Although a considerable amount of information has been generated about airborne organic chemicals in the outdoor urban environment, surprisingly little is known about indoor air, especially in homes and apartments. For many years it was thought that air pollution was an outdoor problem. People were advised to go indoors during "air pollution episodes," and many significant pollutant sources were known to be outside the home, e.g., smokestacks, vegetation, and automobiles. Some studies on selected major air pollutants showed that their indoor concentrations were limited primarily by the rates at which they leaked in from the outside, and it was suggested that if infiltration were eliminated, the pollutants would not be found indoors. Benson et al concluded that indoor levels of vlfur dioxide and carbon monoxide were controlled by the outdoor concentrations. Many years ago, Bloomfield and Isbell found that indoor carbon monoxide levels were elevated in association with heavily traveled streets and attached garages. Thompson et al found that indoor concentrations of PAN, NOx, CO and total oxidant increased when ventilation was increased, and that in the absence of ventilation with the outdoors, their indoor levels fell drastically.