Interpretation of Carboxyhemoglobin and Cyanide Concentrations in Relation to Aviation Accidents

Carbon monoxide (CO) and hydrogen cyanide (HCN) are combustion products of organic material, but their production depends on material constituents and environmental conditions. Non-nitrogenous organic materials generate CO, whereas nitrogenous organic materials also produce HCN. For fire-involved aviation accidents, it is important to determine if the fire occurred during flight or after the crash and to establish the source(s) of the toxic gases. Therefore, this study was pursued. Bio-specimens from aviation accident fatalities (cases) are submitted to the Civil Aerospace Medical Institute for analyses. In blood, CO is analyzed as carboxyhemoglobin (COHb) and HCN as cyanide (CN-). These analytical data are stored in a database, and this database was searched for the period of 1990-2002 for the presence of COHb and CN- in the submitted cases. Out of 5945 cases, there were 223 (4%) cases wherein COHb was > or = 10%. Of the 223 cases, fire was reported with 201, no fire with 21, and undetermined fire status with 1. CN- concentrations were at or above 0.25 ng/mL in 103 of the 201 fire-related cases. None of the 21 non-fire cases had CN-, but nicotine was detected in 9 of the cases. All non-fire cases with COHb>30% (4 cases) were associated with exhaust leaks. Of the 223 cases, COHb-CN- Fractional Toxic Concentration (FTCs) was lethal only in 31 cases with elevated CN- levels. It is concluded that the presence of COHb and CN- in elevated concentrations in the blood of victims who died on impact would indicate an in-flight fire. In the absence of fire and CN-, the elevated COHb concentrations would suggest an exhaust leak, particularly at COHb>20%. Findings of this study also suggest that, in addition to COHb, CN- contributes to the detrimental effects of fire-associated aviation accident fatalities.