Possible mechanisms for the respiratory tract effects of noncarcinogenic indoor-climate pollutants and bases for their risk assessment. Scand J Work Environ Health 1995;21:165-98.This review outlines the effects of pollutants on the lungs. Mechanisms and effects relevant to the assessment of indoor-air risk are especially dealt with. Important mediators have also been considered. Concentration-effect relationships exist for toxic reactions, sensitization reactions, and neurogenic effects. If Harber's Law is used for extrapolations from higher concentrations to the lower indoor-air levels, the indoor-air risk estimate may exceed the real risk. Additivity seems to apply to toxic and neurogenic effects of low doses. Only already sensitized subjects and possible subjects with a profound alpha,-antitrypsin deficiency appear to be extremely sensitive, and a safety factor of 10 seems adequate for the protection of other groups. Thus combining occupational exposure limits (OEL), Harber's Law, and the safety factor suggests that no direct lung effects should be expected from a substance if the exposure level does not exceed 1/40 . OEL.