The incidences of lung neoplasms in female mice were increased
in all exposure groups. Thus, even at 6.25 ppm,
1,3-butadiene is carcinogenic to B6C3F1 mice. Because there
were no exposure levels at which a carcinogenic response was not
induced, it is likely that exposure concentrations below 6.25 ppm
would also cause cancers in laboratory animals. Extrapolation
ofthe dose-response curves to 2 ppm, the proposed OSHA standard
for 1,3-butadiene, results in an estimated 2-fold increase in
lung tumors in female mice. The reduced incidence of lung
neoplasms at 625 ppm compared with the incidence at 200 ppm