The prediction of the probability of death or injury following the inhalation of a toxic gas or vapour is used in risk analysis. The proportion of a population responding for a given endpoint (e.g. lethality) can be related to the received dose using a probit model. Some of the coefficients in the probit equations are based on data from animal testing. Generally, experimental test animals are bred to exhibit low variability. Animal variability in response to toxic exposures may not adequately represent human variability in response to toxic exposures to the tested chemical. It is suggested that some independently established measure of human variability be used in the formulation of the probit equation constants Father than those that arise solely from the fitting of the animal data.