Most air pollution is man-made and derives from poor combustion of fossil or biomass fuels (e.g. exhaust fumes from cars, furnaces or wood stoves). Urban outdoor air pollution refers to the air pollution experienced by populations living in and around urban areas (i.e. cities). Indoor air pollution refers to the pollutants found indoors. The main cause of indoor air pollution is inefficient fuel combustion from rudimentary technologies used for cooking, heating and lighting. There are also natural indoor air pollutants, like radon, and chemical pollutants from building materials and cleaning products that also impact health.