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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules
in a particular case, that a risk that includes an MIR less than the presumptively acceptable level is unacceptable in the light of other health risk factors. Id. at 38045. In other words, risks that include an MIR above 100-in-1 million may be determined to be acceptable, and risks with an MIR
below that level may be determined to be unacceptable, depending on all of the available health information. Similarly, with regard to the ample margin of safety analysis, the EPA stated in the Benzene NESHAP that: EPA believes the relative weight of the many factors that can be considered in selecting an ample margin of safety can only be determined for each specific source category. This occurs mainly because technological and economic factors along with the health-related factors vary from source category to source category. Id. at 38061. We also consider the uncertainties associated with the various risk analyses, as discussed earlier in this preamble, in our determinations of acceptability and ample margin of safety.
The EPA notes that it has not considered certain health information to date in making residual risk determinations. At this time, we do not attempt to quantify the HAP risk that may be associated with emissions from other facilities that do not include the source category under review, mobile source emissions, natural source emissions, persistent environmental pollution, or atmospheric transformation in the vicinity of the sources in the category.
The EPA understands the potential importance of considering an individuals total exposure to HAP in addition to considering exposure to HAP emissions from the source category and facility. We recognize that such consideration may be particularly important when assessing noncancer risk, where pollutant-specific exposure health reference levels e.g., reference concentrations RfCs are based on the assumption that thresholds exist for adverse health effects. For example, the EPA recognizes that, although exposures attributable to emissions from a source category or facility alone may not indicate the potential for increased risk of adverse noncancer health effects in a population, the exposures resulting from emissions from the facility in combination with emissions from all of the other sources e.g., other facilities to which an individual is exposed may be sufficient to result in an increased risk of adverse noncancer health effects. In May 2010, the Science Advisory Board SAB advised the EPA that RTR
assessments will be most useful to
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decision makers and communities if results are presented in the broader context of aggregate and cumulative risks, including background concentrations and contributions from other sources in the area. 5
In response to the SAB
recommendations, the EPA incorporates cumulative risk analyses into its RTR
risk assessments. The Agency 1
conducts facility-wide assessments, which include source category emission points, as well as other emission points within the facilities; 2 combines exposures from multiple sources in the same category that could affect the same individuals; and 3 for some persistent and bioaccumulative pollutants, analyzes the ingestion route of exposure. In addition, the RTR risk assessments consider aggregate cancer risk from all carcinogens and aggregated noncancer HQs for all noncarcinogens affecting the same target organ or target organ system.
Although we are interested in placing source category and facility-wide HAP
risk in the context of total HAP risk from all sources combined in the vicinity of each source, we are concerned about the uncertainties of doing so. Estimates of total HAP risk from emission sources other than those that we have studied in depth during this RTR review would have significantly greater associated uncertainties than the source category or facility-wide estimates. Such aggregate or cumulative assessments would compound those uncertainties, making the assessments too unreliable.
B. How do we perform the technology review?
Our technology review primarily focuses on the identification and evaluation of developments in practices, processes, and control technologies that have occurred since the MACT
standards were promulgated. Where we identify such developments, we analyze their technical feasibility, estimated costs, energy implications, and non-air environmental impacts. We also consider the emission reductions associated with applying each development. This analysis informs our decision of whether it is necessary to revise the emissions standards. In addition, we consider the appropriateness of applying controls to new sources versus retrofitting existing sources. For this exercise, we consider 5 Recommendations of the SAB Risk and Technology Review Methods Panel are provided in their report, which is available at: https
yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPASAB-10-007-unsigned.pdf.

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any of the following to be a development:
Any add-on control technology or other equipment that was not identified and considered during development of the original MACT standards;
Any improvements in add-on control technology or other equipment that were identified and considered during development of the original MACT standards that could result in additional emissions reduction;
Any work practice or operational procedure that was not identified or considered during development of the original MACT standards;
Any process change or pollution prevention alternative that could be broadly applied to the industry and that was not identified or considered during development of the original MACT
standards; and Any significant changes in the cost including cost effectiveness of applying controls including controls the EPA considered during the development of the original MACT
standards.
In addition to reviewing the practices, processes, and control technologies that were considered at the time we originally developed the NESHAP, we review a variety of data sources in our investigation of potential practices, processes, or controls to consider. We also review the NESHAP and the available data to determine if there are any unregulated emissions of HAP
within the source category and evaluate this data for use in developing new emission standards. See sections II.C
and II.D of this preamble for information on the specific data sources that were reviewed as part of the technology review.
C. How do we estimate post-MACT risk posed by the source category?
In this section, we provide a complete description of the types of analyses that we generally perform during the risk assessment process. In some cases, we do not perform a specific analysis because it is not relevant. For example, in the absence of emissions of HAP
known to be persistent and bioaccumulative in the environment PBHAP, we would not perform a multipathway exposure assessment.
Where we do not perform an analysis, we state that we do not and provide the reason. While we present all of our risk assessment methods, we only present risk assessment results for the analyses actually conducted see section IV.B of this preamble.
The EPA conducts a risk assessment that provides estimates of the MIR for cancer posed by the HAP emissions
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