. Furthermore, § 112( c)( 9)( B) indicates that the Administrator can delist both categories and subcategories. The commenter pointed out that the subsection regarding carcinogenic HAP states that a category of sources can be delisted after a determination that " no source in the category" emit pollutants in an amount that poses a lifetime cancer risk greater than one in one million. The criteria for non carcinogens allows for delisting " when no source in the category or subcategory" exceeds an emissions level adequate to protect public health. Commenter IV D 73 added that nothing in the statute or history of EPA's interpretation of § 112( c) precludes subcategorization based on risk, and, in addition, EPA has stated that emission characteristics are factors to be considered when defining categories. Commenter IV D 75 stated that § § 112( c)( 1) and 112( c)( 9) provide EPA the authority to create and delist low risk source categories. The commenter stated that while § 112( c)( 9) is clear in limiting the delisting to categories and subcategories in which all sources meet the applicable tests, the CAA provides the Administrator with significant flexibility to create categories and subcategories as needed to implement § 112. Specifically, § 112( c)( 1) states that " Nothing in the preceding sentence limits the Administrator's authority to establish subcategories under this section, as appropriate." The commenter stated that there is nothing in the statute that limits the criteria the Administrator can use in establishing categories and subcategories. The commenter also pointed out that application of the statutory authority to exclude sources from regulation under § 112( d)( 3) is supported by relevant case law. In the Vinyl Chloride decision ( NRDC v. EPA, 824 F. 2D 1126 ( D. C. Cir. 1987)), the court noted that what is considered a " safe" level is always " marked by scientific uncertainty." The court established a range of acceptable level of risk in establishing limits under prior language in § 112. The commenter suggested that the establishment of an acceptable level of risk could be used to create a low risk subcategory that could be delisted. The commenter also noted that technological or operational differences among sources may also be appropriate of the differences help to discriminate between low risk and highrisk sources. Finally, the commenter added that effective use of § 112( c)( 1) authority to create risk based subcategories would significantly improve the cost effectiveness of the § 112 program without undermining its role in protecting public health and the environment. Commenter IV D 73 stated that in the preambles, EPA expresses uncertainty over whether it has the authority to subcategorize source categories based on risk. The commenter believed that EPA has ample authority, based on § § 112( c)( 1) and 112( d)( 1), to subcategorize based on risk. Section 112( c)( 1) states: " Nothing in the preceding sentence [ relating to following the NSPS program categories and subcategories] limits the Administrator's authority to establish subcategories under this section as appropriate." Thus, the commenter noted, Congress allowed EPA discretion to subcategorize previously created categories, regardless of the criteria that EPA used to create the category in the first place, and to do so at any time. Commenter IV D 73 also pointed out that § 112( d)( 1) provides that EPA " may distinguish among classes, types and sizes of sources" when establishing MACT standards. According to the commenter, the broad terms 225 " classes," " types," and " sizes" indicate that Congress intended that EPA have broad discretion in establishing subcategories and do not preclude EPA from subcategorizing based on risk, since low risk sources could be considered a " class" or " type" of source. The commenter added that the only case to clarify this statutory language recognized the broad discretion it confers on EPA to create subcategories with different emission standards. Sierra Club v. Costle, 657 F. 2D 298 ( D. C. Cir. 1981). The Court noted, "[ t] he required finding that must underlie a variable standard is much broader than a mere determination that uniformity is not achievable." Id. at 321. On this basis, the Court expressly upheld EPA's subcategorization of coal fired power plants based on the sulfur content of fuel. More generally, the commenter stated that the Sierra Club decision confirms EPA's discretion to set differentiated emissions standards for subcategories of sources, even in instances where the strictest standard may be achievable by all sources. Commenter IV D 123 stated that EPA has ample legal authority to subcategorize and delist low risk sources and that EPA has ample legal authority to subcategorize on the basis of risk. The commenter submitted that NRDC's and Earthjustice's contrasting of the terms " category" and " subcategory" offers a distinction that in no way limits EPA's authority to delist low risk sources. The commenter believed the idea that EPA cannot create subcategories based on risk is contradicted by the statutory language, which expressly states that the categories and subcategories EPA creates under § 112 need not match those created under § 111. Further, prior EPA statements do nothing to detract from the Agency's broad discretion to establish categories and subcategories. Commenter IV D 123 stated that the subcategorization factors previously discussed by the Agency justify subcategorization based on risk. The commenter stated further that the authority cited by NRDC does not establish that EPA's discretion to alter subcategorization is limited in anyway, and even if it were, EPA is not bound by any prior position. The arguments that EPA may not delist subcategories for carcinogens ( or sources emitting carcinogens) rest on a formalistic distinction that EPA previously has rejected as meaningless, and that, at any rate, can be remedied with a simple recasting of a subcategory as a category. The commenter asserted that doing so is undisputedly within EPA's authority. Numerous commenters ( IV D 05, IV D 55, IV D 96, IV D 113, IV D 127, and IV D 135) stated that § 112( c)( 9) should not be used to subcategorize based on risk. Commenter IV D 113 stated that subcategory delisting under § 112( c)( 9)( B) is flatly unlawful. The commenter pointed out that § 112( c)( 9)( B) provides that EPA " may delete any source category" from the § 112( c) list upon making certain determinations. The commenter submitted that Congress was well aware of the difference between a " category" and a " subcategory" when it enacted § 112( c). When Congress wished to refer to both subcategories and subcategories, it did so expressly. Commenter IV D 113 stated that by referring only to " category," Congress made plain that § 112( c)( 9)( B) does not allow EPA to delist a " subcategory" for any reason. Commenter IV D 96 stated that EPA's subcategorization theories are unlawful. The commenter presented the following arguments supporting the opinion that § 112( c)( 9) does not authorize EPA to separate identical pollution sources into subcategories that are regulated differently to weed out low risk facilities or reduce the scope/ cost of the standard. EPA may not subcategorize based on risk. Commenter IV D 96 stated that subcategories based solely on risk do not bear a reasonable relationship to Congress' technology based approach or the statutory structure and purposes of § 112, and are not authorized by the CAA. Categories and subcategories are required to be consistent with the categories of stationary sources in § 111. The commenter was not aware of any instance in which EPA has established categories or subcategories based on risk. The commenter stated that EPA routinely defines 226 subcategories based on equipment characteristics ( e. g., technical differences in emissions characteristics, processes, control device applicability, or opportunities for pollution prevention). The commenter pointed out that EPA has not offered any explanation for why reinterpreting the statute to ignore nearly 12 years of settled practices and expectations under the MACT program is reasonable, not why reducing the applicability of HAP emission standards serves Congress's goals in enacting the 1990 CAAA. EPA may not subcategorize to reduce costs. Commenter IV D 96 noted that EPA's discussion of the risk based exemptions is contained in a preamble section entitled, " Can We Achieve the Goals of the Proposed Rule in a Less Costly Manner." This strongly suggested to the commenter that EPA's motivation for considering these risk based approaches, including the § 112( c)( 9) approach, is consideration of cost. Commenter IV D 96 stated that EPA itself has rejected the notion that cost should influence MACT determination, and this prior, consistently applied interpretation better serves the purposes of § 112. The commenter pointed out that in the " Summary of Public Comments and responses on the Proposed NESHAP for Chemical Recovery Combustion Sources at Kraft, Soda, Sulfite, and Stand Alone Semichemical Pulp Mills" ( attached to the comment), the Agency addressed concerns about high costs and low emissions reductions by saying " invoking a de minimis concept for MACT floors would frustrate a primary legislative goal  disregard of costs in determining the MACT floor." When this decision was challenged in court, EPA replied that " sources already took costs into account in taking measures to limit HAP emissions, and Congress built their cost judgements into the statute by mandating that all others in the industry achieve the same limitations. Thus, further consideration of cost factors by EPA is both unnecessary and impermissible." ( The commenter attached portions of the Brief for Respondent Environmental Protection Agency, National Lime Ass'n v. EPA, 233 F. 3D 625 ( D. C. Cir. 2000), at 47 ( July 14, 2000).) The commenter stated that the D. C. Court agreed with EPA on this point. Subcategorizing to set a no control MACT floor is the same as refusing to set a MACT standard because the benefits would be negligible, which is unlawful. EPA may not delist a subcategory of carcinogens. Commenter IV D 96 pointed out that § 112( c)( 9)( B)( i) does not authorize EPA to delist subcategories. Section 112( c)( 9)( B) contains two subsections. Subsection ( i) refers only to categories, and subsection ( ii) refers to both categories and subcategories. The commenter believed that the absence of the term " subcategories" in § 112( c)( 9)( B)( i) indicates a Congressional choice not to permit the Administrator to delist subcategories of sources under § 112( c)( 9)( B). The commenter stated that this is consistent with Congress's decision to require a higher standard to delist categories that emit carcinogens. The commenter added that the § 112( c)( 9)( B)( ii) requirement of less than one in a million lifetime cancer risk for the most exposed individual is a higher and more specific standard than the standard for other HAP. Commenter IV D 113 stated that even if EPA could delist a subcategory, it could not do so based on risk. The commenter observed that § 112( c) states that "[ t] o the extent practicable, categories and subcategories listed under this subsection shall be consistent with the list of source categories established pursuant to section 111 and part C," and the commenter stated that subcategories based on risk would not be consistent with either the § 111 list or part C. The commenter added that EPA has interpreted the statement regarding subcategorizing by " classes, types, and sizes" in § 112( d) to mean that subcategories must share physical characteristics relevant to the degree of pollution control that can be achieved. Because risk is not such a characteristic, EPA may not subcategorize based on risk. The commenter also added that risk 227 based subcategories would be at odds with Congress's purpose in enacting § 112, i. e., requiring technology based standards with a performance based floor, which was intended to overcome the difficulties EPA encountered in completing health based standards. In addition, the commenter stated that EPA has not provided a reason for departing from its current interpretation of the guidelines for establishing subcategories other than to avoid setting emission standards. Thus, subcategorization based on risk, including under the pretense of subcategorization by technology ( which EPA admits to considering), would be unlawful. Commenter IV D 113 stated that EPA did not propose any subcategories for delisting, and if EPA wanted to delist a subcategory, they would have to propose the delisting and allow the public to comment. The commenter added that instead of creating further delays with such a process, EPA should consider that its standards are already late and should focus its resources on completing the overdue standards instead of providing unlawful exemptions for industry groups that wish to avoid cleaning up their hazardous air pollution. Commenter IV D 135 noted that EPA requested comment on establishing a subcategory of facilities within the larger industrial boiler and process heater source category that would meet the risk based criteria for delisting under § 112( c)( 9)( B). The commenter further noted that § 112( c)( 9)( B)( i) authorizes the Administrator to delist a source category even if a source in the source category emits carcinogenic HAPs. However, the Administrator must first find that no source in the category will emit such HAPs at levels that would cause a one in one million lifetime risk of cancer. [ 42 U. S. C. Section 7412( c)( 9)( B)( i).] The commenter asserted that clearly EPA's proposed delisting of a subcategory is inappropriate here because EPA has not made the finding in this proposal that no boiler or process heater source in the source category could emit greater than this amount for all of the carcinogens regulated by this rulemaking. Commenter IV D 05 stated that the only option that appears consistent with the CAA, does not create excessive work for State and local agencies, and may be able to be based on science, is the subcategorization and delisting approach. However, the commenter added that the subcategories should be based on equipment or fuel use, not risk. The commenter added that a subcategory based on site specific risk creates a circular definition and does not make sense. The commenter also stated that subcategory de listing should occur before the compliance date so that facilities don't put off compliance in the hope or anticipation of de listing. Commenter IV D 96 stated that EPA requested comment on the establishment of PCWP subcategories ostensibly based on physical and operational characteristics, but in reality based on risk. The commenter responded that This indirect approach is just a variation on the approach ( direct reliance on risk) that EPA itself notes would disrupt and weaken establishment of MACT floors, and is accordingly unlawful. The commenter stated that even if these approaches were lawful, to the extent that EPA's proposal could be read to suggest that facilities could be allowed to become part of the allegedly low risk subcategory in the future without additional EPA rulemaking, this too would be unlawful. According to the commenter, § 112( c)( 9) provides the EPA Administrator alone the authority to make delisting determinations, and such authority may not be delegated to other government authorities or private parties. The commenter stated that EPA's proposal suggests an approach entirely backward from the statute, i. e., allowing sources to demonstrate after the fact that it belongs in a subcategory that has been delisted under subsection 112( c)( 9), when the statute requires that EPA determine that no source in the category emits cancer causing HAPs above specified levels, or that no source in the category or subcategory emit non carcinogenic HAPs above specified levels, by the time the agency establishes the standard. Commenter IV D 96 noted that EPA provides no explanation of how the suggested approaches 228 would be lawful or workable. Commenter IV D 55 requested that EPA focus on the different circumstances applicable to seasonal agricultural operations to exempt facilities. The commenter claimed that emissions of HAPs such as HCl and acetaldehyde on a seasonal basis do not create any risk to health or safety. The commenter noted that in the past EPA has treated acetaldehyde as a non threshold pollutant, but that in reassessing the carcinogenic dose response relationship for acetaldehyde, EPA is considering reclassifying acetaldehyde as a threshold carcinogen subject to risk exceptions. The commenter believed that EPA should create a risk based exemption for seasonal agricultural facilities which trigger HAP thresholds through natural production of food products. The commenter asserted that the absence of any serious risk of cancer or carcinogenic affects from HCl, a surrogate for non HAPs emissions, indicated that sugar beet processing is a subcategory ideally suited for delisting. Commenter IV D 127 noted that the MACT floor analysis did not define a required control for any small boiler and submitted that the size and type of unit are clearly distinguishable and the risk offered by these units is exceptionally small. The commenter asserted that due to the low risk offered by small equipment, the easy identification of the equipment, and the consistent low risk character of the emissions, small equipment firing any fuel should be subcategorized and delisted from the MACT source category. Commenter IV D 127 believed that boilers and process heaters, of all sizes, firing gaseous fossil fuels and distillate fuel oil represented a low risk class of boilers and process heaters. The commenter stated that due to the low risk offered by equipment firing these fuels, the easy identification and characterization of the fuels, and the consistency of the character of the emissions, equipment firing gaseous fossil fuel and distillate fuel oil should be subcategorized and delisted from the MACT source category. Response: As is explained in response 18.2.1, we have chosen to exercise the authority granted to us pursuant to § 112( d)( 4) of the CAA in establishing health based emission standards for HCl and Mn which are applicable to the large solid fuel fired subcategories. For a discussion of our use of § 112( c)( 9) authority to define a risk based subcategory and delist the subcategory refer to the PCWP NESHAP final preamble. ( See Docket ID No. OAR 2003 0048.)

18.4.2 The Impact on MACT floors of a Low Risk Subcategory Comment: Commenter IV D 05 stated that the preamble discussion of a low risk subcategory on the MACT floors for the entire category sounds like another valid reason not to mix the risk based and technology based standards development. The commenter added that EPA does not address how the " once in, always in" policy would apply. Commenter IV D 96 stated that EPA itself provides an additional compelling reason why the suggested § 112( c)( 9) sub categorization approach is unlawful and arbitrary, i. e., its effect on establishment of MACT floors. According to the commenter, this flaw is so obvious, inherent and contrary to the MACT floor provisions of § 112 and its legislative history, that it proves the undoing of the suggested § 112( c)( 9) exemption. The commenter believed that EPA cannot simultaneously exercise its source category delisting authority consistent with § 112( c)( 9), establish appropriate MACT floors under § 112( d), and establish sub category exemptions in the manner suggested by EPA, because the latter approach contravenes both § 112( c)( 9) and the § 112( d) floor setting process. Commenter IV D 96 stated further that EPA may not subcategorize based upon risk, or 229 delist a subcategory of carcinogens because there is nothing in the CAA authorizing EPA to do either with respect to individual facilities or subcategories after a MACT standard has been established. Section 112' s major source thresholds and statutory deadlines make clear that sources meeting MACT by the time EPA is required to issue MACT standards must install MACT controls and may not subsequently throw them off or be relieved from meeting the MACT level standards. The commenter asserts that while the § 112( f) residual risk process allows EPA to establish more stringent emissions standards, there is nothing in the CAA that suggests EPA possesses authority to relax promulgated MACT standards. Commenter IV D 113 stated that even if EPA could subcategorize based on risk, it would be unlawful for EPA to refuse to consider low risk facilities in the floor calculations since § 112 does not provide any exceptions to its mandate to base floors on the emission levels achieved by the relevant best performing sources. Commenter IV D 135 stated that EPA recognized that a concern with the approach of delisting a subcategory of facilities under § 112( c)( 9)( B) is the effect it would have on the MACT floors. If many of the facilities in the low risk categories are well controlled or have low emissions, removing them from the category could make the MACT floor less stringent for the remaining facilities. The commenter observed that this was inconsistent with the CAA mandate that the MACT floors reflect the emissions limitation achieved by the best performing sources. Commenter IV D 73 supported the concept described in the preambles regarding the establishment of the MACT floor based on the controls for the entire source category. The commenter agreed that considering controls for low risk subcategories could maintain the appropriate stringency of the MACT floor. Once the floor is established, facilities could demonstrate their inclusion in the low risk category that is subsequently listed. Response: We have chosen not to use § 112( c)( 9) authority to delist a subcategory and there are no additional subcategories and no delisting of subcategories, as would be consistent with § 112( c)( 9). We are not attempting to establish or rely upon de minimis authority. Instead we are using § 112( d)( 4) authority to establish health based compliance alternatives for affected facilities in the large solid fuel fired boilers and process heaters subcategory that demonstrate eligibility under the procedures prescribed in Appendix A to the final rule. For affected facilities that demonstrate eligibility for the health based compliance alternatives for HCl and/ or Mn, the parameters used to demonstrate eligibility will be incorporated into title V permits as federally enforceable permit terms and the affected sources will remain subject to the monitoring, reporting, and recordkeeping requirements of the final rule. Additionally, affected sources that demonstrate eligibility for the Mn compliance alternative will be required to comply with the PM limit or the TSM limit based on seven metals ( excluding Mn) instead of eight. Therefore, the affected sources remain in the MACT database and the boiler and process heater MACT floors are not affected.

18.5 RISK ASSESSMENT FRAMEWORK ISSUES: POLLUTANT SPECIFIC BENCHMARKS 18.5.1 Boiler and process heater risk low Comment: Commenter IV D 179 expressed concern about the imposition of very stringent standards for HCl emissions from solid fuel fired boilers. The commenter completed air dispersion modeling of the HCl emission that could be present at their Rock Hill plant that operates five coal fired boilers. Three of the units have a heat input capacity of 176 MMBtu/ hr 230 and the other two have a heat input capacity of 222 MMBtu/ hr. The boilers are equipped with ESPs for particulate control but are not currently controlled for HCl emissions. As input to the dispersion model, commenter IV D 179 estimated the maximum emissions likely to be expected. First, the commenter calculated the maximum HCl emission factor based on the highest chlorine content in coal delivered to the plant over a five year period and assuming it would be totally converted to HCl emissions. Second, the commenter also developed a worstcase emission factor from stack testing of similar boilers at the commenter's plant in Narrows, Virginia which obtain coal from a different source. The resulting worst case emission factor estimates were 0.17 and 0.23 lb/ MMBtu HCl, respectively. Each of these emission factors was applied to the maximum rated capacities of the five coal fired boilers at the Rock Hill site to estimate maximum emissions of HCl for each emission point. The commenter modeled these resulting emission rates using the Industrial Source Complex Short Term Model, Version 3 ( ISCST3) dispersion model. Modeling was performed in accordance with approved procedures documented in EPA's " Guideline on Air Quality Models". The maximum predicted annual concentration was 0.95

g/ m3 HCl. The HQ calculated from the RfC ( 20

g/ m3) is 0.05. Therefore, there is not a concern for long term health effects from the maximum exposure to HCl that can be expected from the Rock Hill plant. Similarly, the maximum predicted short term exposures ( 24, 12, 8, and 1 hour concentrations) were well below any short term exposure guidelines for HCl. Commenter IV D 179 asserted that this study shows that the concentrations of HCl near the Rock Hill facility are well below any thresholds of concern. Yet, the proposed rule would require the installation of very expensive scrubbing technology even when the HAP removed does not pose a health threat to the surrounding community. The commenter estimated that installation of scrubbing technology at the Rock Hill facility will cost three to five million dollars and is not justified given the total lack of environmental risk to human health. Response: As described in Appendix A of subpart DDDDD of 40 CFR part 63, the final rule will include a provision allowing affected sources to make a health based demonstration for emissions of HCl for those boilers and process heaters in the large solid fuel fired subcategory. Affected sources that can demonstrate eligibility for this compliance alternative by either a look up table analysis or site specific modeling will not be required to install additional scrubbing technology. These boilers and process heaters would still be required to meet the PM limit or the alternative TSM emission limits.

Comment: Nine commenters ( IV D 28, IV D 39, IV D 99, IV D 100, IV D 124, IV D 129, IV D 132, IV D 133, and IV D 134) submitted that boilers sized 50 MMBtu or smaller should be exempt from regulation because those boilers do not emit any pollutant that would threaten public health as defined by CAA, § 112( f). The commenters cited CAA § 112( f)( 2) which requires standards applicable to a source or source category that emit[ s] a pollutant classified as a known or possible human carcinogen do not reduce the " lifetime excess cancer risk to the individual most exposed to emissions from a source in the category or subcategory to less than one in one million." The commenters noted that commenter IV D 39 authorized a study of metallic HAP emissions from typical furniture industry wood and coal fired boilers. The emissions estimates in this study were based on EPA emission factors from AP 42 and resulted in emissions exceeding EPA's alternative cumulative metals exemption level. However, Mn was responsible for 91.8 percent of the emissions from wood residue and was the only constituent over the metals exemption threshold. The commenters pointed out that according to EPA, Mn is 231 a category D pollutant that is mainly an irritant in large quantities and is not classifiable as to carcinogenicity in humans. The commenters stated that commenter IV D 39 is in the process of finalizing a study to confirm that small boilers in the furniture industry do not emit enough HAP to create a health based risk. The commenters asserted that not only are small boilers not a health risk, but they do not emit any carcinogens that can be measured above trace amounts while combusting wood fuel. Response: Manganese is a threshold pollutant and has not been suspected of causing cancer. Therefore, cancer risk estimates are not applicable to Mn. As described in Appendix A of subpart DDDDD of 40 CFR part 63, the final rule will include a provision allowing affected sources with boilers and process heaters in the large solid fuel fired subcategory to make a healthbased demonstration for Mn emissions. Affected sources that can demonstrate eligibility for this compliance alternative by either a look up table analysis or site specific modeling will not be required to include Mn in their metals limit ( TSM) under the Boilers MACT but can comply with a TSM limit based on seven metals ( excluding Mn) rather than eight.

Comment: Commenters IV D 26, IV D 104, and IV D 150 presented the results of a detailed modeling analysis of TSM, Hg, and HCl emissions conducted for four sugar mills. Standard EPA models and protocols were followed in the analysis. Maximum annual impacts at the property line for each mill were determined based on the existing mill operations and stack parameters. The maximum total impacts due to each mill for TSM, Hg, and HCl were then compared to the RfC for each HAP, as well as the unit risk factor ( for carcinogens), as shown in Table 4 of the preamble to the proposed rule. The commenters presented two sets of results, one based on fuel analysis and not reflecting any reduction in emissions due to the inherent removal mechanisms ( the fly ash resulting from bagasse fuel burning is highly alkaline, resulting in significant absorption of acid gases) or to existing control devices ( one boiler utilizes an ESP) and the second based on emissions determined from stack testing. The total impact of TSM, Hg, and HCl emissions based solely on fuel analysis, is well below a HI of 1.0, but results in a risk of six in one million for carcinogens ( eight metals, including arsenic, beryllium, cadmium, chromium ( VI), lead, Mn, nickle, and selenium). The total impact of TSM, Hg, and HCl emissions based on stack test results accounting for the effects of existing control devices on sugar mill boilers show a reduction in excess cancer risk to less than one in one million. Commenters IV D 104 and IV D 26 submitted that the purpose of this analysis is to demonstrate that bagasse fuel by its very nature is a clean fuel, is consistent in quality, and emissions of TSM, Hg, or HCl from bagasse fuel burning pose no risk to the public; therefore, bagasse fuel does not warrant regulation in regards to PM/ TSM, Hg, or HCl. The commenters requested that bagasse be specifically exempted from the PM/ TSM, Hg, and HCl limits of the MACT rule. If this exemption is not granted, the commenters supported the option of fuel analysis to demonstrate compliance, but believe that the continuous compliance demonstration requirements and other requirements related to fuel testing must be reduced considerably to be manageable. Response: As described in Appendix A of subpart DDDDD of 40 CFR part 63, the final rule will include a provision allowing affected sources to make a health based demonstration for Mn and/ or HCl emissions. Affected sources that can demonstrate eligibility for the Mn compliance alternative by either a look up table analysis or site specific modeling will not be required to include Mn in their metals limit ( TSM) under the Boilers MACT but can comply with a TSM limit based on seven metals ( excluding Mn) rather than eight. Affected sources that can demonstrate eligibility for the HCl compliance alternative by either look up table analysis or site 232 specific modeling will not be required to install additional scrubbing technology. These are the only health based compliance alternatives included in the final rule. No specific exemption for bagasse fuel has been included in the health based compliance alternatives. All affected sources with boilers and process heaters in the applicable large solid fuel fired subcategory have an opportunity to demonstrate that they can meet the health based compliance alternatives for HCl and/ or Mn. Fuel analysis requirements related to continuous compliance demonstration are addressed elsewhere in this document. When an affected source is demonstrating eligibility for the health based compliance alternatives, fuel sampling and analysis are not included as options for quantifying HCl and/ or Mn emissions levels from boilers and process heaters at the affected source. Affected sources must conduct emissions testing for HCl, Cl 2, and/ or Mn emissions levels.

18.5.2 Dose response values Comment: Commenter IV D 96 stated that § 112( d)( 4) is particularly ill suited to the PCWP and 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. Section 112( d)( 4) does not apply for chemicals that do not have a well defined threshold based on reliable science. Available evidence does not establish a no effect threshold for HF. Commenter IV D 96 stated that IRIS and NATA do not contain HF. EPA acknowledges data suggesting that those with occupational exposure to fluoride have greater than normal incidences of cancer ( www. epa. gov/ ttn/ atw/ hlthef/ hydrogen. html). Registry of Toxic Effects of Chemical Substances ( RTECS) suggests that HF has a mutagenic effect in animals. EPA does not have enough evidence to classify HF as a threshold pollutant or to conclude that HF is not a carcinogen. One limited study ( Derryberry et al, 1963) contains a NOEL for HF but does not account for children's exposure. There is no determined no effect level for children, infants, and fetuses. Available evidence does not establish a no effect threshold for HCl. Commenter IV D 96 stated that IRIS states that HCl " has not undergone a complete evaluation and determination" for cancerous effects. Elsewhere EPA concludes that no information exists on HCl carcinogenicity in humans ( www. epa. gov/ ttn/ atw/ hlthef/ hydrochl. html). RTECS explains that HCl has not reported tumorigenic effect, but could have a mutagenic effect in animals. EPA cannot conclude that HCl does not cause cancer. Rat studies failed to identify a NOEL for HCl. Therefore, the commenter expressed concern that chronic exposure to low levels of HCl could compromise health, especially in sensitive subpopulations. Available evidence does not establish a no effect threshold for manganese compounds. Commenter IV D 96 stated that Mn compounds are on the list of 33 urban air toxics. Regulatory assessments of Mn's toxicological properties prevent EPA from treating the chemical as a threshold pollutant. The IRIS profile for Mn only has lowest observed adverse effect levels ( LOAEL) for several critical effects of inhalation exposure to the substance. California EPA ( CalEPA) Office of Environmental Health Hazard Assessment's ( OEHHA) chronic toxicity summary states that there is no observed adverse effect level ( NOAEL) for the critical effect ( impairment of neurobehavioral function). Available evidence does not establish a no effect threshold for arsenic. Commenter IV D 96 stated that the preamble acknowledges that arsenic is a human carcinogen. EPA's IRIS database indicates that there is no data for establishing a RfC for arsenic exposure by inhalation or 233 other pathways. In addition, CalEPA OEHHA reports no inhalation NOAEL for exposure to arsenic. Available evidence does not establish a no effect threshold for cadmium. Commenter IVD 96 stated that the preamble acknowledges that cadmium is a probable human carcinogen. EPA's IRIS database indicates that there is no data for establishing a RfC for cadmium exposure by inhalation or other pathways. Available evidence does not establish a no effect threshold for chromium. Commenter IVD 96 stated that the preamble acknowledges that chromium IV is a human carcinogen. EPA's IRIS database does not state that thresholds exist for all negative health effects associated with chromium III or chromium IV exposure by inhalation or other pathways. In addition, CalEPA OEHHA reports no inhalation NOAEL for exposure to chromium. Available evidence does not establish a no effect threshold for lead. Commenter IV D 96 stated that the preamble acknowledges that lead is a probable human carcinogen. EPA's IRIS database indicates that there is no data for establishing a RfC for lead exposure by inhalation or other pathways. Available evidence does not establish a no effect threshold for mercury. Commenter IVD 96 stated that the preamble acknowledges that mercury is a possible human carcinogen. EPA's IRIS database indicates that there is no data for establishing a RfC for exposure to mercuric chloride, and the database does not state that thresholds exist for all negative health effects caused by exposure to elemental mercury. In addition, CalEPA OEHHA reports no NOAEL for exposure to mercury. Available evidence does not establish a no effect threshold for nickel. Commenter IV D 96 stated that the preamble acknowledges that nickel refinery subsulfide as a human carcinogen and nickel carbonyl as a probably human carcinogen. EPA's IRIS database indicates that there is no data for establishing a RfC for exposure to any of the four nickel substances that it lists. In addition, CalEPA OEHHA reports no NOAEL for exposure to nickel oxide. Response: As stated previously, we are pursuing establishment of a threshold emission rate for the boilers and process heaters source category under § 112( d)( 4) for specific threshold pollutants emitted from affected sources. We agree that § 112( d)( 4) applies to threshold pollutants. In the absence of specific scientific evidence to the contrary, it has been our policy to classify non carcinogenic effects as threshold effects. RfC development is the default approach for threshold ( or nonlinear) effects. The methodology employed by EPA, which is consistent with that used by other sources we consult, recognizes that while a NOAEL is preferable to a LOAEL for use as the point of departure ( POD) to which uncertainty factors are applied to derive a RfC, a LOAEL may also be used ( or a benchmark concentration derived from benchmark modeling). 1 Use of the LOAEL or other POD does not change the underlying presumption of a nonlinear ( or threshold) dose response relationship. For air toxics risk assessments, we identify pertinent toxicity or dose response values using a default hierarchy of sources, with IRIS being the preferred source, to assist us in identifying the most scientifically appropriate benchmarks for our analyses and decisions. The IRIS process contains internal and external peer review steps and 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. Furthermore, where the IRIS assessment substantially lags the current scientific knowledge, we are committed to consider alternate credible and readily available assessments. RfCs or similar values from other sources are available for all of the HAP listed by the commenter, and we currently rely on these values for 234 assessments of non cancer risks. Comments pertaining to the final PCWP NESHAP are addressed in the promulgation preamble for that rule. ( See Docket ID No. OAR 2003 0048.)

Comment: Commenter IV D 135 stated that HCl applicability cutoffs are not permissible. In the IRIS database, EPA stated that HCl " has not undergone a complete evaluation and determination" for cancerous effects. EPA also stated that " no information" exists on HCl carcinogenicity in humans. In addition, EPA set the RfC at 0.02 milligrams per cubic meter ( mg/ m3) based on rat studies demonstrating adverse effect. These studies failed to establish a NOEL. Response: While the EPA has not developed a formal evaluation of the potential for HCl carcinogenicity ( e. g., for IRIS), the evaluation by the International Agency for Research on Cancer stated that there was inadequate evidence for carcinogenicity in humans or experimental animals and thus concluded that HCl is not classifiable as to its carcinogenicity to humans ( Group 3 in their categorization method). Therefore, for the purposes of the final rule, we have evaluated HCl only with regard to non cancer effects. In the absence of specific scientific evidence to the contrary, it has been our policy to classify non carcinogenic effects as threshold effects. RfC development is the default approach for threshold ( or nonlinear) effects. The methodology employed by EPA, which is consistent with that used by other sources we consult, recognizes that while a NOAEL is preferable to a LOAEL for use as the POD in deriving a RfC, a LOAEL may also be used ( or a benchmark concentration derived from benchmark modeling). 1 Use of the LOAEL rather than a NOAEL does not change the underlying presumption of a nonlinear ( or threshold) dose response relationship. A NOEL is not recommended for use in developing a RfC.

Comment: Commenter IV D 05 stated that EPA should consider formaldehyde and acetaldehyde as carcinogens unless a reassessment classifies them as threshold pollutants. Commenter IV D 148 stated that the proposals inappropriately use draft guidelines and toxicity profiles that have not been subject to public review and/ or are not publicly available. The commenter was particularly concerned with the reference to 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. Response: We agree 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. 2 For air toxics risk assessments, we identify pertinent toxicity or dose response values using a default hierarchy of sources, with IRIS being the preferred source, to assist us in identifying the most scientifically appropriate dose response values for our analyses and decisions. 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 to ensure that we have consistent and scientifically sound assessments. Furthermore, where the IRIS assessment substantially lags the current scientific knowledge, we are committed to consider alternative credible and readily available assessments. For our use, these alternatives need to be grounded in publicly available, peer reviewed information. We are not using information that does not meet these requirements. We agree with the commenter's statement that formaldehyde and acetaldehyde are considered probable human carcinogens. However, both of these HAP can also 235 cause non carcinogenic ( threshold) effects. For the final rule, we are applying § l12( d)( 4) only to HAP that can result in threshold effects. Affected sources conducting risk assessments should refer to Appendix A of subpart DDDDD of 40 CFR part 63 and other analytic tools such as the " Air Toxics Risk Assessment Reference Library" ( if appropriate for the specific source) for guidance on choosing appropriate dose response values.

Comment: Commenter IV D 14 agreed with EPA's choice to derive their data from IRIS, 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. EPA can only rely on what is currently published and underwent either peer review or agency review. The issue of changing health based guideline values will always be a concern once health based regulations are promulgated. Response: We agree with the commenter that the issue of changing health based guideline values is a general challenge in setting health based regulations. However, we are committed to setting such regulations that reflect current scientific understanding, to the extent feasible. Where we identify pollutant assessments that substantially lag current understanding, we have committed to consider alternative credible, current, and readily available assessments, developed using a process that includes external peer review and, where appropriate, organization consensus. Affected sources conducting risk assessments should refer to Appendix A of subpart DDDDD of 40 CFR part 63 and other analytic tools such as the " Air Toxics Risk Assessment Reference Library" ( if appropriate for the specific source) for guidance on choosing appropriate dose response values.

Comment: Commenter IV D 40 asserted that the proposal in the preamble that would permit the provision of CAA § 112( d)( 4) to " be applied to both threshold and nonthreshold pollutants using the one in a million cancer risk level for decision making for nonthreshold pollutants" was not acceptable. The commenter noted that separate calculations are made of cancer risks and noncancer HQ and stated that decisions to remediate a site contaminated with threshold pollutants or to establish threshold level based emission standards should not be based on cancer risk estimates. The commenter believes that if a remedial decision were based on the cancer risk alone, no action could be taken at a site, e. g., with a HI for exposure via the soil ingestion pathway that is greater than one. The commenter submitted that as a conservative measure, a comparison of concentrations of threshold HAP emitted from boilers and process heaters to risk based concentrations could be made, and suggested the EPA Region 3 Risk Based Concentration table would be a good reference for this comparison. Regarding the application of ` an ample margin of safety' to the threshold levels, the commenter requested that further description of the safety margin be provided to ensure a reviewer that the procedure will adequately protect human health. Response: For the final rule, we are applying § l12( d)( 4) only to threshold pollutants. In the modeling, we compare predicted exposure concentrations for specified HAP to appropriate dose response values. As stated above, 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 sound dose response values for our analyses and decisions. The IRIS process includes 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 236 default hierarchy that recognizes the desirability of these qualities in ensuring that we have consistent and scientifically sound assessments. For the final rule, we are considering an HI ( HCl and Cl 2) or an HQ ( Mn) of 1.0 to provide an ample margin of safety for protecting public health under CAA section 112( d)( 4). 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 HCl and Cl 2 assumes additivity in mixtures of chemicals that target the same organ system ( i. e., respiratory system). Affected sources attempting to utilize the health based compliance alternative for HCl will calculate an HI for HCl and Cl 2 as described in Appendix A to subpart DDDDD. Affected sources attempting to utilize the health based compliance alternative for Mn will calculate an HQ for Mn as described in Appendix A to subpart DDDDD.

Comment: Commenters IV D 122 and IV D 123 believe that the IRIS RfC for HCl is conservative and provides an ample margin of safety. In both cases where the EPA declined to regulate HCl emissions as part of a NESHAP, EPA concluded that HCl is a threshold pollutant and used the RfC as the primary benchmark for risk assessment. Response: We agree with the commenter's statement that HCl is a threshold pollutant and that the IRIS RfC for HCl is appropriate for use in this analysis.

Comment: Commenters IV D 26, IV D 104, and IV D 150 asserted that a § 112( d)( 4) risk based exemption is warranted for Mn, citing EPA precedent that two criteria must be met: ( 1) the pollutant must be health based and not subject to large uncertainty; and ( 2) the pollutant does not create significant or widespread environmental effects. The commenters claimed that Mn meets both of these criteria. The commenters stated that EPA's Office of Research and Development has established an inhalation RfC for Mn, which is retrievable from the EPA's IRIS. The IRIS monograph for Mn identifies impairment of nuero behavioral function, a threshold effect, as the health endpoint of concern, and establishes an RfC of 0.00005 mg/ m3 ( 0.05

g/ m3) for this endpoint by applying a conservative adjustment factor of 1000 to the identified threshold for this health effect. Regarding the creation of significant or widespread environmental effects, the commenters claimed that Mn is ubiquitous in the environment and is an essential element for humans, animals, and plants. According to the commenters, acute exposures to Mn are very rare, and toxicity from inhaled Mn particles has been reported only under extreme occupational exposures ( such as mining). Chronic ( i. e, 2 3 years and longer) Mn exposure at levels significantly higher than the RfC ( i. e., from 0.14 to 30 mg/ m3) is reported to cause manganism, a form of central nervous system toxicity. This exposure range is approximately 2,800 to 600,000 times greater than the RfC. The commenters also stated that there are no known human reproductive or teratogenetic effects of Mn exposure. Additionally, the commenters noted that data indicate that air concentrations of Mn have been decreasing over the past 30 years. The commenters stated that there is little likelihood of chronic or widespread exposure to Mn at concentrations above the noadverse effect threshold, given the above factors and the fact that Mn emissions from combustion sources subject to this rulemaking are quite low, and concentrations of Mn in exhaust gases are very low due to the typically high volume of the exhaust gases. In addition, ambient concentrations of Mn around sugar mills are expected to be very low due to stack height effects, topography, meteorology, and other site specific factors. Response: We agree that a health based compliance alternative is appropriate for Mn, 237 which is a threshold pollutant, under CAA § 112( d)( 4) in cases where affected sources are able to demonstrate that their emissions of Mn pose little or no threat to human health. We agree that the IRIS RfC for Mn is appropriate for use in this analysis, and for the final rule, we are basing the health based compliance alternative on the RfC value, as described in Appendix A of subpart DDDDD of 40 CFR part 63. For air toxics risk assessments, we identify pertinent toxicity or dose response values using a default hierarchy of sources, with IRIS being the preferred source, to assist us in identifying the most scientifically appropriate benchmarks for our analyses and decisions. The IRIS process contains internal and external peer review steps and represents EPA consensus values. The critical effect for the Mn RfC, representing the most sensitive toxicological endpoint, is impairment of neurobehavioral function. With regard to human health hazard associated with acute exposures, we have performed an acute assessment of Mn emissions from boiler and process heater affected sources. Our results suggest that acute emissions of Mn from boilers and process heaters are unlikely to adversely affect human health. Therefore, affected sources attempting to utilize the health based compliance alternative for Mn are not required to perform an acute assessment. To identify HAP with potential to cause multimedia and/ or environmental effects, we have identified HAP with significant potential to persist in the environment and to bioaccumulate. This list does not include Mn. Therefore, we do not believe that emissions of Mn from affected sources will pose a significant risk to the environment, and affected sources attempting to comply with the health based alternatives for Mn are not required to perform an ecological assessment.

Comment: Commenters IV D 122 and IV D 123 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. The EPA's mandate under the statute's " ample margin of safety" language is to provide a reasonable amount of protection in light of scientific uncertainties. The CAA does not define " ample margin of safety" explicitly. However, 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 commenters 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. This is the approach called for by the Senate Report accompanying the 1990 CAA Amendments: For some pollutants a MACT emissions limitation may be far more stringent than is necessary to protect public health and the environment. For some of the hazardous air pollutants listed under subsection ( b) it is possible to establish a " no observable effects level" ( NOEL) below which human exposure is presumably " safe." This NOEL or health effects threshold may be higher than the level of emissions which can be achieved with the application of maximum achievable control technology . . . [ W] here health thresholds are well established, for instance in the case of ammonia, and the pollutant presents no risk of other adverse health effects, the Administrator may use the threshold with an ample margin of safety ( and not considering cost) to set emissions limitations for sources in the category or subcategory. S. Rep. No. 228, 101st Cong. Sess. 171 ( 1990). The legislative history of CAA § 112( d)( 4) thus demonstrates that Congress intended the adequate margin of safety determination to rest on a finding of a level of exposure below which adverse effects are not seen in toxicological studies ( i. e., the NOEL), reduced by multiplicative 238 factor( s) to provide a reasonable degree of protection in light of scientific uncertainties. This is exactly what is done in deriving an RfC or similar inhalation health benchmark. Commenters IV D 61, IV D 122, and IV D 123 stated that EPA's derivation of the RfC contains multiple layers of conservatism to account for scientific uncertainty. The commenters believe that RfCs and similar inhalation health benchmarks already incorporate sufficient uncertainty factors to fulfill or exceed the ample margin of safety mandate of CAA § § 112( d)( 4) and ( c)( 9). The commenters noted that the RfC is defined as the concentration of a pollutant in air which a person  including sensitive individuals  can breathe 24 hours a day every day for 70 years without experiencing an adverse health effect. The RfC is calculated by taking the most conservative animal data and employing a number of uncertainty factors which assume that humans generally are an order of magnitude more sensitive to the effect than animals, that some humans are an order of magnitude more sensitive than others, and that the dose level at which no effect will be seen in a study is an order of magnitude lower than the lowest dose level at which an effect was seen. Safety factors may also be added where a study was of shorter than lifetime duration, or to account for an " inadequate" database. The range of aggregated safety factors usually is between 300 and 3,000. Commenters IV D 26, IV D 104, and IV D 150 noted that individual facility exposure modeling would be required to demonstrate that the health benchmark is not exceeded and believe that the combination of conservative air dispersion modeling techniques and a conservative human health benchmark ensure that, where a source meets the requirement for a risk based compliance option, human health will be protected with an ample margin of safety. Commenters IV D 26, IV D 104, and IV D 150 stated that a HI of 1 or higher is appropriate due to a variety of factors, including the extensive conservatism built into both the derivation of the RfCs and the exposure modeling, and the limited exposures to these HAP through pathways. The commenters noted that the range of aggregated safety factors built into the RfC usually is between 300 and 3,000, resulting in the RfC being an extremely conservative number. Commenters IV D 60, IV D 62, and IV D 119 submitted that a HI of 1 is appropriate for HCl since EPA's derivation of the RfC in general contains multiple layers of conservatism that fulfills, and in many cases exceeds, the ample margin of safety mandate of § 112( d)( 4). The commenters pointed out that EPA's RfC for HCl is 20

g/ m3, whereas EPA notes in its IRIS entry for HCl that an expert panel identified a range of 300 to 3000

g/ m3 as a reasonable estimate of the no effect level in humans. The HCl health benchmark of 20

g/ m3 incorporates uncertainty factors of 300. The commenters believe that given that EPA's RfC is an order of magnitude lower than these experts' lowest estimate of a no effect level in humans, the HCl RfC properly is viewed as a highly conservative ( i. e., health protective) value and does not warrant a further reduction through a regulatory mechanism that would allocate only a portion of the HI to major sources. Commenter IV D 73 stated that uncertainty is already considered in the establishment of RfC from which the HI is derived. The commenter stated that the uncertainty factors used in the NATA are large, and because of the considerable uncertainty adjustments that are already applied, it is highly unlikely that an ample margin of safety would ever have to include more uncertainties than are already incorporated in the RfCs. In some cases, the uncertainty corrections are too conservative. Commenter IV D 186 stated that EPA should use RfC and reference doses ( RfDs) as the threshold for deciding whether a MACT standard for a specific pollutant is warranted. The RfCs 239 and RfDs are set conservatively by first determining a NOEL and then reducing that level by an uncertainty factor. This method of setting RfCs and RfDs assures that public health is protected by " an ample margin of safety." Commenter IV D 186 added that EPA should use § 112( d)( 4) in the boiler and process heater MACT to limit the number of affected sources subject to limits for threshold pollutants like HCl. Public exposure to HCl from most industrial boilers should be well below the RfC for HCl. Response: The final rule will utilize CAA § 112 ( d)( 4) rather than § 112 ( c)( 9). We agree that the CAA does not define " ample margin of safety" explicitly. The CAA does, however, in § 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 involves determination of an " acceptable risk" and includes a presumptive limit on maximum individual risk, the second step 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. In considering the extrapolation of the ample margin of safety objective described for linear cancer risk to the management of risk for nonlinear effects, EPA considers 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. Allowable emission limits under this health based compliance alternative are derived with consideration of RfC values, as described in Appendix A of subpart DDDDD of 40 CFR part 63. 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 RfCs and comparable values are derived from assessments of pertinent toxicological information to identify the lowest POD ( 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 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. 1 Affected sources can demonstrate compliance with the health based alternative as described in Appendix A of subpart DDDDD of 40 CFR part 63. Affected sources are not restricted to a particular modeling approach. Rather, they may choose to perform a relatively refined modeling analysis which will require more effort but will produce results that are less uncertain and less conservative ( i. e., less likely to overestimate risk) than a screening level assessment.

Comment: Commenters IV D 122 and IV D 123 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. Commenters IV D 122 and IV D 123 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 240 approach. 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" and incorporates a number of conservative assumptions. Actual average concentrations are likely to be much lower. 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. Indeed, 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: Affected sources performing site specific modeling are referred to Appendix A of subpart DDDDD of 40 CFR part 63. This appendix refers to the EPA " Air Toxics Risk Assessment Reference Library," which describes possible approaches to conducting site specific modeling. We discussed a tiered analytical approach in the preamble to the proposed rule, beginning with relatively simple look up tables and followed by increasingly more site specific but more resource intensive tiers of analysis, with each tier being more refined. In the final rule, we are adopting a somewhat different approach for meeting the requirements of CAA § 112( d)( 4). The basis for this approach stems from the general air toxics assessment approach presented in the Residual Risk Report to Congress, which was developed with full consideration of EPA risk assessment policy, guidance, and methodology. 2 The approach for demonstrating eligibility is discussed in section 18.8. Affected sources can demonstrate eligibility for the health based emission limits by using site specific emissions test data and look up tables that were developed using health protective input parameters. These look up tables are included in Appendix A of subpart DDDDD of 40 CFR part 63. Affected sources that cannot demonstrate eligibility based on the health protective screening assessment ( i. e., look up tables) may used more refined sitespecific risk assessments as described in Appendix A of subpart DDDDD of 40 CFR part 63 and in other analytical tools such as the " Air Toxics Risk Assessment Reference Library," ( which may be appropriate for specific sources). A more refined analysis requires more effort, but produces results that are less uncertain and less conservative ( i. e., less likely to overestimate risk). 2 18.5.3 HAP to be included Comment: Multiple commenters ( IV D 26, IV D 60, IV D 62, IV D 73, IV D 104, IVD 119, IV D 122, IV D 123, IV D 150, and IV D 166) presented modeling examples and discussed HAP to include in modeling. Commenters IV D 122 and IV D 123 stated that many coal fired boilers would need to install a scrubber to meet the proposed HCl limit, and HF would also be controlled by the scrubber. The commenters stated that, based on modeling performed by the CIBO, risk from HF is small compared to that for HCl. The commenters contended that HF emissions are not likely to drive risk for industrial boilers. Risk modeling indicating that HCl levels are below the health benchmark would also indicate that HF is below the benchmark. Commenters IV D 60, IV D 62, IV D 73, and IV D 119 presented the results of dispersion modeling conducted at two sources in order to establish whether typical solid fuel fired sources could meet the RfC provided by EPA in Table 4 ( 68 Fed. Reg. 1690). Since EPA indicates that it is using HCl as a surrogate for other acid gas HAP, the test case modeling also included dispersion modeling for HF in order to characterize its highest concentration versus HCl 241 relative to the respective RfCs. Dispersion modeling was conducted using the ISCST3. All modeling was done in accordance with approved procedures codified in 40 CFR 51, Appendix W. For each source, the models were run using two stack HCl concentrations: ( 1) the proposed rule limit of 0.09 lb/ MMBtu and ( 2) the highest conceivable coal chloride content for the facility with minimal capture in the fabric filter installed for PM control. The commenters presented the modeling results for each of the two sources. For each source, the commenters claimed that the HCl modeling results indicate that there is an insignificant impact due to emissions of HCl from those coal fired boilers in a rural setting, and that a requirement to install HCl emissions controls on those boilers would provide insignificant benefit at a very high cost. Similar results were found for the HF modeling at each source. The commenters, and commenter IV D 166, asserted that based on the analysis of modeling these specific sources, it is apparent that there are likely many solid fuel fired affected sources that could take advantage of a flexible modeling option to prove insignificant impact and meet MACT intent without installation of expensive control technology. The commenters stated that the results also demonstrate that the inherently lower fluorine content in coals results in much lower HQ for HF emissions than seen for HCl emissions, indicating an even higher margin of safety for HF. Therefore, the applicability and modeling criteria for HCl as a surrogate for acid gas HAP are adequate without requiring additional applicability and/ or modeling criteria for other acid gas HAP. Commenters IV D 73 and IV D 166 added that it is apparent from the modeling exercise that a solid fuel fired source located in complex terrain may not be able to demonstrate an HQ on an annual basis of less than 1.0 relative to the RfC. Therefore, there will be inherent limitations to the ability of affected sources to use this type of applicability exemption criteria. Commenter IV D 73 also described risk modeling done for a facility that includes three hazardous waste incinerators and seven coal fired boilers. The HCl and Cl 2 emissions rates were based on worst case testing involving chlorine spikes. Aggregate modeling results for the incinerators/ boilers showed an HQ of 0.00897 for total HCl and chlorine ( approximately one tenth of the reference air concentration). The commenter believes other boilers would also show low risk associated with HCl and Cl 2, and therefore, it would be inappropriate to impose HCl limits on boilers. Commenters IV D 73 and IV D 166 stated that EPA should clarify that sources wishing to use the § 112( d)( 4) emissions limit must only demonstrate compliance with risk based thresholds for those HAP from Table 4 ( 68 FR 1690) that are emitted from the source. EPA states in the preamble that nine HAP account for 68 percent of total HAP emissions from the source category, and HAP emissions vary significantly from source to source depending on fuel use and combustion device configuration. Therefore, the commenters believe that those HAP identified in Table 4 are the only HAP that could potentially be emitted in significant quantities from a source within the category, and for purposes of demonstrating compliance with a riskbased standard, only those pollutants should be considered. The commenters added, for example, that a boiler MACT facility that emits HF, HCl, mercury, chromium, formaldehyde, and lead would demonstrate risk based compliance with § 112 ( d)( 4) emissions limitations for threshold pollutants ( HF, HCl, and mercury) and/ or compliance with de minimis emissions limitations for non threshold pollutants ( chromium, formaldehyde, and lead). Commenters IV D 26, IV D 104, and IV D 150 also believe the tiered approach should be extended to individual HAP. For example, if a facility could demonstrate that the impact of its TSM, Hg, or HCl emissions were individually below a given fraction of the RfC ( e. g., 242 20 percent), or that the HI for TSM, Hg or HCl were below a given fraction of 1 ( e. g., 0.2), then the facility would be exempt from the emission limits and control requirements of the rule as they apply to TSM, Hg, or HCl. The commenters pointed out that it makes little sense to require controls and other requirements on identified categories of HAP ( i. e., TSM, Hg, or HCl) when emissions of such HAP pose no threat to the public and do not contribute significantly to the overall HI for the facility. Therefore, the commenters supported an approach that would allow individual HAP or groups of HAP ( such as TSM) to be exempted from regulation if they pose no risk to the public. Response: Health based compliance alternatives are included in the final rule only for HCl and Mn. When developing MACT, the focus was on the most prevalent HAP because the rule was technology based. However, health based decisions must take HAP toxicity into account as well as emissions. Our rationale for selecting HCl as a surrogate for emissions of other acid gases and inorganic HAP for MACT purposes is provided in the preamble to the proposed rule ( 68 FR 1660; January 13, 2003.) Affected sources can demonstrate compliance with the health based alternative for HCl using a look up table analysis or site specific modeling as described in Appendix A of subpart DDDDD of 40 CFR part 63. Affected sources conducting site specific modeling may use an approach of their choice provided that approach is transparent, scientifically valid, and has undergone peer review. We have required measurement of Cl 2 emissions from emission points as described in Appendix A of subpart DDDDD of 40 CFR part 63 because our own modeling found that not all affected sources are eligible for the compliance alternative and that risk is driven primarily by chronic Cl 2 emissions. Our analysis demonstrated that other respiratory toxicants and acid gases emitted by boilers and process heaters do not contribute significantly to the HI. Affected sources can demonstrate compliance with the health based alternative for Mn using a look up table analysis or site specific modeling as described in Appendix A of subpart DDDDD of 40 CFR part 63. We acknowledge that terrain characteristics can affect the outcome of a risk assessment. However, a combination of factors, only one of which is terrain characteristics, will determine whether an affected source is able to utilize the health based compliance alternative in the final rule. 3 18.6 RISK ASSESSMENT ANALYSES 18.6.1 Background and multipathway exposure Comment: Multiple commenters ( IV D 26, IV D 60, IV D 61, IV D 62, IV D 73, IVD 75, IV D 104, IV D 119, IV D 122, IV D 123, IV D 150, IV D 184) opposed use of background concentration and multipathway exposure modeling. Commenters IV D 61, IV D 122, and IV D 123 stated that consideration of background and multipathway exposures is not required by law and is not necessary for sound policy. The commenters explained that the exclusive focus on the emissions from a source in making regulatory decisions under CAA § 112 is evident in all of the statutory provisions on which EPA would rely to implement the risk based mechanisms ( i. e., § 112( d)( 4), 112( c)( 9)( B), or EPA's de minimis authority). As a result, EPA has no legal obligation to consider background or multi pathway exposures. Moreover, the statutory focus on the MACT regulated source further means that there is no legal obligation to model risks from the entire facility, but rather only the 243 MACT regulated portion of the facility. Commenters IV D 123 and IV D 122 referenced House Rep. No. 101 490, Part 1, at 327 and stated that the CAA's legislative history does not support a requirement to consider other exposures. Commenters IV D 123, IV D 61, and IV D 122 also noted that 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. Commenters IV D 60, IV D 61, IV D 62, IV D 119, IV D 122, and IV D 123 noted that EPA has chosen not to regulate HCl emissions under § 112( d)( 4) authority in NESHAP for chemical recovery combustion sources at pulp mills [ 63 FR 18754, 18765 ( April 15, 1998)] and for the chlorine production source category [ 67 FR 44713 ( July 3, 2002)]. In both instances, EPA's exposure assessment focused on exposures attributable to the MACT affected sources alone, without consideration of background and multipathway HCl exposures. The commenters supported this approach. Based on the results of EPA's Cumulative Exposure Project, the commenters stated that compared to the RfC, background concentrations of HCl are negligible components of inhalation exposure. Commenters IV D 122 and IV D 123 stated that § 112( d)( 4) does not require the complete absence of a hazard and that all substances have adverse effects on health and the environment at sufficiently high doses  hence the fundamental tenet of toxicology that " the dose makes the poison." Accordingly, § 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 emission and whether the risk posed by any source in the category, by itself, exceeds one in a million cancer risk. Commenters IV D 122 and IV D 123 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. 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 an HI of 1.0 ( or higher) is amply protective of public health and is warranted under EPA's statutory mandate, the consideration of background concentration is not appropriate. Commenters IV D 122 and IV D 123 disagreed that EPA should consider multipathway exposures of HCl. The commenters stated that HCl is neither persistent or bioaccumulative. In solution, HCl readily dissociates to H+ and Cl. The commenters stated that there is no indication that ingestion of HCl ( from food, water, or soil) causes risk to human health. The commenters stated that dietary and drinking water exposure to HCl is insignificant. According to commenters IV D 60, IV D 61, IV D 62, and IV D 119, multipathway exposures to HCl are negligible and need not be considered. The commenters asserted that there is no indication that ingestion exposure to HCl causes any risk to human health ( the IRIS database contains no oral reference dose for HCl). Commenter IV D 75 stated that the CAA does not give EPA the authority to consider background concentrations; MACT standards must be based only on emissions from the regulated source and not existing background levels. The commenter stated that § 112 can be distinguished from other statutory provisions where EPA has been given authority to consider background sources, both in the CAA and in other environmental legislation. Where Congress intended EPA to consider background sources, the authority is clearly granted. The commenter provided several CAA examples where authority to consider background concentrations was explicitly 244 granted, including regulations to meet NAAQS; § 112( k), relating to urban air toxics from area sources; and § 169A, relating to visibility pollution. The commenter also provided examples of other environmental legislation where Congress explicitly granted authority to consider background concentrations. The commenter concluded that where intended, Congress granted such authority explicitly in the language of the statutes, and in any rulemaking to develop a riskbased alternative limit for the SCALDT source category, the consideration of background concentrations is inappropriate and is not specifically required by § 112( d) of the CAA. Commenter IV D 73 stated that making an allowance for other exposures under § 112 is not necessary to protect public health. The commenter added that consideration of exposures from other sources places a disproportionate burden on major sources. Legislative history does not support the consideration of exposures from other source types when setting risk based criteria. The commenter stated that multi pathway risk assessment should be required only for those HAP that have the potential for causing significant multi pathway exposure. Commenters IV D 26, IV D 104, and IV D 150 supported the position that the appropriate HI limit is 1, or higher. That is, whether facilities qualify for risk based compliance options should depend on a comparison of HAP exposures, attributable to their emissions alone, to the RfC or equivalent health benchmark. The commenters believe that including background sources in determining the HI is not appropriate. Commenter IV D 184 stated that EPA should use realistic exposure assumptions the characterize risk accurately; specifically, the focus should be on inhalation unless there is HAPspecific or site specific reason to expand the analysis to include other pathways of concern. Response: Commenters discussed the use of § 112( d)( 4) and § 112( c)( 9) of the CAA for the Boiler rule health based compliance alternative. This rule is relying not on CAA § 112( c)( 9), but on § 112( d)( 4), which requires us to evaluate ecological and multimedia human exposures when determining the applicability of § 112( d)( 4). To identify potential multipathway and/ or ecological concerns, we have identified HAP with significant potential to persist in the environment and to bioaccumulate. However, this list does not include Mn, HCl, or Cl 2 which are the only HAP relevant for the health based compliance alternative under the final rule. Therefore, facilities attempting to comply with the health based alternative in the final rule are not required to perform a multipathway analysis. We are not considering background emissions in the health based compliance alternatives for HCl and/ or Mn. Rather, as we indicated in the Residual Risk Report to Congress, we intend to assess facility wide emissions of HAP in future residual risk actions under § 112( f)( 2), as appropriate and to the extent it is reasonable and appropriate to do so. See also, 54 Fed. Reg. at 38,059 ( Sept. 14, 1989) ( benzene NESHAP). Affected sources attempting to utilize this health based compliance alternative for HCl must measure emissions of HCl and Cl 2 and compare the HI to 1.0, as described in Appendix A to subpart DDDDD. Affected sources attempting to utilize the health based compliance alternative for Mn must measure emissions of Mn and compare the HQ to 1.0, as described in Appendix A to subpart DDDDD. One commenter also made reference to the SCALDT rule; we are not incorporating a risk based option into the SCALDT rule.

Comment: Multiple commenters ( IV D 05, IV D 14, IV D 40, IV D 96, IV D 113, IVD 135) supported multipathway exposure modeling. Commenter IV D 05 provided the example of mercury, a persistent, bioaccumulative, and 245 toxic ( PBT), from coal combustion. The commenter stated that it would be inappropriate to exclude PBT sources from MACT without consideration of non inhalation pathway exposures and environmental impacts. The commenter recommended that as a precaution, PBT sources should not be granted exemptions from MACT, which provides a level playing field. Commenter IV D 14 stated that analyses like the concentration based applicability threshold approach do not address non inhalation exposures or adverse effects on the environment. Commenter IV D 14 stated that allowing individual facilities to monitor the HAP backgrounds for use in their own analysis would require oversight and evaluation by State and local agencies to insure proper site selections and analytical methods and would be costly to administer and, therefore, not acceptable. Commenter IV D 96 stated that EPA must consider all ways that a HAP could harm public health or the environment. With regard to EPA's request for comment on the " appropriateness and necessity" of accounting for non inhalation exposures, the commenter stated that § 112( d)( 4) refers to pollutants " for which a health threshold has been established." As this language and the legislative history make clear, it refers to pollutants that have no adverse health or environmental effects. See 5 Legislative History at 8511. Thus, § 112( d)( 4) necessarily requires EPA to consider all possible ways that a pollutant could affect human health or the environment. As EPA has recognized repeatedly in the past, 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. Moreover they bioaccumulate in wildlife and food sources, poisoning people and animals alike. See, e. g., 64 Fed. Reg. 52828, 53014 ( September 30, 1999); 64 Fed. Reg. 31898, 31908 31909 ( June 14, 1999); 63 Fed. Reg. 14182, 14193 ( March 28, 1998); 61 Fed. Reg. 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). To evaluate whether a pollutant is a threshold pollutant and what its health threshold and ample margin of safety must be, therefore, EPA must consider all the potential health and environmental effects of deposition, persistence and bioaccumulation of that pollutant. EPA would contravene § 112( d)( 4) by considering only health effects caused by inhalation. Commenter IV D 113 stated that EPA appears to assume that it only needs to consider inhalation risks, but the EPA does not demonstrate or even claim that people are exposed only by inhalation to HAP from the source category. Without such a demonstration, it is reasonable to conclude that other pathways may result in exposures, and, therefore, other pathways must be considered. Commenter IV D 135 stated that all exposure pathways must be considered. Section 112( d)( 4) necessarily requires EPA to consider all possible ways that a pollutant could affect human health. To evaluate whether a pollutant is a threshold pollutant and what its threshold may be, EPA must consider all potential health effects, including non inhalation pathways such as ingestion and indirect contamination of food by pollutants such as mercury and dioxin. Commenter IV D 40 noted that metals and HF account for approximately 4 percent and 5 percent, respectively, of the HAP emissions from boilers and process heaters and stated that exposure to these HAP can result in noncancer health effects ranging from skin irritation to central nervous system damage. The commenter pointed out that in addition, exposure to a subset of these HAP ( i. e., arsenic, cadmium, hexavalent chromium, and nickle) can result in cancer. The commenter submitted that EPA is proposing to base the NESHAP on the direct inhalation pathway, which assumes that all of the HAP emissions are available for inhalation and 246 that no other routes of exposure are applicable. The commenter has found that EPA currently does not have an Fv value for HF, but that since it is described as a colorless gas, assumed it is entirely in the gaseous state and available only through the direct inhalation pathway. Conversely, the commenter stated that all of the metal HAP emitted by boilers and process heaters, with the exception of mercury, were entirely in the particulate state when emitted. The metals are available for direct inhalation, but will undergo deposition over time. The commenter stated that once deposition has occurred, the chemical is available for uptake through indirect exposure pathways, and uses the incidental ingestion of soil pathway as an example. The commenter also stated that incidental soil ingestion rates vary among age groups, with young children ingesting more than adults. The commenter asserted that the potential risks to public health from boiler and process heater emissions should be explicitly addressed and consideration should be given to both direct and indirect exposure pathways. The commenter also requested that impacts from deposition of the HAP emissions be adequately addressed to ensure the proper protection of human health. Response: We agree that exposures via all relevant routes need to be considered for pollutants identified as PBT. To identify HAP with potential to cause multimedia and/ or environmental effects, we have identified HAP with significant potential to persist in the environment and to bioaccumulate. This list does not include HCl, Cl 2, or Mn, which are the only HAP relevant for the health based compliance alternative in the final rule. Therefore, affected sources attempting to comply with the health based alternatives for these HAP are not required to perform a multimedia assessment. Additionally, a screening level analysis conducted by the EPA indicates that acute impacts of these HAP from boiler and process heater facilities are highly unlikely. For these reasons, we do not anticipate that emissions of HCl, Cl 2, or Mn from boiler and process heat facilities will pose a significant risk to the environment, and affected sources attempting to comply with the health based alternatives for these HAP are not required to perform an ecological assessment. MACT limits for Hg are not affected by the health based compliance alternative. Affected sources able to demonstrate that they are low risk for HCl and Cl 2 or for Mn would still be subject to a PM limit; however, they would be subject to a TSM limit based on seven HAP metals ( excluding Mn) rather than the TSM limit based on eight metals.

Comment: Commenter IV D 148 stated that EPA has 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 (" Framework for Cumulative Risk Assessment" ( EPA/ 630/ P 02/ 001A, April 23, 2002), and chemical mixtures ( EPA/ 630/ R 00/ 002). 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. Commenter IV D 96 stated that ( if § 112( d)( 4) may properly be read to create an individualized exemption to an otherwise applicable MACT standard) EPA's methodologies for determining the contribution of other sources to the overall hazard are deeply flawed. The EPA's proposed alternative methodologies for determining the contribution of other sources to cumulative risk are untenable. The first and second approaches ( HI of 1 and HI of 0.2) would allow exemptions based on blanket assumptions about exposure, but EPA provides no basis for making any assumption. The third option suggests relying on existing estimates of background 247 levels of certain HAP, but these information sources are neither designed nor adequately precise to be used as the basis of regulatory applicability determinations. The EPA itself cautions about NATA that the 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). NATA does no estimate exposure to a number of HAP ( e. g., HF, HCl). The ATSDR profiles offer generalized assessments, but are not specific enough to establish as baseline for a given facility. Commenter IV D 14 stated that the use of NATA to determine background concentrations is unacceptable. EPA has clearly stated at a number of public meetings that NATA is not to be used to make regulatory decisions. 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. 3, 4 There are few mixtures ( e. g., coke oven emissions), however, for which such assessments are available. When mixture specific dose response assessments are not available, a component by component approach is recommended. The method for component data depends on a judgement 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 " Air Toxics Risk Assessment Reference Library," assumptions about toxicologic similarity are made in order to choose among risk assessment methods. In general, for the term " toxicologic similarity," 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. 3 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 toxicity by different routes. To reflect these differences, the HI is then usually developed for each exposure route of interest, and for a single target organ or organ system. A mixture may then be assessed by several HI, each representing one route and one toxic effect or target organ. 3 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. Pending the availability of such data for the HAP components of the mixture being assessed, the method employed is to aggregate HAP HQ by target organ to generate a target organ specific hazard index ( TOSHI). Affected sources attempting to utilize the health based compliance alternative for HCl will calculate an HI for HCl and Cl 2 as described in Appendix A to subpart DDDDD. Affected sources attempting to utilize the health based compliance alternative for Mn will calculate an HQ for Mn as described in Appendix A to subpart DDDDD. We agree that it is inappropriate to use the NATA national scale assessment for determining local background concentrations of particular HAP. We are not considering background emissions in the health based compliance alternatives for HCl and/ or Mn. Rather, as we indicated in the Residual Risk Report to Congress, we intend to assess facility wide emissions of HAP in future residual risk actions under § 112( f)( 2), as appropriate and to the extent it is reasonable and appropriate to do so. See also, 54 Fed. Reg. at 38,059 ( Sept. 14, 1989) ( benzene NESHAP). 248 Comment: Commenter IV D 135 stated that EPA's HI proposal is unworkable. It is inappropriate to exclude sources from regulation on the basis of a calculated HI for the following reasons: ( a) emissions, ambient concentrations and risks vary over time and cannot be captured by a single analysis to predict exposure, ( b) modeling exercises always involve simplifying assumptions that may or may not accurately reflect the level of pollution that people are exposed to, ( c) relying only on published health thresholds does not account for exposures and risks from compounds that have not been evaluated or for which the data are inconclusive, ( d) evaluating exposure on the basis of ambient exposure does not account for all pathways of exposure, and ( e) variations in background exposures would be unaccounted for, thus underestimating the level of pollution that people are exposed to. Response: Affected facilities attempting to comply with the health based alternatives will be required to assess their emissions under worst case conditions as described in Appendix A to subpart DDDDD. If facilities undergo any alterations that may affect their HAP emissions as described in Appendix A, they are required to re certify their low risk status. We discussed a tiered analytical approach in the preamble to the proposed rule, beginning with relatively simple look up tables and followed by increasingly more site specific but more resource intensive tiers of analysis, with each tier being more refined and less conservative. In the final rule, we are adopting a somewhat different approach for meeting the requirements of CAA § 112( d)( 4). The basis for this approach stems from the general air toxics assessment approach presented in the Residual Risk Report to Congress, which was developed with full consideration of EPA risk assessment policy, guidance, and methodology. 2 The approach for demonstrating eligibility is discussed in section 18.8. For air toxics risk assessments, we identify pertinent toxicity or dose response values using a default hierarchy of sources, with IRIS being the preferred source, to assist us in identifying the most scientifically appropriate dose response values for our analyses and decisions. 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. Further, where the IRIS assessment substantially lags the current scientific knowledge, we consider alternate credible and readily available assessments. For our use, these alternatives need to be grounded in publicly available, peer reviewed information. We are not using information that does not meet these requirements. Health based compliance alternatives are available for HCl and for Mn in the final rule. EPA has identified those HAP with greatest potential to cause multimedia and/ or environmental impacts. This list does not include Mn, HCl, or Cl 2. Therefore, affected sources attempting to comply with the health based alternatives in the final rule are not required to perform a multipathway analysis. We are not considering background emissions in the health based compliance alternatives for HCl and/ or Mn. Rather, as we indicated in the Residual Risk Report to Congress, we intend to assess facility wide emissions of HAP in future residual risk actions under § 112( f)( 2), as appropriate and to the extent it is reasonable and appropriate to do so. See also, 54 Fed. Reg. at 38,059 ( Sept. 14, 1989) ( benzene NESHAP).

Comment: Commenter IV D 154 stated that the proposal is critically flawed because riskbased exemptions ignore the cumulative risk that comes from exposure to multiple air toxics sources ( e. g., hundreds of combustion engines and boilers within a city) and do not protect public 249 health. Although many individual sources may pose a risk below a designated threshold, the accumulation of these pollutants can be hazardous. Addressing this problem will require a general reduction in air toxics emissions across large and small sources, not just those sources for which a high local risk can be demonstrated. The commenter stated that NATA indicates that air toxics exposures are already high throughout the country. Commenter IV D 154 stated that there are five HAP emitted from boilers/ process heaters that are predicted to be at concentrations that pose greater than 10 6 risk across New Jersey, and at least four other pollutants have county averages greater than 10 6. Much of this risk comes from area sources. Response: We are not considering background emissions in the health based compliance alternatives for HCl and/ or Mn. Rather, as we indicated in the Residual Risk Report to Congress, we intend to assess facility wide emissions of HAP in future residual risk actions under § 112( f)( 2), as appropriate and to the extent it is reasonable and appropriate to do so. See also, 54 Fed. Reg. at 38,059 ( Sept. 14, 1989) ( benzene NESHAP). The only HAP relevant for the health based compliance alternatives for boiler and process heater facilities are HCl, Cl 2, and Mn. These HAP pose noncancer risks to human health. Affected sources able to demonstrate that they meet the health based compliance alternatives will still be required to install controls for other pollutants covered under the final rule.

Comment: Commenter IV D 40 offered comments on each option proposed by EPA to establish a HI limit. The commenter asserted that the first, second, and third options should be removed from consideration. The commenter stated that the first option ( HI of 1) assumes no other exposures to threshold HAP from emissions from nearby sources or from uptake via other exposure pathways. The commenter believes the second option ( a default percentage such as HI of 0.2) has a high level of uncertainty because it does not take into account site characteristics, such as the predominant wind speed and direction, topography, climate and other related parameters affecting the dispersion of emissions. The commenter pointed out that the third option ( using data available through EPA or other scientific sources to establish background concentrations) has two major limitations: ( 1) use of historically collected data to estimate current background conditions, which may be different because of, for example, the construction of new sources, the development of new monitoring devices, or a change in atmospheric conditions, and ( 2) the likely use of regional background concentration data, which may or may not accurately reflect the local air concentrations. The commenter believes that the final option ( permit facilities to estimate their own specific background concentrations and apply a default percentage based on their findings to ensure their emissions would not result in an HI of greater than one) was clearly the best approach for establishing a facility's HI limit. The commenter believes that this option would result in accurate site specific estimations of background concentrations and an accurate estimation of a facility's default percentage. Response: We are not considering background emissions in the health based compliance alternatives for HCl and/ or Mn. Rather, as we indicated in the Residual Risk Report to Congress, we intend to assess facility wide emissions of HAP in future residual risk actions under § 112( f)( 2), as appropriate and to the extent it is reasonable and appropriate to do so. See also, 54 Fed. Reg. at 38,059 ( Sept. 14, 1989) ( benzene NESHAP). Affected sources attempting to utilize the health based compliance alternative for HCl must measure emissions of HCl and Cl 2 and compare the HI to 1.0, as described in Appendix A to 250 subpart DDDDD. Affected sources attempting to utilize the health based compliance alternative for Mn must measure emissions of Mn and compare the HQ to 1.0, as described in Appendix A to subpart DDDDD.