Extrapolation of the Benzene Inhalation Unit Risk Estimate to the Oral Route of Exposure (External Review Draft)

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The National Center for Environmental Assessment-Washington Division, Office of Research and Development, prepared this document on the Extrapolation of the Benzene Inhalation Unit Risk Estimate to the Oral Route of Exposure. This document will serve as a source for updating the oral cancer unit risk estimate for benzene in the Integrated Risk Information System (IRIS).

Given the absence of oral dose-response information, the authors propose a simple method for extrapolating benzene-induced cancer risk from the inhalation to the oral route. The method is based on the relative efficiency of benzene absorption across pulmonary and gastrointestinal barriers. Substantial literature on pulmonary absorption in humans and a few laboratory animal species exists. Data on oral absorption in humans are lacking; hence extrapolation is based on gastrointestinal absorption studies in several experimental animal species. A review of the relevant literature suggests absorption efficiencies of 50% and 100% for inhalation and oral routes of exposure, respectively. Application of these absorption factors to the current inhalation unit risk range of 2.2 X 10 to 7.8 X 10^-6 per µg/m3 results in a proposed range for the oral unit risk of 4.4 X 10^-7 to 1.6 X 10^-6/µg/L.

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