The Importance of Carbon Monoxide in the Toxicity of Fire Atmospheres

This paper addresses the issue of determining toxic fire hazard in a manner that is relevant to the large-scale fire scenarios that cause most lethalities, ventilation controlled flashover fires.

The work involves (a) a literature study of background information on the toxicity of carbon monoxide (CO) to humans, (b) two very extensive forensic studies (with 2 241 and 2 673 cases, respectively) on human lethality involving CO in fires and non-fires, (c) statistical analyses of the forensic data, to ensure full separation of variables, and (d) an analysis of the literature on the effects of different parameters in fire atmospheres and in small-scale tests.

This work addresses five issues essential to fire hazard assessment: (1) Relative role of toxicants other than CO in causing fire fatalities and critical carboxyhemoglobin (COHb) values representing lethality. (2) Differences in populations between fire victims and those dying in other CO-containing atmospheres. (3) Comparison of modern fire atmospheres, containing smoke from man-made materials, with traditional fire atmospheres. (4) CO yields in flashover fires and effects of fuel chemistry. (5) CO yields in small-scale toxicity tests and fuel effects.

The work has shown that: (1) The toxicity of fire atmospheres is determined almost exclusively by the amount of CO. The lethal CO threshold level depends on the physical condition of the victim, but COHb values > 20% can produce lethality with no other apparent cause. (2) Fire and non-fire CO victim populations are very different: fire victims are much older or much younger and more infirm, and thus more sensitive to CO than those in nonfire exposures. (3) Replacing large amounts of natural and traditionally used materials by man-made materials has made no difference to fire atmosphere toxicity. (4) CO concentrations in large-scale flashover fire atmospheres are determined by oxygen availability and such variables, but are little affected by chemical composition of fuels. (6) Small-scale tests give excessively low CO yields so that they cannot be used to predict toxic fire hazard for ventilation controlled flashover fires, although post computational CO concentration corrections can make these tests useful as part of fire hazard assessment calculations.