Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations: Initial Report

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In response to a request from the U.S. Army, a committee convened by the National Research Council (NRC) conducted the first in a sequence of studies evaluating the combined health effects of low-level exposure to two chemicals Army personnel are likely to be exposed to in firing tank weapons. The Army sought information on whether the two chemicals, hydrogen cyanide and carbon monoxide, result in similar health impacts and should be assessed together when establishing exposure limits. Based on a review of the scientific literature, the report finds that the biochemical health impacts of the chemicals are similar and that the Army's proposed approach to setting exposure limits is appropriate. Because previous research has focused on high exposures, this initial NRC report recommends that futher neurological studies at low concentrations of exposure to the chemicals be conducted.
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Product Details

  • ISBN-13: 9780309113663
  • Publisher: National Academies Press
  • Publication date: 2/1/2008
  • Pages: 42
  • Product dimensions: 8.30 (w) x 10.80 (h) x 0.30 (d)

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Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations

Copyright © 2008 National Academy of Sciences
All right reserved.

ISBN: 978-0-309-11366-3

Chapter One


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 ifthe 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.

COHb%/10% + 15-min avg. HCN (ppm)/4.7 ppm = HQ.

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.

measured data and various running averages.

While the toxicity of combined exposures to HCN and CO is important to understand, the Army should also consider concurrent exposures to other chemicals, e.g., other combustion gases, diesel exhaust, which may have additional effects on the tank crew.


Excerpted from Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations Copyright © 2008 by National Academy of Sciences. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents


1 Introduction....................4
2 Mechanisms of Carbon Monoxide and Hydrogen Cyanide Toxicity....................7
3 A Brief Review of Hydrogen Cyanide and Carbon Monoxide Toxicity....................9
4 Summary of the Effects of Combined Exposure to Carbon Monoxide and Hydrogen Cyanide and Recommendation for Combined-Exposure Risk Assessment....................11
5 Pharmacokinetics and Mathematical Modeling for Assessing Toxicity of Mixtures of Chemicals....................13
6 Appropriateness of Measurement of Blood or Air Levels of Cyanide....................16
7 Conclusions and Recommendations....................18
Appendix Biographic Information on the Committee on Combined Exposures to Hydrogen Cyanide and Carbon Monoxide in Army Operations....................23

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