EPA interim final report on WTC background study, April 2003

NYC 9/11 Public Portal Document

Averaging Time (AT) For non-carcinogens, AT is the exposure duration expressed in days. For carcinogens, exposure is averaged over a 70-year lifetime (the factor on which the cancer slope factors are based), and the AT is 70 years, in days (25,550).

Dermal Absorption Fraction (ABSd) This parameter is chemical-specific. Dermal absorption fractions of 0.06 for PCBs and 0.03 for dioxins from soil were first proposed in USEPA, 1992 and more recently adopted in EPA 2001b. Michaud et al (1994) used 0.02 for dioxins and 0.03 for PCBs uptake from a sooty surface, based on the ranges of estimated ABSd values for soil. The Binghamton panel used a range of values for PCBs (0.01, 0.1, and 0.5) and dioxins (0.01 and 0.1) (Kim and Hawley, 1985).

Reported ranges for dermaruptake for PCBs in solvent vehicles are reported to range from 15 to 56%, with most of the values clustering around 20% (ATSDR, 1993). Reported ranges for 2,3,7,8-TCDD in solvent vehicles are reported to range from 1 to 40% (ATSDR, 1988). Therefore, it seems that even if absorption from the wall material might be enhanced by residual solvent, the maximum possible absorption of 100% would be unrealistic even for worst-case exposure.

The values recommended here of 3% for dioxins and 13% for PAHs are based on EPA, 2001b.

Toxicity Values There are two toxicity values that can be used to calculate screening values, a Reference Dose (RfD) for non-carcinogenic compounds and a Cancer Slope Factor (CSF) for carcinogenic compounds. The RfD is defined as an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. It can be derived from a NOAEL, LOAEL, or benchmark dose, with imcertainty factors generally applied to reflect limitations of the data used. The CSF is defined as an upper bound, approximating a 95% confidence limit, on the increased cancer risk from a lifetime exposure to an agent. This estimate, usually expressed in units of proportion (of a population) affected per mg/kg/day, is generally reserved for use in the low-dose region of the dose-response relationship, that is, for exposures corresponding to risks less than 1 in 100. As the cancer endpoint was more sensitive, CSFs were used instead of RflDs. The CSFs used to calculate the clearance criteria are: 1.0 E+6 for dioxin (Dioxin Reassessment, EPA, 2001) and 7.3 E+0 for PAHs (IRIS, 2002).

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