Biomass fuels (wood, agricultural waste, and dung) are used by about half the world's population as a major, often the only, source of domestic energy for cooking and heating. The smoke emissions from these fuels are an important source of indoor air pollution, especially in rural communities in developing countries. These emissions contain important pollutants that adversely affect health-such as suspended particulate matter and polycyclic organic matter which includes a number of known carcinogens, such as benzo[a]pyrene, as well as gaseous pollutants like carbon monoxide and formaldehyde.Exposure to large amounts of smoke may present a health risk that is of a similar order of magnitude to the risk from tobacco smoke. The effects on health arising from exposure to air pollution are reviewed, based on what has been reported in the literature so far. Further and more detailed information on exposures and on the epidemiological aspects is urgently required.The persons most frequently affected are women who do the cooking for households in rural villages; they suffer from impaired health due to prolonged and repeated contact with these harmful pollutants. When they are pregnant, the developing fetus may also be exposed and this leads to the risk of excess deaths. In the developing countries, exposure to biomass fuel emissions is probably one of the most important occupational health hazards for women. A conservatively estimated 300-400 million people worldwide, mostly in the rural areas of developing countries, are affected by these problems.