Many potent air toxins are in the smoke of burning forest and range biomass. Firefighters are exposed to the smoke of both wildfire and prescribed fire. A comprehensive assessment of these exposures is not yet possible due to insufficient data. Preliminary data on firefighter exposures to carbon monoxide and formaldehyde at four prescribed burns of Western United States natural fuels are presented. At these well-managed prescribed burns, occupational exposure limits for these compounds were not exceeded. In the exposure samples taken, formaldehyde was correlated to carbon monoxide emissions. The firefighters' exposures to these compounds relative to workplace standards are discussed.