4.7 parts per million (ppm) to minimize the potential for headache; nausea; nasal, throat, and pulmonary irritation; and enlargement of the thyroid gland, which can result from low-concentration exposure (ACGIH 2001).
In addition to the individual threshold levels, the Army uses the following hazard quotient (HQ) approach, which assumes the effects of CO and HCN at low levels are additive. An HQ equal to or greater than 1.0 indicates an overexposure.
To determine whether the air quality inside armored-vehicle cabins can meet the exposure guidelines under deployed conditions, the Army test fires the vehicles’ weapons and measures the concentration of potentially harmful gases resulting under various operational scenarios. When an armored vehicle is first tested, no personnel are usually in the cabin and weapons are fired remotely. Before, during, and after firing, air is sampled remotely from the crew compartments and concentrations of specific gases are measured. Subsequent testing involves weapons firing by personnel within the vehicle cabin followed by air sampling and analysis (Bazar and Kluchinsky 2008; M. Bazar CHPPM, personal commun., September 23, 2008). To evaluate the stream of test data, the Army calculates COHb levels at the end of each data interval (3 or 5 seconds) using the instantaneous CO concentrations and the COHb concentration from the end of the previous interval. The 15-minute (min) HCN average is based on a running average calculated at the end of each data interval. The 15-min HCN average concentration is used because HCN exposures have been observed to be transient and to clear quickly after a round is fired. CO concentrations exhibit a spike when a round is fired and also quickly decline, but COHb begins to accumulate in the blood of exposed subjects after several rounds.
In addition to evaluating test data, the Army provides predictions for proposed training and operational scenarios. The predictions are used for adjusting the proposed firing rates and patterns to keep weapons-emissions exposure below the specified levels or for verifying the need to use personnel protective equipment. The predictions are based on the worst-case scenario of CO exposure levels per round (expressed in parts per million–minutes) from the proposed hatch-position and ventilation configuration. The buildup and decay of COHb is calculated over the course of the scenario. The HQ is then calculated with the highest estimated COHb value and the highest value of the 15-min running HCN average from the relevant scenario.
In summary, to assess the potential for health effects of CO and HCN combined exposures at low levels, the Army first determines if either or both of the limits of 10% COHb or 4.7-ppm HCN are exceeded. If so, the scenario fails and the HQ calculation is not needed. If COHb and HCN are within acceptable limits, then the HQ calculation is performed; the scenario fails if the HQ equals or exceeds 1.0. The method used allows the HQ results to be consistent with the single results.
In 2005, the Department of Defense (DOD) requested that the National Research Council assess (NRC) the Army’s proposed guidance for assessing the adverse effects resulting from the combined simultaneous exposures to low levels of CO and HCN in weaponized armored vehicles. The NRC was asked to prepare two reports. In response, the NRC 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. The committee’s Statement of Task is presented in Box 1-1.
In its initial report (NRC 2008), the committee evaluated the Army’s proposed guidance on using the HQ approach as part of its HHAs of military systems. The committee concluded in its initial report