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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules for very reactive pollutants or larger particles. For all factors, we reduce uncertainty when possible. For example, with respect to census-block centroids, we analyze large blocks using aerial imagery and adjust locations of the block centroids to better represent the population in the blocks. We also add additional receptor locations where the population of a block is not well represented by a single location.

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d. Uncertainties in Dose-Response Relationships There are uncertainties inherent in the development of the dose-response values used in our risk assessments for cancer effects from chronic exposures and noncancer effects from both chronic and acute exposures. Some uncertainties are generally expressed quantitatively, and others are generally expressed in qualitative terms. We note, as a preface to this discussion, a point on dose-response uncertainty that is stated in the EPAs 2005 Guidelines for Carcinogen Risk Assessment; namely, that the primary goal of EPA actions is protection of human health;
accordingly, as an Agency policy, risk assessment procedures, including default options that are used in the absence of scientific data to the contrary, should be health protective the EPAs 2005 Guidelines for Carcinogen Risk Assessment, pages 1
through 7. This is the approach followed here as summarized in the next paragraphs.
Cancer UREs used in our risk assessments are those that have been developed to generally provide an upper bound estimate of risk.18 That is, they represent a plausible upper limit to the true value of a quantity although this is usually not a true statistical confidence limit. In some circumstances, the true risk could be as low as zero; however, in other circumstances the risk could be greater.19 Chronic noncancer RfC and reference dose RfD values represent chronic exposure levels that are intended to be health-protective levels.
To derive dose-response values that are intended to be without appreciable risk, the methodology relies upon an uncertainty factor UF approach,20
18 IRIS glossary https ofmpub.epa.gov/sor_
internet/registry/termreg/searchandretrieve/
glossariesandkeywordlists/search.do?details=&
glossaryName=IRIS%20Glossary.
19 An exception to this is the URE for benzene, which is considered to cover a range of values, each end of which is considered to be equally plausible, and which is based on maximum likelihood estimates.
20 See A Review of the Reference Dose and Reference Concentration Processes, U.S. EPA, December 2002, and Methods for Derivation of
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which considers uncertainty, variability, and gaps in the available data. The UFs are applied to derive dose-response values that are intended to protect against appreciable risk of deleterious effects.
Many of the UFs used to account for variability and uncertainty in the development of acute dose-response values are quite similar to those developed for chronic durations.
Additional adjustments are often applied to account for uncertainty in extrapolation from observations at one exposure duration e.g., 4 hours to derive an acute dose-response value at another exposure duration e.g., 1 hour.
Not all acute dose-response values are developed for the same purpose, and care must be taken when interpreting the results of an acute assessment of human health effects relative to the dose-response value or values being exceeded. Where relevant to the estimated exposures, the lack of acute dose-response values at different levels of severity should be factored into the risk characterization as potential uncertainties.
Uncertainty also exists in the selection of ecological benchmarks for the environmental risk screening assessment. We established a hierarchy of preferred benchmark sources to allow selection of benchmarks for each environmental HAP at each ecological assessment endpoint. We searched for benchmarks for three effect levels i.e., no-effects level, threshold-effect level, and probable effect level, but not all combinations of ecological assessment/
environmental HAP had benchmarks for all three effect levels. Where multiple effect levels were available for a particular HAP and assessment endpoint, we used all of the available effect levels to help us determine whether risk exists and whether the risk could be considered significant and widespread.
For a group of compounds that are unspeciated e.g., glycol ethers, we conservatively use the most protective dose-response value of an individual compound in that group to estimate risk. Similarly, for an individual compound in a group e.g., ethylene glycol diethyl ether that does not have a specified dose-response value, we also apply the most protective dose-response value from the other compounds in the group to estimate risk.
Inhalation Reference Concentrations and Application of Inhalation Dosimetry, U.S. EPA, 1994.

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e. Uncertainties in Acute Inhalation Screening Assessments In addition to the uncertainties highlighted above, there are several factors specific to the acute exposure assessment that the EPA conducts as part of the risk review under section 112
of the CAA. The accuracy of an acute inhalation exposure assessment depends on the simultaneous occurrence of independent factors that may vary greatly, such as hourly emissions rates, meteorology, and the presence of a person. In the acute screening assessment that we conduct under the RTR program, we assume that peak emissions from the source category and reasonable worst-case air dispersion conditions i.e., 99th percentile cooccur. We then include the additional assumption that a person is located at this point at the same time. Together, these assumptions represent a reasonable worst-case actual exposure scenario. In most cases, it is unlikely that a person would be located at the point of maximum exposure during the time when peak emissions and reasonable worst-case air dispersion conditions occur simultaneously.
f. Uncertainties in the Multipathway and Environmental Risk Screening Assessments For each source category, we generally rely on site-specific levels of PBHAP or environmental HAP
emissions to determine whether a refined assessment of the impacts from multipathway exposures is necessary or whether it is necessary to perform an environmental screening assessment.
This determination is based on the results of a three-tiered screening assessment that relies on the outputs from modelsTRIM.FaTE and AERMODthat estimate environmental pollutant concentrations and human exposures for five PBHAP dioxins, POM, mercury, cadmium, and arsenic and two acid gases HF and HCl. For lead, we use AERMOD to determine ambient air concentrations, which are then compared to the secondary NAAQS standard for lead. Two important types of uncertainty associated with the use of these models in RTR risk assessments and inherent to any assessment that relies on environmental modeling are model uncertainty and input uncertainty.21
21 In the context of this discussion, the term uncertainty as it pertains to exposure and risk encompasses both variability in the range of expected inputs and screening results due to existing spatial, temporal, and other factors, as well as uncertainty in being able to accurately estimate the true result.

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