Abatement of indoor air pollution must be included in all efforts to protect public health from environmentally related illness. Recent studies show that indoor air pollutant concentrations are often higher than corresponding outdoor levels. This paper reviews the present state of knowledge of indoor pollutant concentrations, their time dependence, and their relationship with indoor sources, energy-conserving measures, and indoor activity patterns. Pollutants of primary concern are organic compounds, respirable suspended particulates, nitrogen dioxide, and allergens. Knowledge of carbon monoxide, assessed. Finally, typical formaldehyde, radon, and infectious agents are concentrations of carbon monoxide, asbestos, nonasbestos mineral fibers, ozone, and sulfur dioxide in residences and other indoor environments are reviewed. Present knowledge is based on unrelated, anecdotal studies of the indoor air environment. It is concluded that a systematic assessment of the indoor air quality issue is warranted.