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uncertainty is driven largely by species differences in butadiene uptake and metabolism used in the slope factor.

The purpose of this study is twofold:

It is neither the intent of this paper to evaluate any cause and effect relationship between 1,3 butadiene exposure and cancer in humans, nor to provide the most scientifically defensible cancer potency estimates for 1,3-butadiene. Risks referred to in this paper are hypothetical estimates useful for regulatory purposes. These estimates assume as a matter of regulatory policy that a low-dose linear carcinogenic response to butadiene occurs in humans. Actual risks would be zero if butadiene is not carcinogenic to humans at these exposure levels.

Texaco initiated this evaluation in 1990 (Radian Corporation, 1990). That assessment focused on increasing the precision of the EPA screening level risk estimates based on more realistic representation of emissions, dispersion and exposure after completion of the Butadiene Modernization Project. This project centered around changing the extraction solvent used in the distillation process and in changing the "once-through" cooling water system to a recirculating cooling tower system in order to reduce butadiene emissions. Although based on site-specific information wherever possible, the risk assessment noted several sources of uncertainty that impacted interpretation of the risk estimates. Primary sources of uncertainty were identified as estimated emissions rates, assumptions and algorithms associated with dispersion modeling analysis, assumptions used to calculate inhalation exposure, and the theoretical estimate of the carcinogenic potency of butadiene, if any, in humans.

The Butadiene Modernization Project, now largely completed, has resulted in a process that is cleaner from both a product purity and environmental perspective. Butadiene emissions have been reduced more than 90 percent. Repeating the prior EPA screening level analysis predicts a maximum individual cancer risk after completion of this project in the range of 5-10 in 1000 based on a 70 year exposure to the maximum predicted annual-average ground level concentration 200 meters from the center of the plant. The current study was initiated to reexamine some of the sources of uncertainty in the risk estimates and to update the risk estimates, using the most site-specific and chemical-specific information available (Radian 1992a) The resulting risk estimates range from 3 in