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OCR for page 18
Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations: Final Report 5 Is There Dose-Related Performance Degradation Resulting from Combined Exposures to Carbon Monoxide and Hydrogen Cyanide? As stated in its first report, the committee concludes that consideration should be given to the potential interaction between hydrogen cyanide (HCN) and carbon monoxide (CO) that could affect crew performance and health. This conclusion is based primarily on the possible additivity of the effects of both chemicals on attentiveness and reaction times. However, there is not enough evidence available at this time for the committee to make a conclusive assessment. In addition, the assessment is complicated by the presence of other combustion products released both from weapons firing and from combusting vehicle fuel, and the committee recognizes that the potential hazardous effects of these other products, especially in combination with CO and HCN, may also be important. Hence, although the committee still considers the hazard quotient approach to risk assessment valid, and it reiterates its recommendation that the approach be used until further findings suggest otherwise, the committee concludes that additional information is needed to better understand possible synergisms between CO, HCN, and other combustion products that could help refine this risk assessment. The information available on exposures to low levels of these substances is scarce at best. A review of the literature suggests that, aside from reports on overexposures due to fires or suicides, there is a dearth of published studies on the neurobehavioral effects of HCN in humans and essentially nothing in nonhuman primates. There may also be epidemiologic data that could be helpful in addressing the effects of low concentrations of CO and HCN combined, but the committee is unaware of any recent experimental information in the current published literature that would directly assist in resolving this question with respect to the low concentrations of interest to the Army. The studies conducted to date on co-exposures to CO and HCN primarily in animals have focused on high levels of exposure. There is the possibility of a synergism between the two chemicals at these levels. The committee discussed this potential problem with respect to the exposures in weaponized armored vehicles and concluded that the interaction, based on the known mechanisms of action of these agents, is more likely to be additive than synergistic if neither agent alone is the driver for the adverse effect. However, the problem cannot be addressed with greater certainty because of the inability to extrapolate to the low-exposure concentrations with respect to humans. Few studies have been conducted on other possible modes of interaction—for example, an effect of either agent, alone or in combination, on such factors as respiratory rate increases, in addition to possible increases in respiration based on workload. From a practical standpoint, to address the likelihood of synergy with respect to possible decrements in performance, it is necessary to understand how low concentrations of CO and HCN would act individually. This understanding will require additional information from studies specifically designed to examine such effects as attentiveness and reaction times. The information could then be used as a frame-
OCR for page 19
Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations: Final Report work for follow-up studies on combinations of the two agents to determine whether there are interactions involving any of those effects. Experiments on combined human exposures to CO and HCN, similar to those suggested in Chapter 3 for CO alone, would be ideal; however, the health risks posed to the subjects due to the high toxicity of HCN would likely be unacceptable. An alternative would be to design neuro-behavioral experiments in nonhuman primates to test the effect of HCN in combination with CO. An additional or alternative approach considered viable by the committee is to conduct field studies in actual armored vehicles, where CO and HCN would be measured simultaneously along with crew performance (for example, reaction times). As stated in Chapter 4, however, such field experiments are unlikely to give definitive results because of multiple uncontrollable variables. Recommendation The committee recommends that, as more data become available on the actual combined exposure concentrations of CO and HCN and durations of such exposures, the Army seeks to understand how exposures to additional chemicals might affect the interaction of CO and HCN. Examples of additional chemicals are ammonia, particulate matter (including combustion-derived nanoparticles), and other components of diesel exhaust. Furthermore, it recommends that information or experimental results related to such potential interactions be reviewed by an independent body to assist in setting priorities on such interactions, including those that might be synergistic.