experience notable discomfort, irritation, or certain asymptomatic, nonsensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure.

AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects, or an impaired ability to escape.

AEGL-3 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience life-threatening health effects or death.

Airborne concentrations below the AEGL-1 represent exposure levels that could produce mild and progressively increasing odor, taste, and sensory irritation, or certain asymptomatic, nonsensory effects. With increasing airborne concentrations above each AEGL, there is a progressive increase in the likelihood of occurrence and the severity of effects described for each corresponding AEGL. Although the AEGLs represent threshold levels for the general public, including sensitive subpopulations, it is recognized that certain individuals, subject to idiosyncratic responses, could experience the effects described at concentrations below the corresponding AEGL.

Carbon monoxide (CO) is a tasteless, nonirritating, odorless, and colorless gaseous substance. The main source of CO production is the combustion of fuels. Exposure at the workplace occurs in blast-furnace operations in the steel industry and when gasoline- or propane-powered forklifts, chain-saws, or other machines are used in confined spaces, such as companies, tunnels and mines. Environmental exposure to CO can occur while traveling in motor vehicles (9-25 ppm and up to 35 ppm); visiting urban locations with heavily traveled roads (up to 50 ppm); or cooking and heating with domestic gas, kerosene, coal, or wood (up to 30 ppm); as well as in fires and by environmental tobacco smoke. Endogenous CO formation during normal metabolism leads to a background carboxyhemoglobin (COHb) concentration of about 0.5-0.8%. Smokers are exposed to considerable CO concentrations leading to a COHb of about 3-8%.

CO binds to hemoglobin, forming COHb, and thereby renders the hemoglobin molecule less able to bind oxygen. Because of this mechanism, the oxygen transport by the blood and the release of bound oxygen in the tissues are decreased. Tissue damage results from local hypoxia. Organs with a high oxygen requirement, such as the heart and the brain, are especially sensitive for this effect.

AEGL-1 values were not recommended because susceptible persons may experience more serious effects (equivalent to AEGL-2) at concentrations that do not yet cause AEGL-1 effects in the general population.