Development of a Unit Risk Factor for 1,3‐Butadiene Based on an Updated Carcinogenic Toxicity Assessment

The Texas Commission on Environmental Quality (TCEQ) has developed an inhalation unit risk factor (URF) for 1,3‐butadiene based on leukemia mortality in an updated epidemiological study on styrene‐butadiene rubber production workers conducted by researchers at the University of Alabama at Birmingham. Exposure estimates were updated and an exposure estimate validation study as well as dose‐response modeling were conducted by these researchers. This information was not available to the U.S. Environmental Protection Agency when it prepared its health assessment of 1,3‐butadiene in 2002. An extensive analysis conducted by TCEQ discusses dose‐response modeling, estimating risk for the general population from occupational workers, estimating risk for potentially sensitive subpopulations, effect of occupational exposure estimation error, and use of mortality rates to predict incidence. The URF is 5.0 × 10−7 per μg/m3 or 1.1 × 10−6 per ppb and is based on a Cox regression dose‐response model using restricted continuous data with age as a covariate, and a linear low‐dose extrapolation default approach using the 95% lower confidence limit as the point of departure. Age‐dependent adjustment factors were applied to account for possible increased susceptibility for early life exposure. The air concentration at 1 in 100,000 excess leukemia mortality, the no‐significant‐risk level, is 20 μg/m3 (9.1 ppb), which is slightly lower than the TCEQ chronic reference value of 33 μg/m3 (15 ppb) protective of ovarian atrophy. These values will be used to evaluate ambient air monitoring data so the general public is protected against adverse health effects from chronic exposure to 1,3‐butadiene.