A mathematical model for estimating the likelihood of incapacitation and lethality from the inhalation of toxic gases is presented. The model computes an internal dose, equal to retained toxic gas per body mass, which is used to extrapolate outcomes across species. Account is taken for ventilation changes due to species, activity, and chemical response. The internal dose is correlated with each outcome using a cumulative, log-normal, probability distribution, which allows the estimation of tolerances for any population incidence. No internal interactions of gases are modeled and probabilities are combined independently. The model compares favorably with combined gas and large animal data.