Chemical, biological and physical contamination of indoor air. It may result in adverse health effects. In developing countries, the main source of indoor air pollution is biomass smoke which contains suspended particulate matter (SPM), nitrogen dioxide (NO2), sulphur dioxide (SO2), carbon monoxide (CO), formaldehyde and polycyclic aromatic hydrocarbons (PAHs). In industrialized countries, in addition to NO2, CO, and formaldehyde, radon, asbestos, mercury, human-made mineral fibres, volatile organic compounds, allergens, tobacco smoke, bacteria and viruses are the main contributors to indoor air pollution.