Abstract
This paper describes a unique system for the systematic evaluation of volatile chemicals that may be released to the atmosphere in large quantities. Chemicals were ranked for their potential to cause death or serious injury in a surrounding community in the event of a large release following a rail accident. Two parameters were scored in the risk model: (1) toxicity and (2) exposure. Toxicity evaluations were based on data on acute lethality and serious sublethal effects. The decision rules used to score the chemicals and the assumptions relied on in the selection of the underlying data are detailed. The potential for exposure was evaluated using a simplified model based on vapor pressure and molecular diffusion. The rankings based on these scores and the overall risk scores were compared to rankings based on two other sophisticated exposure models: Mackay, an empirical model of evaporation, and SPILLS, a Gaussian puff dispersion model in use by the Association of American Railroads. The rank orders predicted by each model of a chemical's propensity to evaporate were in excellent agreement with one another. Because the process of evaluation and review of the underlying data for ranking is still underway, the ranking is confidential.