Indoor concentrations of pollutants often exceed outdoor concentrations, presenting a very real and dangerous health threat. Adverse health impacts of indoor concentrations of nitrogen dioxide from gas-fired stoves, formaldehyde from home insulation, benzo(a)pyrene from fireplaces, and radioactive radon gas that can be emitted from concrete, brick, stone, and even the soil under building foundations have been measured. Effects of energy conservation measures that reduce ventilation on indoor air quality are discussed. Legal and political barriers that interfere with EPA's authority to control indoor pollutant levels are examined. (3 drawings, 3 graphs)