In addition to evaluating test data, the Army also provides predictions for proposed training and operational scenarios. The predictions are used for adjusting the proposed training and operational scenarios. The predictions are used for adjusting the proposed firing rates and patterns to keep weapons emissions exposure below the desired levels or verifying the need for use of personnel protective equipment. The predictions are based on the worst-case CO exposure levels per round (expressed in ppm-minutes) from the proposed hatch position/ventilation configuration. The build-up and decay of COHb is calculated over the course of the scenario. The HQ is then calculated with the highest estimated COHb value and highest value of the 15-minute running HCN average from the relevant scenario.

In summary, the Army used three criteria to evaluate the data. If one or both of the 10% COHb and 4.7 ppm HCN limits is exceeded, then the scenario fails and the HQ calculation is essentially not applicable. If COHb and HCN are within acceptable limits, then the HQ calculation is performed as the third criterion. The method employed allowed the HQ results to be consistent with the singular results. Although the Army assumes a linear relation between biological effects and COHb and HCN concentrations that may not be true, it was successful in providing an additional degree of protection above the singular benchmarks.

In 2005, the Department of Defense requested that the National Research Council assess the Army’s proposed guidance for assessing the adverse effects resulting from the combined simultaneous exposures to low-levels of CO and HCN. The potential for combined exposures results from routine firing of guns in enclosed but ventilated spaces in the military environment such as armored tanks. In response, the National Research Council convened the Committee on Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations under the oversight of the Committee on Toxicology to assess the Army’s proposed guidance.

Both CO and HCN are well known intoxicants with established guidelines for safe levels of exposure. Adherence to these guidelines for either of these intoxicants alone would lead to engineering designs, administrative controls, and use of personal protective devices. These controls would ensure an acceptable working environment. Safe levels of exposure to each of the intoxicants may be lower if the combined effects of exposure are additive to more than additive. Hypothetically the design requirements could be predicated based upon the toxicological mechanisms of CO and HCN being independent, additive, or synergistic. The three different scenarios would lead to variation in the resulting designs for ventilation systems, etc.


The committee’s Statement of Task is as follows:

A committee of the National Academies’ Committee on Toxicology will assess potential toxic effects from combined exposures to low-levels of CO and HCN and evaluate the Army’s proposed guidance on assessing combined exposures in Health Hazard Assessments (HHAs) of military systems. The committee will specifically determine the following:

  • Does the hazard presented from combined exposure to HCN and CO at low levels warrant their combined assessment or is the individual assessment of each chemical sufficiently protective of health?

  • If the combined exposure assessment of HCN and CO is warranted at low levels, is the HQ approach, discussed in the technical context section, a reasonable method of assessment? Should it be modified or improved (i.e., use of a blood CN benchmark instead of the ACGIH TLV-C)?

  • Is the approach discussed in the technical context section appropriate or an alternative assessment method should be developed and validated through either field or laboratory study?



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