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

Abstract

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 x 10(-7) per microg/m(3) or 1.1 x 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 microg/m(3) (9.1 ppb), which is slightly lower than the TCEQ chronic reference value of 33 microg/m(3) (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.

Appraisal of levels and patterns of occupational exposure to 1,3-butadiene

“…The occupational exposure to 1,3-butadiene mainly occurred in the manufacture of chemicals and chemical products, including the manufacture of other organic basic chemicals and the manufacture of synthetic rubber. In this sector, indeed, a low mean exposure level was measured, comparable to the exposure limit suggested for the general population (ie, that causes 10-6 excess risk of leukemia), with no substantial differences among occupations (16,17). On the other hand, a wide range of co-exposure patterns were reported among chemical industry workers.…”

A comprehensive review of occupational and general population cancer risk: 1,3-Butadiene exposure–response modeling for all leukemia, acute myelogenous leukemia, chronic lymphocytic leukemia, chronic myelogenous leukemia, myeloid neoplasm and lymphoid neoplasm

“…In addition to Sielken and Valdez-Flores [22,23], TCEQ [25], Grant et al [10] contain cancer risk assessments based on the most recent update of the UAB study. These TCEQ assessments present results for several exposure-response models.…”

A Chronic Reference Value for 1,3-Butadiene Based on an Updated Noncancer Toxicity Assessment

“…While an exact partitioning of the 17-fold difference may not be possible, there are science-based and logical explanations accounting for most of the differences. The TCEQ has also developed a unit risk factor (URF) of 5.0 × 10 −7 per µg/m 3 (1.1 × 10 −6 per ppb) based on leukemia mortality in an updated epidemiological study on styrenebutadiene rubber production workers (TCEQ, 2008a;Grant et al, 2009). The corresponding air concentrations at 10 −4 , 10 −5 , and 10 −6 excess cancer risk are as follows: • The 10 −4 -risk air concentration is 200 µg/m 3 (91 ppb).…”

“…The purpose of this study is to present the procedures used in the noncarcinogenic chronic toxicity assessment of BD applicable to the general population. The procedures used for BD's updated carcinogenic toxicity assessment based on updated information were previously published (Grant et al, 2009).…”