Development of an inhalation unit risk factor for ethylene dichloride.

The Texas Commission on Environmental Quality (TCEQ) conducts up-to-date carcinogenic assessments for chemicals emitted in Texas. An inhalation unit risk factor (URF) was developed for ethylene dichloride (EDC, CAS 107-06-2) based on tumorigenicity results observed in a 2-year animal inhalation study conducted by Nagano et al. More specifically, the incidence of combined mammary gland tumors (adenomas, fibroadenomas, adenocarcinomas) in female rats demonstrated a statistically significant dose-response relationship, was amenable to benchmark concentration (BMC) modeling, was ultimately determined to be the most sensitive tumorigenic effect in the most sensitive species and sex, and was utilized as the carcinogenic endpoint for the development of the URF. The 95% lower confidence limit of the BMC at the 10% excess risk level (BMCL10 of 40.1 ppm) was determined for calculation of the URF. The resulting URF based on increased incidence of combined mammary gland tumors in female rats is 1.4E-02 per ppm (3.4E-03 per mg/m(3)). The lifetime air concentration corresponding to a no significant excess risk level of 1 in 100 000 is 0.71 ppb (2.9 µg/m(3)), which is considered sufficiently health-protective for use in protecting the general public against the potential carcinogenic effects of chronic exposure to EDC in ambient air.