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7 Moving Forward The U.S. Army performs an important role in ensuring that the health of personnel in armored vehicles is not compromised by co-exposures to carbon monoxide (CO) and hydrogen cyanide (HCN) during weapons firing. Given the current weight of evidence, the committee clearly supports the Armyâs Center for Health Promotion and Preventive Medicine (CHPPM) hazard quotient approach, as indicated in the first report. (The committee recognizes that alternative approaches may provide additional insights into the conjoined effects of exposure to two or more substances. However, until further findings suggest otherwise, we are confident that the hazard quotient approach provides an appropriate method for estab- lishing exposure limits for combined exposures.) This chapter provides several issues the Army should consider as it moves forward in developing exposure guidelines and taking other actions to eliminate or control health hazards to military personnel. CHPPMâs efforts are focused on testing armored vehicles during the design phase (prior to de- ployment). Weapons are fired under various controlled scenarios to estimate exposures that crew mem- bers might experience within deployed vehicles. As a result of its discussion with various Department of Defense representatives, it became clear to the committee that the Armyâs health-hazards-assessment ac- tivity in relation to the development of new models of armored vehicles and the use of scenarios to mimic possible crew exposures to CO, HCN, and other gases could benefit substantially from greater communi- cation with and feedback from groups involved with personnel training and field deployment. These groups may include the Armyâs Human Factors Engineering Program, instructors associated with training armored-vehicle crews in the field, and health personnel involved with those crews during actual deploy- ment. These groups could serve as valuable resources regarding the signs and symptoms associated with actual experiences in the different uses of these vehicles and under varying ventilation conditions. Feed- back might yield information obtained from crew members, either real or perceived, on specific func- tional decrements in the field. Because the design or modification of the Armyâs armored vehicles is an iterative process, the committee concludes that the Armyâs future efforts to detect and mitigate potentially hazardous exposures within armored vehicles could benefit from advice, both positive and negative, from additional perspectives. EXPOSURE AND EFFECTS RESEARCH The committee identified the need for human experiments to assess the validity of the Coburn- Forster-Kane (CFK) equation at low or spiking concentrations of CO or under conditions of rapid changes in ventilation. The committee also identified the need for human experiments to investigate the effects of CO exposures on neuropsychological end points, such as visual and reaction-time decrements, relevant to real-life scenarios in armored vehicles. The experiments that we recommend would expose humans to concentrations of CO that are within the range of typical conditions encountered by military personnel 22
Moving Forward when firing weapons from inside armored vehicles. It is important to note that these studies involving re- search on human subjects must comply with federal and other applicable regulations for the protection of human subjects of research. Protocols for research involving human subjects should receive approval by a certified Institutional Review Board. OTHER CONSIDERATIONS FOR FUTURE RESEARCH As more data become available as a result of the committeeâs recommended experiments, DOD should consider how other factors may affect the relationship between exposure and effects. Factors That May Modify Effects of Carbon Monoxide Because of the weakness of the database regarding adverse effects of small increases in body CO on brain functions, such as those involved in decision making and alertness, it is not possible at this time to make conclusions about the extent to which environmental factors, such as temperature and humidity, can confound relationships between CO exposures and performance. However, environmental factors can alter CO uptake that can potentially translate into adverse effects of CO. Effect of Altitude The partial pressure of oxygen (O2) in the ambient air is reduced as altitude increases. This, in turn, reduces the partial pressure of O2 in the inspired air as well as the volume of O2 transported to hu- man tissues by hemoglobin in the blood. The result is tissue hypoxia, the severity of which is determined by the absolute altitude. As described by the CFK equation, CO both competes with O2 for hemoglobin, thus reducing the amount of O2 that can be transported, and increases the affinity of hemoglobin for O2 so that O2 is released less readily in the tissues. Thus, reduced O2 transport to the tissues due to altitude may be further reduced by inhaled CO (McGrath, 2000). The resulting tissue hypoxia would be exacerbated by stress and altitude-induced hyperventilation, which would increase CO uptake and cause respiratory alka- losis, further reducing O2 transport to the tissues. Thus, possible adverse effects from CO exposure of military personnel within armored vehicles might be amplified at altitude. Effect of Previous High Exposure to CO CO uptake at a given inspired CO concentration might possibly decrease as a result of increased carboxyhemoglobin (COHb) in tobacco smokers or as a result of previous exposure to high inspired CO concentrations due to previous gun firings. It will be important to consider the extent to which adaptation via tobacco smoking may protect soldiers against detrimental effects of small increases in COHb due to CO uptake secondary to cannon firings. Use of Computational Models to Assess Multiple Exposures Future computational efforts for estimating the health risks posed by exposure to combustion products in armored vehicles could include evaluation of the Toxic Gas Assessment (TGAS) model. The TGAS models (versions 1.0 and 2.0) were developed to predict the onset of death or immediate incapaci- 23
Combined Exposures to HCG and CO in Army Operations: Final Report tation resulting from the inhalation of gases released during fires ( for example, CO, HCN, nitrogen diox- ide, hydrochloric acid, acrolein, and carbon dioxide) at high concentrations and in any combination, as well as from a decrease in available O2 (Stuhmiller et al. 2006). Thus, TGAS models account for several factors that influence the breathing rate. The TGAS models would probably need to be restructured and calibrated to estimate the combustion-product-exposure conditions presented to the committee and also to estimate internal dosimetrics, such as COHb resulting from CO exposure. 24