Time-to-tumour risk assessment for 1,3-butadiene based on exposure of mice to low doses by inhalation.

The excess risk for cancer due to lifetime occupational exposure to 1,3-butadiene at the proposed US Occupational Safety and Health Administration standard of 2 ppm was estimated on the basis of a quantitative risk assessment. The risk assessment was based on a recent study by the US National Toxicology Program of the carcinogenicity of butadiene in B6C3F1 mice, using exposure concentrations ranging from 6.25 to 625 ppm butadiene and controls. Cancer risks were estimated using a multistage Weibull time-to-tumour model; the dose was based on the external butadiene concentration, owing to the low-dose linearity of butadiene metabolism. The parameters of the time-to-tumour model were estimated for seven tumour sites in male mice and nine in female mice. The risk estimates were extrapolated from mice to humans on the basis of body weight raised to the three-fourths power, and the median lifespan of mice was equated to a human lifespan of 74 years. Estimates of excess risk for lifetime occupational exposure (8 h/day, 5 days/week, 50 weeks/year, for 45 years) to 2 ppm butadiene ranged from 0.2 per 10,000 workers, based on female mouse heart haemangiosarcomas, to 600 per 10,000 workers, based on female mouse lung tumours. An analysis was performed to assess the effects of varying the modelling assumptions (incidental versus fatal tumours, inclusion or exclusion of the 625-ppm dose group, and basis for interspecies scaling) on the risk estimates from the female mouse lung tumour model. Depending on the assumptions, estimates of lifetime excess risk derived from the female mouse lung model ranged from 60 per 10,000 to 1600 per 10,000. These results suggest that exposures to butadiene in the work place should be reduced to the lowest feasible level.