Meta-Analysis of Rat Lung Tumors from Lifetime Inhalation of Diesel Exhaust

Estimating the carcinogenic potential of exposure to diesel-engine exhaust particulates (DEPs) is problematic. In rats, high concentrations of DEPs ($>\ 1,000\ \mu {\rm g}/{\rm m}^{3}$) inhaled over a lifetime result in excess lung tumors. However, data for rats exposed to DEP at concentrations not associated with lung overload are consistent with no tumorigenic effect. Individual rat studies have only a limited number of exposure groups; therefore, we combined the tumor data from eight chronic inhalation studies in a meta-analysis. Statistical analysis identified a threshold of response between 200 and 600 μ g/ m3average continuous lifetime exposure, consistent with biological-effect thresholds reported by other investigators. Our exposure-response analysis of all rats with $