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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules
populations provides the basis of human exposure calculations U.S.
Census, 2010. In addition, for each census block, the census library includes the elevation and controlling hill height, which are also used in dispersion calculations. A third library of pollutant-specific dose-response values is used to estimate health risk.
These are discussed below.

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b. Risk From Chronic Exposure to HAP
In developing the risk assessment for chronic exposures, we use the estimated annual average ambient air concentrations of each HAP emitted by each source in the source category. The HAP air concentrations at each nearby census block centroid located within 50
km of the facility are a surrogate for the chronic inhalation exposure concentration for all the people who reside in that census block. A distance of 50 km is consistent with both the analysis supporting the 1989 Benzene NESHAP 54 FR 38044 and the limitations of Gaussian dispersion models, including AERMOD.
For each facility, we calculate the MIR
as the cancer risk associated with a continuous lifetime 24 hours per day, 7 days per week, 52 weeks per year, 70
years exposure to the maximum concentration at the centroid of each inhabited census block. We calculate individual cancer risk by multiplying the estimated lifetime exposure to the ambient concentration of each HAP in micrograms per cubic meter mg/m3 by its unit risk estimate URE. The URE is an upper-bound estimate of an individuals incremental risk of contracting cancer over a lifetime of exposure to a concentration of 1
microgram of the pollutant per cubic meter of air. For residual risk assessments, we generally use UREs from the EPAs Integrated Risk Information System IRIS. For carcinogenic pollutants without IRIS
values, we look to other reputable sources of cancer dose-response values, often using California EPA CalEPA
UREs, where available. In cases where new, scientifically credible doseresponse values have been developed in a manner consistent with EPA
guidelines and have undergone a peer review process similar to that used by the EPA, we may use such doseresponse values in place of, or in addition to, other values, if appropriate.
The pollutant-specific dose-response values used to estimate health risk are available at https www.epa.gov/fera/
dose-response-assessment-assessinghealth-risks-associated-exposurehazardous-air-pollutants.

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To estimate individual lifetime cancer risks associated with exposure to HAP
emissions from each facility in the source category, we sum the risks for each of the carcinogenic HAP 10 emitted by the modeled facility. We estimate cancer risk at every census block within 50 km of every facility in the source category. The MIR is the highest individual lifetime cancer risk estimated for any of those census blocks. In addition to calculating the MIR, we estimate the distribution of individual cancer risks for the source category by summing the number of individuals within 50 km of the sources whose estimated risk falls within a specified risk range. We also estimate annual cancer incidence by multiplying the estimated lifetime cancer risk at each census block by the number of people residing in that block, summing results for all of the census blocks, and then dividing this result by a 70-year lifetime.
To assess the risk of noncancer health effects from chronic exposure to HAP, we calculate either an HQ or a target organ-specific hazard index TOSHI.
We calculate an HQ when a single noncancer HAP is emitted. Where more than one noncancer HAP is emitted, we sum the HQ for each of the HAP that affects a common target organ or target organ system to obtain a TOSHI. The HQ is the estimated exposure divided by the chronic noncancer dose-response value, which is a value selected from one of several sources. The preferred chronic noncancer dose-response value is the EPA RfC, defined as 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 10 The EPAs 2005 Guidelines for Carcinogen Risk Assessment classifies carcinogens as: carcinogenic to humans, likely to be carcinogenic to humans, and suggestive evidence of carcinogenic potential. These classifications also coincide with the terms known carcinogen, probable carcinogen, and possible carcinogen, respectively, which are the terms advocated in the EPAs Guidelines for Carcinogen Risk Assessment, published in 1986 51
FR 33992, September 24, 1986. In August 2000, the document, Supplemental Guidance for Conducting Health Risk Assessment of Chemical Mixtures EPA/630/R00/002, was published as a supplement to the 1986 document. Copies of both documents can be obtained from https
cfpub.epa.gov/ncea/risk/
recordisplay.cfm?deid=20533&CFID=70315376&
CFTOKEN=71597944. Summing the risk of these individual compounds to obtain the cumulative cancer risk is an approach that was recommended by the EPAs SAB in their 2002 peer review of the EPAs National Air Toxics Assessment NATA
titled NATAEvaluating the National-scale Air Toxics Assessment 1996 Dataan SAB Advisory, available at https yosemite.epa.gov/sab/
sabproduct.nsf/214C6E915BB04
E14852570CA007A682C/$File/ecadv02001.pdf.

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an appreciable risk of deleterious effects during a lifetime https
iaspub.epa.gov/sor_internet/registry/
termreg/searchandretrieve/
glossariesandkeywordlists/
search.do?details=&
vocabName=IRIS%20Glossary. In cases where an RfC from the EPAs IRIS is not available or where the EPA determines that using a value other than the RfC is appropriate, the chronic noncancer dose-response value can be a value from the following prioritized sources, which define their dose-response values similarly to the EPA: 1 The Agency for Toxic Substances and Disease Registry ATSDR Minimum Risk Level https
www.atsdr.cdc.gov/mrls/index.asp; 2
the CalEPA Chronic Reference Exposure Level REL https oehha.ca.gov/air/
crnr/notice-adoption-air-toxics-hotspots-program-guidance-manualpreparation-health-risk-0; or 3 as noted above, a scientifically credible dose-response value that has been developed in a manner consistent with the EPA guidelines and has undergone a peer review process similar to that used by the EPA. The pollutant-specific dose-response values used to estimate health risks are available at https
www.epa.gov/fera/dose-responseassessment-assessing-health-risksassociated-exposure-hazardous-airpollutants.
c. Risk From Acute Exposure to HAP
That May Cause Health Effects Other Than Cancer For each HAP for which appropriate acute inhalation dose-response values are available, the EPA also assesses the potential health risks due to acute exposure. For these assessments, the EPA makes conservative assumptions about emission rates, meteorology, and exposure location. As part of our efforts to continually improve our methodologies to evaluate the risks that HAP emitted from categories of industrial sources pose to human health and the environment,11 we revised our treatment of meteorological data to use reasonable worst-case air dispersion conditions in our acute risk screening assessments instead of worst-case air dispersion conditions. This revised treatment of meteorological data and the supporting rationale are described in more detail in Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in Support of the 2020
Risk and Technology Review Proposed Rule, and in Appendix 5 of the report:
11 See, e.g., U.S. EPA. Screening Methodologies to Support Risk and Technology Reviews RTR: A
Case Study Analysis Draft Report, May 2017.
https www3.epa.gov/ttn/atw/rrisk/rtrpg.html.

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