Validation of additive models for lethal toxicity of fire effluent mixtures

Fractional Effective Dose (FED) calculation models for incapacitation and lethality are based upon a concept that overall toxicity results from the effects of a few key toxic gases and that the interactions between the component gases are essentially additive (with a possible multiplicative [synergistic] effect of CO2). A body of experimental data exists for common polymeric materials for lethal toxicity in rats (30 min LC50 + 14 days post exposure observation) combining animal data with chemical test atmosphere analysis. This data set has been used to compare LC50 concentrations calculated using two multi-gas FED models, the Levin N-Gas model and the Purser rat LC50 model, with the measured animal LC50 results. The Purser rat LC50 model has then been used to calculate LC50 concentrations for a range of common materials decomposed under conditions considered typical of four types of compartment fires as specified in ISO 19706: non-flaming oxidative, well-ventilated flaming, under-ventilated flaming and post-flashover under-ventilated flaming. It is concluded that both FED models give reasonably good predictions of overall toxicity for flaming fires, while the Purser LC50 model, which includes a term for the toxicity of organic irritants, provides a good fit for both flaming and non-flaming decomposition effluent toxicities. The results therefore confirm that for a range of common polymers, toxicity can be predicted in terms of the additive effects of a small number of key toxic gases, especially for flaming fires, while the generalized mix of organic compounds make a contribution to the toxicity mainly under non-flaming conditions, in which they form a significant proportion of the overall effluent mixture.