Possible implications from results of animal studies in human risk estimations for benzene: nonlinear dose-response relationship due to saturation of metabolism.

To date, all risk assessment studies on benzene have been based almost exclusively on epidemiological data. We have attempted a more integrated and quantitative evaluation of carcinogenic risk for humans, trying to utilize, in addition to the epidemiological data, all data available, specifically data on metabolism, genotoxicity, and carcinogenicity in small rodents. An integrated evaluation of the globality of the available data seems to suggest a progressive saturation of metabolic capacity both for man and rodents between 10 and 100 ppm. The most susceptible target cells seem to be different in humans (predominant induction of myelogenous leukemia) and small rodents (induction of a wide variety of tumors). Nevertheless, both epidemiological and experimental carcinogenicity data tend to indicate a flattening of the response for the highest dosages, again suggesting a general saturation of mechanisms of metabolic activation, extended to different target tissues. From a quantitative point of view, the data suggest a carcinogenic potency at 10 ppm two to three times higher than that computable by a linear extrapolation from data in the 100 ppm range. These observations are in accord with the recent proposal of the European Economic Community of reducing benzene time-weighted average occupational levels from 10 to 5 ppm.