In developed and developing countries, indoor air pollution is gaining increasing prominence as a public health problem. Time-activity studies and exposure surveys have shown the dominant contributions of indoor environments to population exposures for many pollutants. Mounting epidemiological evidence documents adverse health effects of indoor pollutants and risk assessments indicate that indoor carcinogens may contribute substantially to the population's burden of lung and other cancers. Unacceptable indoor air quality has also been identified as a common cause of symptoms.