Exposures to carbon monoxide, hydrogen cyanide and their mixtures: Interrelationship between gas exposure concentration, time to incapacitation, carboxyhemoglobin and blood cyanide in rats

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Abstract
Carbon monoxide (CO) and hydrogen cyanide (HCN) are generated during aircraft interior fires in sufficient amounts to incapacitate cabin occupants. For typical post-crash and in-flight fires, minimum protection periods of 5 and 35 min, respectively, have been suggested for breathing devices to protect the occupants from smoke. Relationships of blood carboxyhemoglobin (COHb) and cyanide (CN−) levels to incapacitation have not been well defined for these gases. Therefore, time to incapacitation (ti) and blood COHb and CN− at incapacitation were examined in rats exposed to CO (5706 ppm for 5-min ti; 1902 ppm for 35-min ti), HCN (184 ppm for 5-min ti; 64 ppm for 35-min ti) and their mixtures (equipotent concentrations of each gas that produced 5- and 35-min ti). Blood CO and HCN uptakes were evaluated at the two concentrations of each gas. With either gas, variation in ti was higher for the 35-min ti than the 5-min ti. The COHb level reached a plateau prior to incapacitation at both CO concentrations, and COHb levels at the 5- and 35-min ti were different from each other. Blood CN− increased as a function of both HCN concentration and exposure time, but CN− at the 5-min ti was half of the 35-min ti CN− level. The HCN uptake at the high concentration was about three times that at the low concentration. In the high concentration CO–HCN mixture, ti was shortened from 5 to 2.6 min; COHb dropped from 81 to 55% and blood CN− from 2.3 to 1.1 μg ml−1. At the low-concentration CO–HCN mixture, where ti was reduced from 35 to 11.1 min, COHb decreased from 71 to 61% and blood CN− from 4.2 to 1.1 μg ml−1. Any alteration in the uptake of either gas by the presence of the other was minimal. Our findings suggest that specific levels of blood COHb and CN− cannot be correlated directly with the incapacitation onset and that post-mortem blood COHb and CN− levels should be evaluated carefully in fire victims.