Air in homes, workplaces, and schools often has higher levels of pollution than outdoor air. Although governments and international bodies such as the World Health Organization define healthy air in terms of the air quality at a fixed point outdoors, indoor air quality, especially in the home, is a much better indicator of the effects of air pollution on public health. The average person spends approximately 93 percent of his or her time indoors, five percent in transit, and only two percent outdoors.24 Presently, air quality standards are specified for each pollutant in terms of its concentration in outdoor air or its mass in a fixed volume of outdoor air. No measurements of long-term indoor air quality are available for the home or elsewhere.

Traditional indoor air pollution sources include heating and cooking equipment that use fossil fuels and biofuels, environmental tobacco smoke, household cleaning solutions used or stored in the home, radon, lead, and biological pollutants such as bacteria, viruses, mold, dust mites, and animal dander. According to an EPA study, major stationary and mobile sources accounted for only two to 25 percent of personal exposure to volatile organic compounds and pesticides. Smoking, dry-cleaned clothes, and chloroform from heated water in the home, on the other hand, were found to be two to five times larger sources of exposure than outdoor emissions sources.25

Calculations for the United States indicate that one gram of indoor particulate matter emissions can have a greater effect on total exposure of the population than one kilogram (1000 grams) released by a power plant from a relatively high stack.26 The average "nonsmoking" household's in-home concentrations of PM-10 between 1940 and 1990 declined 91 percent, according to EPA estimates of residential fuel combustion emissions divided by the corresponding number of occupied housing units. Further, 99 percent of the improvements occurred before 1970, before the imposition of federal regulations on stationary sources.27