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.