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Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations: Initial Report
been published by governmental and professional groups, they have the same limited databases, especially for the interaction.
As noted, since the actions of the two toxicants apparently are additive, the use of the hazard quotient approach proposed by the army is appropriate. The use of the Coburn-Foster-Kane (CFK) model for the prediction of COHb levels related to air concentrations appears to have a solid scientific basis. Since the spikes observed in air levels of CO within the tank during firing scenarios were of such short duration taking into account respiratory rate and tidal volume, the committee concluded that it was not overly concerned about them in comparison with longer, possibly increasing, COHb levels. However, because of conditions where ambient CO concentrations are changing rapidly, it is recommended that the use of CFK equation to calculate COHb levels needs to be verified with blood COHb measurements. It was also noted by the Army that their monitoring data from gun firing scenarios would suggest that cyanide levels would be expected to be below levels of concern most of the time. That, is they would normally not be expected to add substantially to the hazard quotient.
An ancillary question was whether or not the blood level of cyanide, rather than air exposure, should be used in the hazard quotient calculation. The committee concluded that use of a blood level would be problematical based on the lack of good data to support a model such as the CFK model for CO. Furthermore, there is no simple method for determining cyanide levels in blood, and there are a number of technical difficulties in obtaining and handling blood samples. In addition, the rapid metabolism of cyanide makes correlation of air levels and blood levels difficult. Finally there are a host of confounding environmental factors which may influence cyanide levels. The committee, therefore, recommends that the Army continue to use the air level for cyanide rather then a blood level.
Because most studies on the combined toxicity of CO and HCN have been carried out at high concentrations and have focused on lethality and/or incapacitation, which makes extrapolation to the low-levels of exposure and more subtle toxicity end points of interest to the Army is difficult. Therefore, the Army should conduct neurological studies of sensory and motor performance at lower concentrations of HCN and CO.
In conducting its review and evaluation, the committee provides the following additional recommendations for the army to consider. One is to search relevant military documents regarding exposure and biological monitoring that may not be in the open, published literature, especially with regard to cyanide levels. A second is that the Army should consider that, while the binary system is important, other potential exposures such as exposure to diesel combustion products may need to be considered with respect to health of the tank crew.