Overview of Final MATS Appropriate

and Necessary (A & N) Finding

November 1, 2011

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Structure of A & N Finding

• Introduction (will be added, not in the current interagency 

review draft)

• Summary of A &N finding as reported in the preamble to 

the proposed rule

• Peer review of national-scale Hg risk analysis (Hg Risk TSD)

• Revisions to the Hg Risk TSD and results

• Peer review of the nickel and chromium risk case studies

• Revisions to the non-Hg HAP inhalation risk case studies 

and results

• Response to public comments

• Affirmation of the A & N finding

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Summary of A &N Finding:

It is appropriate to regulate Hazardous Air Pollutants 

(HAP) from U.S. EGUs

• Hg and non-Hg HAP emissions are hazardous to public health, and U.S. 

EGU emissions cause and/or contribute to these hazards

• Hg and some non-Hg HAP are hazardous to the environment

• U.S. EGUs are the largest domestic source of Hg, HCl, and HF emissions in 

the U.S., and a significant source of other HAP emissions

• Hg emissions Hg emissions from individual EGUs causes e om individual EGUs causes excess local deposition dir local deposition directly 

around the EGU that would not be reduced by regional emission 

reductions, requiring controls at all U.S. EGU sources that emit Hg

• Hg emissions from EGUs contribute to future risk because Hg is persistent 

in the environment , and Hg can be re-emitted from previous deposition

• Delaying regulation already resulted in hundreds of additional tons of Hg 

being emitted to the environment

• Effective controls for Hg and non-Hg HAP are available for U.S. EGU 

sources

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Summary of A &N Finding

It is necessary to regulate HAP from U.S. EGUs

• Hg emissions from EGUs remaining in 2016 still pose a hazard to public 

health and the environment in the absence of regulations under Section 

112 that would be required to be implemented at that time

• U.S. EGU emissions of Hg after imposition of the requirements of the CAA 

are projected to be 29 tons per year in 2016, the same as the level of Hg 

emitted today

• EGUs are the largest source of Hg in the U.S. and thus contribute to the 

risk associated with exposure to MeHg

• EGUs are the largest source of Hg in the U.S. and thus contribute to the 

risk associated with exposure to MeHg

• We cannot be certain that the identified cancer risks attributable to EGUs 

will be addressed through imposition of the requirements of the CAA, 

• The environmental hazards posed by acidification will not be fully 

addressed through imposition of the CAA

• Regulation under section 112 is the only way to ensure that HAP emissions 

reductions that have been achieved since 2005 remain permanent

• Direct control of Hg emissions affecting U.S. deposition is only possible 

through regulation of U.S. emissions; we are unable to control global 

emissions directly 

4
Risk Results Supporting the A & N Finding as 

Presented in the Proposal

• National scale Hg risk analysis

– 12% of modeled watersheds receive enough Hg from EGU emissions 

alone to cause a public health hazard, even without taking into 

account Hg deposition from other sources. 

– In 22% of modeled watersheds, Hg from EGUs added with Hg from 

other emission sources causes a public health hazard, and EGUs are 

responsible for a significant amount of that Hg (more than 5%). 

– In total, up In total, up to 28% of to 28% of modeled U modeled U.S. watersheds have populations that tions that 

are at risk from Hg emitted by U.S. EGUs

• Non-Hg HAP inhalation risk case studies

– 4 out of 16 case study EGUs have estimated risks of greater than 1 in a 

million

– Highest risk for oil-fired unit was 10 in 1 million, driven by nickel 

emissions

– Highest risk for coal-fired unit was 8 in 1 million, driven by chromium 

six emissions

– Non-cancer risks were all small, with hazard index values less than 1

5
Peer Review of the Hg Risk TSD

• Peer review was conducted in accordance with 

OMB’s Final Information Quality Bulletin for Peer 

Review

• 22-member SAB peer review panel, including 

members representing 16 academic institutions, 

4state health or environmental agencies, 1 

federal agency, and 1 utility industry organization

• Public meetings held in June, August, and 

September, final letter issued September 29

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General Findings of the SAB Review

• SAB report was generally supportive regarding the methods and data 

used:

– SAB concluded that EPA’s risk assessment was an “objective, reasonable, and 

credible determination of the potential for a public health hazard from Hg 

emitted from U.S. EGUs”

– SAB also “regards the design of the risk assessment as suitable for its intended 

purpose, to inform decision-making regarding an ‘appropriate and necessary 

finding’ for regulation of hazardous air pollutants from coal and oil-fired EGUs”

• The review noted that “The SAB finds the current draft document 

deficient, however, because of its lack of transparency in describing key 

analytical methods and findings.” Thus, SAB support is contingent on 

significant revisions to the TSD to improve the clarity and presentation of 

methods, results, and uncertainties

• Three general themes of recommendations were

– Improve clarity of TSD in terms of methods and presentation of results

– Expand discussion of sources of variability and uncertainty

– De-emphasize IQ loss as an endpoint in the TSD

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Response to SAB Recommendations

• Revised TSD addresses all of the recommendations from the SAB

• Major responses:

– Improved overall clarity and comprehensiveness of the TSD

– Clarified focus on watersheds with populations potentially at risk

– IQ analysis has been moved to an appendix

– Expanded discussion of fish tissue data, and added additional data for 

key states

– Examined sensitivity amined sensitivity of results to alternativ of results to alternative fish tissue fish tissue Hg 

concentration percentiles

– Expanded discussion of uncertainty related to fish consumption rates

– Included additional information about the number of watersheds 

excluded due to screening criteria for source populations

– Enhanced discussion of assumption of linear relationship between 

changes in Hg deposition and changes in fish tissue Hg concentrations 

at the watershed level

– Expanded tables of sources of variability and uncertainty

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Revised Hg TSD Risk Results

• Revisions to quantitative results reflect 3 recommendations 

from the SAB:

– Added 824 additional watersheds based on fish tissue Hg 

sample data obtained from states and the National Listing of 

Fish Advisories, for a total of 3,141 watersheds, a 33% increase

– Applied a 0.95 adjustment factor to account for the fraction of 

total fish Hg that is MeHg

– Included all watersheds with fish samples that meet the filtering 

criteria in representing potential risks for high fish consuming 

– Included all watersheds with fish samples that meet the filtering 

criteria in representing potential risks for high fish consuming 

female populations

• 99th percentile consumption rate: number of watersheds 

with populations potentially at risk increased from 672 to 

917 (36%). Percent of watersheds increased from 28 to 

29%.

• 95th percentile consumption rate: Percent of watersheds 

with populations potentially at risk increased from 22 to 

24%.

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Revised Hg TSD Sensitivity Analyses

(comparisons with revised primary results)

• Excluding states with the potential for historical 

loadings of Hg from non-air sources resulted in an 

increase in the percent of watersheds with populations 

potentially at risk from 29 to 33 percent

• Including only watersheds in the top 25th percentile of 

U.S. EGU deposition r GU deposition resulted in an incr in an increase in the ease in the 

percent of watersheds with populations potentially at 

risk from 29 to 30 percent

• Using the 50th instead of the 75th percentile of fish 

tissue Hg concentrations at each watershed resulted in 

a decrease in the percent of watersheds with 

populations potentially at risk from 29 to 26 percent

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Peer Review of Approach for Estimating Nickel 

and Chromium Cancer Risks

• Peer review focused on approaches and rationale to derive 

inhalation cancer risks from chromium and nickel from EGUs

• Letter peer-review process, 3 independent external peer reviewers 

representing government, academic, and private sectors

• Reviewers comments on chromium

– considered hexavalent chromium as the species likely to be driving 

cancer risks

considered hexavalent chromium as the species likely to be driving 

cancer risks

– EPA’s use of average of range of speciation data as reasonable when 

no specific speciation data

– Agreed that there is high uncertainty associated with variability in the 

speciation data available for chromium

• Reviewer comments on nickel

– Agreed that nickel compounds are carcinogenic as a group

– One reviewer recommended evaluation of additional speciation 

literature that suggests low levels of sulfidic nickel emitted from EGUs

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Response to Chromium and Nickel 

Peer Review

• EPA reviewed additional studies on chromium speciation 

and determined that more recent speciation data is unlikely 

to reduce uncertainty of chromium speciation, and that 

confidence in default speciation profiles is low

• EPA treats all nickel compounds as carcinogenic as a group

– EPA decided t A decided to use 100 per o use 100 percent of the curr of the current IRIS URE t IRIS URE for 

nickel subsulfide, rather than assuming that 65 percent of the 

total mass of emitted nickel might be nickel subsulfide, as used 

in previous analyses

– EPA also considers it reasonable to use a value that is 50 percent 

of the IRIS URE for nickel subsulfide for providing an estimate of 

the lower end of a plausible range of cancer potency values for 

different mixtures of nickel compounds

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Revised Non-Hg Case Study Results

• EPA also received public comments on the non-Hg case studies, 

including a risk assessment prepared by EPRI

• In response to the peer review and public comments revisions were 

made to the emissions estimates, dispersion modeling, and risk 

characterization

– Key changes include changes in emissions, changes in stack 

parameters for some facilities, use of updated versions of the 

dispersion model, and use of 100 sion model, and use of 100 percent of the IRIS URE f cent of the IRIS URE for nick or nickel 

subsulfide

• Number of case study facilities with cancer risks greater than 1 in 1 

million increased from 4 to 6 out of 16 case study facilities 

– 4 of the 6 were coal-fired units

– 2 case studies had risks driven by oil-fired units

• Highest cancer risk for the oil-fired facility increased from 10 to 20 

in a million, driven by nickel emissions

• Highest cancer risk for the coal-fired facilities decreased from 8 to 5 

in a million, driven by hexavalent chromium emissions 

13
Response to Comments

• Received comments on legal, technical, and 

policy aspects of the A &N finding

• Organization of section:

– Comments on legal aspects of the finding

– Comments on technical analyses

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Comment Categories for Legal Aspects

• History of Section 112(n)(1)(a)

• Definition of terms “appropriate” and “necessary”

• Regulation of all HAP

• Consideration of health and environmental effects

• Consider Consideration of costs

• Consideration of CAA requirements in making 

necessary finding

• Listing of U.S. EGU under Section 112

• The 2000 finding

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Comment Categories for Technical and Policy Aspects

• Modeling of Hg emissions – global and U.S. 

• Hg deposition modeling and analysis of excess local deposition 

• Hg risk analysis

– Assumed linear relationship between changes in Hg deposition and 

fish tissue Hg concentrations

– Fish consumption rates and fish tissue Hg concentrations

– Cooking loss adjustment factor

– Behavior of subsistence fishing populations

– Fish consumption rates

– MeHg reference dose and Hg health effects studies

• Non-Hg HAP inhalation cancer risk case studies

– Non-Hg HAP emissions

– Nickel and chromium risk

– Alternative analysis of risk by EPRI

• Ecosystem impacts related to acid gases, e.g. HCl

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Affirmation of the A & N Finding

• Hg

– The percent of modeled watersheds with populations potentially at risk has 

increased from 28 to 29 percent in the revised risk assessment, and

– The revised analysis includes 36 percent more watersheds covering a much 

larger part of the U.S.

– We conclude that the finding that emissions of Hg from U.S. EGUs are a hazard 

to public health is affirmed by the revised risk analysis

• Non-Hg HAP

– The lifetime cancer risk for the oil-fired EGU facility has increased from 10 to 

20 in a million

– The number of coal-fired EGU facilities with cancer risks greater than one in a 

million has increased from three to five

– The highest risk coal-fired facility still has cancer risks of five in a million, which 

is greater than the one in a million benchmark

– We conclude that the finding that emissions of non-Hg HAP from U.S. EGUs 

are a hazard to public health is affirmed by the revised case studies

• These revised analyses, along with the other unchanged elements of the 

finding from the proposal, lead EPA to affirm its finding that it remains 

appropriate and necessary to regulate coal- and oil-fired EGUs under CAA 

section 112, and the inclusion of coal- and oil-fired EGUs on the section 

112(c) list of source categories regulated under CAA section 112.

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