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Read more ›The U.S. Army’s Health Hazard Assessment (HHA) Program is a Medical Department initiative that supports the Army acquisition process by evaluating potential health hazards during the design and development of materiel systems. Weapons emissions evaluated by the program include carbon monoxide (CO), hydrogen cyanide (HCN), oxides of nitrogen, sulfur dioxide, ammonia, and carbon dioxide. Typically, these chemicals are evaluated on an individual basis against their respective medical criteria that may include military-specific standards. However, additive or synergistic toxic effects among the chemicals must also be considered. Therefore, the Army is considering the simultaneous exposures of crew members in enclosed vehicles to CO and HCN generated from firing of conventional munitions from a 30-mm cannon.
Both CO and HCN are well known toxicants with established guidelines for safe levels of exposure. Adherence to these guidelines for either of these toxicants alone leads to engineering designs, administrative controls, and use of personal protective devices to ensure an acceptable working environment. However, safe levels of exposure to each of the toxicants may need to be lower if the combined effects of exposure are additive or more than additive. Hypothetically, the design requirements could be based upon the toxicologic mechanisms of CO and HCN being independent, additive, or synergistic. The three different scenarios would lead to differences in the resulting designs for ventilation systems, etc.
The potential for combined exposures results from firing of guns in enclosed (but ventilated) spaces in a military environment such as armored tanks. Because of concerns for the health effects of the personnel simultaneously exposed to HCN and CO, the U.S. Army’s Center for Health Promotion and Preventive Medicine prepared a report titled Assessment of Combined Health Effects of Hydrogen Cyanide and Carbon Monoxide at Low Levels for Military Occupational Exposures. That report provides guidance to assess combined exposures in HHAs of military systems.
The weight of available evidence indicates that the toxic effects of inhaled CO and HCN at lethal and incapacitating levels are additive. Whether similar additive effects hold true at lower concentrations and longer time periods that military personnel may experience, while also in the presence of other combustion gases, is not known. No relevant chronic or low-level exposure studies were found in the literature. In 1981, a military standard established the Army’s COHb limits of 5% for aviation crew members to protect against visual effects and 10% for all other military personnel. The exposure criterion for HCN is the current American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLC) ceiling of 4.7 ppm on the basis of anoxia, central-nervous-system, irritation, lung, and thyroid effects.
In addition to singular or individual evaluations of CO and HCN, the following hazard quotient (HQ) approach using singular benchmarks was employed in the Army’s HHA report.
