The Army reports that exposures to HCN appear to be low most of the time such that HCN may not contribute substantially to the HQ calculation for HCN and CO. By implication, if this is true, then HCN exposures may not warrant assessment. In practice if a chemical is 5% or less of occupational exposure limit as a maximum exposure, the chemical may be considered as minor contributor to the toxicity of a mixture of chemicals. In the current assessment, it is not clear that HCN exposure is low enough to warrant the elimination of monitoring activities, and as long as HCN is monitored, it should be included in the HQ calculation.
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5 Pharmacokinetics and Mathematical Modeling for Assessing Toxicity of Mixtures of Chemicals ."
Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations: Initial Report . Washington, DC: The National Academies Press,
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