From a hygienic view-point optimum indoor air quality can be characterized as the complete absence of pollutants. The most important sources of such pollutants are reviewed, amongst them those entering a room from outside, those generated by man and his activities and those emanating from different materials such as building materials, furnishing, cleansing and polishing agents. In view of achieving acceptable indoor conditions all emissions have to be kept as low as possible. Carbon dioxide and body odors as well as pollutants, the concentrations of which cannot be reduced to non-relevant levels by product control, should be eliminated by ventilation measures. However, such measures should not exceed a given limit, since in order to minimize energy consumption, air exchange rates have to be kept at a minimum. As a result of the investigations, it is concluded that the minimum ventilation rate should be derived from the acceptable CO/sub 2/ concentration. The concentrations of all other pollutants (e.g., formaldehyde) should be limited by product control or source removal. With 75 refs., 10 tabs., 19 figs.