ernmental and professional groups, they have the same limited databases, especially for the interaction. Therefore, the possibility that independence or subadditive responses may occur cannot be discounted.
However, in light of the weak database of relevant studies, the committee agrees that assuming an additive response is the most reasonable approach. In assessing the toxic effects of these two compounds, it is prudent to expect that the additive effects of combined exposure observed with high concentrations would occur in subjects exposed to low concentrations. Therefore, the committee concluded that until further findings suggest otherwise, the Army’s use of the HQ approach or hazard index is reasonable in establishing exposure limits for personnel simultaneously exposed to CO and HCN. The committee stated that the HQ should be used for calculating the risk of adverse effects from exposure to CO and HCN combined. The approach involves assessment of CO exposure using the CFK equation.
The CFK equation has not, however, been evaluated in environments with rapidly changing CO concentrations, such as in the air within crew compartments of armored vehicles. Therefore the committee recommended that the Army assess the validity of the CFK model in that context.
Regarding HCN, the committee considered available evidence of HCN toxicity and the appropriateness of measurements of blood or air concentrations of HCN in the HHA. The committee identified several reasons why measurement of air concentrations is a better benchmark to use (for example, the lack of reported rapid or simple methods for the determination of HCN in biologic fluids). Also, there seemed to be no compelling reason why blood measurements of HCN would be a better predictor of adverse effects than measurement of ambient air concentrations. The added difficulties associated with the measurement and interpretation of blood HCN concentrations indicated that this measurement should not be selected as a routine monitoring method. Therefore, the committee concluded that the Army’s use of air concentrations of HCN, rather than blood HCN concentrations, in the HHA is reasonable. Also, the committee recommended that the Army conduct further neurologic studies on sensory and motor performance at low concentrations of HCN and CO. In addition, the committee recommended that the Army consider concurrent exposures to other chemicals that may have additional effects on the armored-vehicle crew.
The exposure information provided to the committee by CHPPM focused mainly on CO. As noted in the committee’s initial report, the Army reported that exposures to HCN appear to be low most of the time, indicating that HCN may not contribute substantially to the HQ calculation for HCN and CO. As a result, the committee focused much of its consideration on CO with the understanding that actual exposures involve a multi-chemical mixture.
In the committee’s final report, Chapter 2 discusses whether there is a role for the use of portable multi-agent monitors to assess the armored-vehicle environment during varied operations.
Chapter 3 discusses the types of experiments needed to answer whether the CFK equation is valid (1) for assessing COHb levels at low and or spiking levels of CO or (2) under conditions of rapid changes in ventilation. Appendix B reviews past assessments of using the CFK equation for estimating various exposure concentrations, durations, and conditions. Appendix C provides details on methods for additional experiments recommended by the committee to assess the CFK equation.
Chapter 4 discusses whether there is dose-related performance degradation resulting from exposure to CO. Chapter 5 discusses whether there is dose-related performance degradation resulting from combined exposures. Chapter 6 discusses the potential for other deleterious end points of these exposures. Each of the chapters also recommends studies that may provide useful information for developing exposure limit guidelines for combined exposures.
Chapter 7 presents considerations for the Army as it moves forward in developing exposure guidelines. The chapter also considers environmental factors that might modify responses to CO exposure that are not currently taken into account by the Army, and it discusses whether computational models could be used to assess the effect of multiple exposures.