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Prioritization of Data Sources for Chronic Exposure
Table 1 includes chronic dose-response values for all air toxics for which at least one of the assessment sources has developed a weight-of-evidence or dose-response assessment. However, many air toxics have chronic dose-response assessments developed by more than one of these agencies. Because different agencies developed these assessments at different times for purposes that were similar but not identical, it is inevitable that the results are not totally consistent. In some cases interagency differences are substantial, especially between assessments that were done many years apart. In lieu of chemical-specific decisions to resolve these discrepancies, EPA has applied a consistent priority scheme to the sources of chronic dose-response information described above.
Draft RfCs, RfDs, and UREs under development for the EPA IRIS process were given first priority on a case-by-case basis, where such assessments have already undergone external peer review and subsequent revision. Consistent with EPA's commitment to sound science, Table 1 does not include draft assessments that have not undergone peer review. This practice conforms to the 1996 changes to the IRIS review process requiring such external peer review.
Where externally peer reviewed IRIS draft assessments were not selected to supersede existing EPA IRIS values, Table 1 shows current IRIS information. For substances lacking current IRIS assessments, ATSDR chronic MRLs (available only for noncancer effects) received next preference, followed by CalEPA chronic RELs and UREs. Where ATSDR or CalEPA assessments did not exist, Table 1shows HEAST assessments.
For some carcinogenic substances (e.g., dichlorvos, propylene dichloride) that lack inhalation assessments from these sources,
inhalation UREs are derived from oral carcinogenic potency estimates.
(This is not done for noncancer effects.) The conversion from oral risk
(per mg/kg/d oral intake) to inhalation risk (per ug/m3 inhaled) is based
on EPA's standard assumptions of a 70-kg body mass and 20 m3/d inhalation
rate, as follows:
Although conversion of oral dose-response information to inhalation exposure is not optimal risk assessment practice, the alternative would be to omit these substances altogether from any quantitative inhalation risk estimates, thereby making a de facto assumption of zero carcinogenic potency by the inhalation route. Although we recognize that these values possess substantial uncertainty (which varies among HAPs according to available data and chemical-specific characteristics), we regard the use of oral-to-inhalation conversion as preferable to "zero potency" for screening-level risk assessments. We do not recommend oral-to-inhalation conversion for assessments that may lead to regulatory actions.