EPA interim final report on WTC background study, April 2003

NYC 9/11 Public Portal Document

Table 2. Exposure parameters for calculating clearance criteria for air samples.

Input Parameter Value Explanation Risk 1 X 10"* See Appendix C AT-(Averaging Time - 25,550 days See Appendix D Carcinogens) of LT ED (Exposure Duration) 30 years Upper-bound Estimate of Time in Residence [EPA, 1989] EF (Exposure Frequency) 365 days/yr Days in Residence [EPA, 1989] lUR (Inhalation Unit Risk) 2.3 E -01 (asbestos) IRIS,2002 2.86 E+5 (dioxin)* Dioxin Reassessment, EPA, 2001 7.3 E 0 (PAHs)* IRIS, 2002 *IUR was calculated by route-to-route extrapolation of oral Slope Factor

The cancer risk level (1x10"^) employed herein reflects the upper bound of the acceptable risk range (10"^ to 10"®) establish^ in EPA’s Superfimd regulation [Federal Register, 1990]. Practical Quantitation Limits (PQLs, the lowest level that can be reliably achieved within specified limits of precision and accuracy during routine laboratory operating procedures [EPA, 1992]) and anticipated background levels [ATSDR, 1995] dictated the selection of the risk level at 1x10"^. A more detailed discussion of this subject can be found in Appendix C.

For non-carcinogenic compounds, airborne contaminant concentration is compared to the Reference Concentration (RfC). The RfC is an estimate of a chronic exposure concentration for the human population, including sensitive subpopulations, that is likely to be without an appreciable risk of deleterious effects during a lifetime [EPA, 1989]. That comparison (i.e, chronic exposure concentration (CEC) divided by the RfC) is called the Hazard Quotient (HQ).

HQ = CEC/RfC

where CEC is the daily averaged air concentration, calculated as: AC * [EF*ED/AT-NC], where AC is the average air concentration during the period of exposure (pg/m^), EF is the exposure frequency (days), ED is exposure duration (years), AT-NC is the averaging time for non­ carcinogens, and RfC is the reference concentration, expressed in units of 1/concentration.

According to EPA guidelines [EPA, 1989], if the HQ is greater than one, there may be concern for potential health effects. Therefore, for Tier IH (clearance level), the benchmark screening level was set at an HQ of 1. For Tier I, which equates to a sub-chronic exposure scenario, an HQ of 10 was used, in accordance with EPA’s Hazard Evaluation Handbook [EPA, 1997]. An HQ of 10 accoimts for the fact that chronic toxicity criteria (RfDs/RfCs) are being applied to sub-chronic exposure scenarios (i.e., exposure not expected to exceed 6 months to one year in duration). Accordingly, an HQ of 10 was used for non-carcinogens to reflect a similar (i.e..

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