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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules
proceeded to the next step of the evaluation. Except for lead, the human health risk screening assessment for PB
HAP consists of three progressive tiers.
In a Tier 1 screening assessment, we determine whether the magnitude of the facility-specific emissions of PBHAP
warrants further evaluation to characterize human health risk through ingestion exposure. To facilitate this step, we evaluate emissions against previously developed screening threshold emission rates for several PB
HAP that are based on a hypothetical upper-end screening exposure scenario developed for use in conjunction with the EPAs Total Risk Integrated Methodology. Fate, Transport, and Ecological Exposure TRIM.FaTE
model. The PBHAP with screening threshold emission rates are arsenic compounds, cadmium compounds, chlorinated dibenzodioxins and furans, mercury compounds, and polycyclic organic matter POM. Based on the EPA
estimates of toxicity and bioaccumulation potential, these pollutants represent a conservative list for inclusion in multipathway risk assessments for RTR rules. See Volume 1, Appendix D at https www.epa.gov/
sites/production/files/2013-08/
documents/volume_1_reflibrary.pdf. In this assessment, we compare the facility-specific emission rates of these PBHAP to the screening threshold emission rates for each PBHAP to assess the potential for significant human health risks via the ingestion pathway. We call this application of the TRIM.FaTE model the Tier 1 screening assessment. The ratio of a facilitys actual emission rate to the Tier 1
screening threshold emission rate is a screening value SV.
We derive the Tier 1 screening threshold emission rates for these PB
HAP other than lead compounds to correspond to a maximum excess lifetime cancer risk of 1-in-1 million i.e., for arsenic compounds, polychlorinated dibenzodioxins and furans, and POM or, for HAP that cause noncancer health effects i.e., cadmium compounds and mercury compounds, a maximum HQ of 1. If the emission rate of any one PBHAP or combination of carcinogenic PBHAP in the Tier 1
screening assessment exceeds the Tier 1
screening threshold emission rate for any facility i.e., the SV is greater than 1, we conduct a second screening assessment, which we call the Tier 2
screening assessment. The Tier 2
screening assessment separates the Tier 1 combined fisher and farmer exposure scenario into fisher, farmer, and
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gardener scenarios that retain upperbound ingestion rates.
In the Tier 2 screening assessment, the location of each facility that exceeds a Tier 1 screening threshold emission rate is used to refine the assumptions associated with the Tier 1 fisher and farmer exposure scenarios at that facility. A key assumption in the Tier 1
screening assessment is that a lake and/
or farm is located near the facility. As part of the Tier 2 screening assessment, we use a U.S. Geological Survey USGS
database to identify actual waterbodies within 50 km of each facility and assume the fisher only consumes fish from lakes within that 50 km zone. We also examine the differences between local meteorology near the facility and the meteorology used in the Tier 1
screening assessment. We then adjust the previously-developed Tier 1
screening threshold emission rates for each PBHAP for each facility based on an understanding of how exposure concentrations estimated for the screening scenario change with the use of local meteorology and the USGS lakes database.
In the Tier 2 farmer scenario, we maintain an assumption that the farm is located within 0.5 km of the facility and that the farmer consumes meat, eggs, dairy, vegetables, and fruit produced near the facility. We may further refine the Tier 2 screening analysis by assessing a gardener scenario to characterize a range of exposures, with the gardener scenario being more plausible in RTR evaluations. Under the gardener scenario, we assume the gardener consumes home-produced eggs, vegetables, and fruit products at the same ingestion rate as the farmer.
The Tier 2 screen continues to rely on the high-end food intake assumptions that were applied in Tier 1 for local fish adult female angler at 99th percentile fish consumption 16 and locally grown or raised foods 90th percentile consumption of locally grown or raised foods for the farmer and gardener scenarios 17. If PBHAP emission rates do not result in a Tier 2 SV greater than 1, we consider those PBHAP emissions to pose risks below a level of concern.
If the PBHAP emission rates for a facility exceed the Tier 2 screening threshold emission rates, we may conduct a Tier 3 screening assessment.
16 Burger, J. 2002. Daily consumption of wild fish and game: Exposures of high end recreationists.
International Journal of Environmental Health Research, 12:343354.
17 U.S. EPA. Exposure Factors Handbook 2011
Edition Final. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R09/052F, 2011.

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There are several analyses that can be included in a Tier 3 screening assessment, depending upon the extent of refinement warranted, including validating that the lakes are fishable, locating residential/garden locations for urban and/or rural settings, considering plume-rise to estimate emissions lost above the mixing layer, and considering hourly effects of meteorology and plume-rise on chemical fate and transport a time-series analysis. If necessary, the EPA may further refine the screening assessment through a sitespecific assessment.
For further information on the multipathway assessment approach, see the Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in Support of the Risk and Technology Review 2020 Proposed Rule, which is available in the docket for this action.
5. How do we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect, Environmental HAP, and Ecological Benchmarks The EPA conducts a screening assessment to examine the potential for an adverse environmental effect as required under section 112f2A of the CAA. Section 112a7 of the CAA
defines adverse environmental effect as any significant and widespread adverse effect, which may reasonably be anticipated, to wildlife, aquatic life, or other natural resources, including adverse impacts on populations of endangered or threatened species or significant degradation of environmental quality over broad areas.
The EPA focuses on eight HAP, which are referred to as environmental HAP, in its screening assessment: Six PB
HAP and two acid gases. The PBHAP
included in the screening assessment are arsenic compounds, cadmium compounds, dioxins/furans, POM, mercury both inorganic mercury and methyl mercury, and lead compounds.
The acid gases included in the screening assessment are HCl and hydrogen fluoride HF.
HAP that persist and bioaccumulate are of particular environmental concern because they accumulate in the soil, sediment, and water. The acid gases, HCl and HF, are included due to their well-documented potential to cause direct damage to terrestrial plants. In the environmental risk screening assessment, we evaluate the following four exposure media: Terrestrial soils, surface water bodies includes water-

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