The document is an evaluation of the health effects of nitrogen dioxide (NO2) and ozone (O3) intended to serve as a basis for establishing Swedish air quality standards. The specific effects of nitrogen dioxide and ozone are reviewed on the basis of published studies on animals, tissues and cells, controlled studies on humans, and epidemiologic studies. The focus is on the importance of the gases in relation to sensitive groups, outdoor-indoor exposure relationships, the question of short- or long-term limit values, and their combined effects with other pollutants. The minimum adverse effect level for human short-term exposure is assessed to be 900 micrograms/m3 for nitrogen dioxide and 200 micrograms/m3 for ozone; for sensitive persons these values should possibly be even lower. Large safety factors should be added to these values before they are used for air pollution control purposes.