EXPOSURES TO CARBON-MONOXIDE, HYDROGEN-CYANIDE AND THEIR MIXTURES - INTERRELATIONSHIP BETWEEN GAS EXPOSURE CONCENTRATION, TIME TO INCAPACITATION, CARBOXYHEMOGLOBIN AND BLOOD CYANIDE IN RATS

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 (t(i)) and blood COHb and CN- incapacitation were examined in rats exposed to CO (5706 ppm for 5-min t(i); 1902 ppm far 35-min t(i)), HCN (184 ppm for 5-min t(i); 64 ppm for 35-min t(i)) and their mixtures (equipotent concentrations of each gas that produced 5- and 35-min t(i)). Blood CO and HCN uptakes were evaluated at the two concentrations of each gas. With either gas, variation in t(i) was higher for the 35-min t(i) than the 5-min t(i). The COHb level reached a plateau prior to incapacitation at both CO concentrations, and COHb levels at the 5- and 35-min t(i) were different from each other, Blood CN- increased as a function of both HCN concentration and exposure time, but CN- at the 5-min t(i) was half of the 35-min t(i) 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, t(i) was shortened from 5 to 2.6 min; COHb dropped from 81 to 55% and blood CN- from 2.3 to 1.1 mu g ml(-1). At the low-concentration CO-HCN mixture, where t(i) was reduced from 35 to 11.1 min, COHb decreased from 71 to 61% and blood CN- from 4.2 to 1.1 mu g 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.