It assumed the effects at low levels were additive. An HQ equal to or greater than 1.0 indicated an overexposure.

The Army used the following criteria to evaluate the data involving combined exposures to CO and HCN: if both or either 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.

In 2005, the Department of Defense requested that the National Research Council evaluate the Army’s proposed guidance for assessing the adverse effects resulting from combined exposures to low-levels of HCN and CO, and recommend exposure limit guidelines for combined exposures to these chemicals. In response, the National Research Council convened the Committee on Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations with oversight from the Committee on Toxicology to address the task assigned to it.

The committee’s Statement of Task is as follows:

An ad hoc committee under the oversight of the standing Committee on Toxicology (COT) will assess potential toxic effects from combined exposures to low-levels of HCN and CO. In its first report (i.e., this report), the committee will evaluate the Army's proposed guidance on assessing combined exposures. The ad hoc committee will specifically determine the following in its initial report:

  1. 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?

  2. If the combined exposure assessment of HCN and CO is warranted at low levels, is the hazard quotient 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)?

In its second report, to be completed next year, the committee will determine the following:

  1. 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?

  2. What improvements are needed in the Army's proposed methodology for assessing these combined exposures? The committee will also provide recommendations that will yield more precise measurements of gases which might be useful in hazard assessment.

  3. What exposure limit guidelines are appropriate for combined exposures to these chemicals?


After receiving a briefing from the Army and evaluating published literature on the adverse effects of CO and HCN, both individually and in combination, in animals and in humans, the committee arrived at the following overall conclusions and recommendations for its initial report.

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