Fire smoke toxicity: The effect of nitrogen oxides

Measurements of the toxic potency of fire effluents are required for fire-safety engineering and fire-hazard assessments. Toxic potency values may be generated using chemical analysis data and/or by animal protocols. The current ISO methods of calculating toxic potency values from chemical analysis data assume that the nitrogen oxides present in inhaled fire effluents are nitrogen dioxide, a highly irritant acid gas. Observations from real scale fire tests and bench scale tests which simulate the different fire stages show that, in some situations and particularly in proximity of the fire, the nitrogen oxides will be predominately nitric oxide but this can gradually change to nitrogen dioxide as the effluent moves away from the fire. Nitric oxide has a very different toxic potency and effect when compared to nitrogen dioxide. This paper considers the formation of nitric oxide and nitrogen dioxide in fire effluents, their potential toxic effects and the consequential need to reconsider the methods of calculating toxic potency values.