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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules Technical Support Document for Acute Risk Screening Assessment. This revised approach has been used in this proposed rule and in all other RTR
rulemakings proposed on or after June 3, 2019.
To assess the potential acute risk to the maximally exposed individual, we use the peak hourly emission rate for each emission point,12 reasonable worst-case air dispersion conditions i.e., 99th percentile, and the point of highest off-site exposure. Specifically, we assume that peak emissions from the source category and reasonable worstcase air dispersion conditions co-occur and that a person is present at the point of maximum exposure.
To characterize the potential health risks associated with estimated acute inhalation exposures to a HAP, we generally use multiple acute doseresponse values, including acute RELs, acute exposure guideline levels AEGLs, and emergency response planning guidelines ERPG for 1-hour exposure durations, if available, to calculate acute HQs. The acute HQ is calculated by dividing the estimated acute exposure concentration by the acute dose-response value. For each HAP for which acute dose-response values are available, the EPA calculates acute HQs.
An acute REL is defined as the concentration level at or below which no adverse health effects are anticipated for a specified exposure duration. 13
Acute RELs are based on the most sensitive, relevant, adverse health effect reported in the peer-reviewed medical and toxicological literature. They are designed to protect the most sensitive individuals in the population through the inclusion of margins of safety.
Because margins of safety are incorporated to address data gaps and uncertainties, exceeding the REL does not automatically indicate an adverse health impact. AEGLs represent threshold exposure limits for the general
public and are applicable to emergency exposures ranging from 10 minutes to 8
hours.14 They are guideline levels for once-in-a-lifetime, short-term exposures to airborne concentrations of acutely toxic, high-priority chemicals.
Id. at 21. The AEGL1 is specifically defined as the airborne concentration expressed as ppm parts per million or mg/m3 milligrams per cubic meter of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure.
The document also notes that Airborne concentrations below AEGL1 represent exposure levels that can produce mild and progressively increasing but transient and nondisabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects. Id.
AEGL2 are defined as the airborne concentration expressed as parts per million or milligrams per cubic meter of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape. Id.
ERPGs are developed for emergency planning and are intended as healthbased guideline concentrations for single exposures to chemicals. 15 Id. at 1. The ERPG1 is defined as the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to 1 hour without experiencing other than mild transient adverse health effects or without perceiving a clearly defined, objectionable odor. Id. at 2. Similarly, the ERPG2 is defined as the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or
12 In the absence of hourly emission data, we develop estimates of maximum hourly emission rates by multiplying the average actual annual emissions rates by a factor either a categoryspecific factor or a default factor of 10 to account for variability. This is documented in Residual Risk Assessment for Mercury Cell Chlor-alkali Plants Source Category in Support of the 2020 Risk and Technology Review Proposed Rule, and in Appendix 5 of the report: Technical Support Document for Acute Risk Screening Assessment.
Both are available in the docket for this rulemaking.
13 CalEPA issues acute RELs as part of its Air Toxics Hot Spots Program, and the 1-hour and 8hour values are documented in Air Toxics Hot Spots Program Risk Assessment Guidelines, Part I, The Determination of Acute Reference Exposure Levels for Airborne Toxicants, which is available at https oehha.ca.gov/air/general-info/oehha-acute8-hour-and-chronic-reference-exposure-level-relsummary.

14 National Academy of Sciences, 2001. Standing Operating Procedures for Developing Acute Exposure Levels for Hazardous Chemicals, page 2.
Available at https www.epa.gov/sites/production/
files/2015-09/documents/sop_final_standing_
operating_procedures_2001.pdf. Note that the National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances ended in October 2011, but the AEGL program continues to operate at the EPA and works with the National Academies to publish final AEGLs https
www.epa.gov/aegl.
15 ERPGS Procedures and Responsibilities. March 2014. American Industrial Hygiene Association.
Available at: https www.aiha.org/get-involved/
AIHAGuidelineFoundation/EmergencyResponse PlanningGuidelines/Documents/
ERPG%20Committee%20Standard %20Operating%20Procedures%20%20-%20March %202014%20Revision%20%28Updated%2010-22014%29.pdf.

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developing irreversible or other serious health effects or symptoms which could impair an individuals ability to take protective action. Id. at 1.
An acute REL for 1-hour exposure durations is typically lower than its corresponding AEGL1 and ERPG1.
Even though their definitions are slightly different, AEGL1s are often the same as the corresponding ERPG1s, and AEGL2s are often equal to ERPG
2s. The maximum HQs from our acute inhalation screening risk assessment typically result when we use the acute REL for a HAP. In cases where the maximum acute HQ exceeds 1, we also report the HQ based on the next highest acute dose-response value usually the AEGL1 and/or the ERPG1.
For this source category, we used a default acute emissions multiplier of 10
as we have no information to suggest another factor to account for variability in hourly emissions data is more appropriate.
In our acute inhalation screening risk assessment, acute impacts are deemed negligible for HAP for which acute HQs are less than or equal to 1, and no further analysis is performed for these HAP. In cases where an acute HQ from the screening step is greater than 1, we assess the site-specific data to ensure that the acute HQ is at an off-site location. For this source category, the data refinements employed consisted of estimating the highest HQ that might occur outside facility boundaries with the use of satellite imagery of the facility with receptor locations. These refinements are discussed more fully in the Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in Support of the 2020 Risk and Technology Review Proposed Rule, which is available in the docket for this source category.
4. How do we conduct the multipathway exposure and risk screening assessment?
The EPA conducts a tiered screening assessment examining the potential for significant human health risks due to exposures via routes other than inhalation i.e., ingestion. We first determine whether any sources in the source category emit any HAP known to be persistent and bioaccumulative in the environment, as identified in the EPAs Air Toxics Risk Assessment Library see Volume 1, Appendix D, at https
www.epa.gov/fera/risk-assessment-andmodeling-air-toxics-risk-assessmentreference-library.
For the Mercury Cell Chlor-Alkali Plant source category, mercury emissions were the only PBHAP
emitted by the source category, so we
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