This assessment was conducted to review the new information that has become available since EPA’s 1985 health assessment of 1,3-butadiene.
1,3-Butadiene is a gas used commercially in the production of styrene-butadiene rubber, plastics, and thermoplastic resins. The major environmental source of 1,3-butadiene is the incomplete combustion of fuels from mobile sources (e.g., automobile exhaust). Tobacco smoke can be a significant source of 1,3-butadiene in indoor air.
This assessment concludes that 1,3-butadiene is carcinogenic to humans by inhalation, based on the total weight of evidence. The specific mechanisms of 1,3-butadiene-induced carcinogenesis are unknown, however, it is virtually certain that the carcinogenic effects are mediated by genotoxic metabolites of 1,3-butadiene.
Animal data suggest that females may be more sensitive than males for cancer effects; nevertheless, there are insufficient data from which to draw any conclusions on potentially sensitive subpopulations.
The human incremental lifetime unit cancer (incidence) risk estimate is based on extrapolation from leukemias observed in an occupational epidemiologic study. A twofold adjustment to the epidemiologic-based unit cancer risk is then applied to reflect evidence from the rodent bioassays suggesting that the epidemiologic-based estimate may underestimate total cancer risk from 1,3-butadiene exposure in the general population.
1,3-Butadiene also causes a variety of reproductive and developmental effects in mice; no human data on these effects are available. The most sensitive effect was ovarian atrophy observed in a lifetime bioassay of female mice. Based on this critical effect and using the benchmark concentration methodology, an RfC (i.e., a chronic exposure level presumed to be “without appreciable risk” for noncancer effects) was calculated.
In summary, the EPA’s conclusions about the health effects of 1,3-butadiene are:
· 1,3-Butadiene is carcinogenic to humans by inhalation.
· The unit cancer risk estimate is 0.08/ppm (based primarily on linear modeling and extrapolation of human data). This incorporates an adjustment factor of 2 to address concerns for sensitive populations. The corresponding estimate of the chronic exposure level of 1,3-butadiene resulting in extra cancer risk of 10-6 (i.e., 1 in a million) is 0.01 ppb.
· A chronic RfC (0.9 ppb), an acute reference value (7 ppb), and a subchronic reference value (7 ppb) are presented for noncancer effects.
