Simple, but conservative, atmospheric transport models are used in the initial stages of a hazard screening methodology to determine a preliminary hazard rank. The hazard rank is one indicator of the additional effort, if any, that must be applied to determine if a system is safe. Simple methods avoid prolonged calculations at this early stage when details of potential accidents may be poorly defined. The models are used to simulate the consequences resulting from accidental releases of toxic substances. Instantaneous and constant-rate releases are considered. If a release takes place within a relatively small enclosure, the close-in transport is approximated by assuming the airborne material is instantaneously mixed with the volume of air within this enclosure. For all other situations and large distances, the transport is estimated with simple atmospheric dispersion models using published values of dispersion coefficients for large distances, and values based on turbulent diffusion theory for close-in distances. Consequences are assessed by defining exposure levels that are equivalent to negligible, reversible, and irreversible health effects. The hazard rank is related to the number and location of people within each category of health effects.