Physiologically based pharmacokinetic models provide a useful means of evaluating the impact of different exposure scenarios upon certain components of the risk assessment. In the paper, a physiologically based pharmacokinetic model used for risk assessment purposes in 1987, was used to determine pulmonary absorption fractions the amount of metabolite formed at various exposure concentrations. The effect of various model parameters such as the metabolic rate constants and the blood to air partition ratios were examined. The paper illustrates the usefulness of such models to test the impact of various assumptions and exposure scenarios upon risk. They are not a replacement for soundly determined data but can be an important adjunct in the interpretation and use of such data.