Often used in open fires or poorly ventilated stoves, solid fuel burning is a significant source of indoor air pollution. Solid fuel smoke contains thousands of substances, many of which are hazardous to human health. The most well understood of these substances are carbon monoxide (CO); small particulate matter; nitrous oxide; sulfur oxides; a range of volatile organic compounds, including formaldehyde, benzene and 1,3-butadiene; and polycyclic aromatic compounds, such as benzo-a-pyrene, which are thought to have both short and long term health consequences.