Most people spend approximately 90 per cent of their time indoors, more in the case of elderly and sick people. The indoor environment can thus influence health through exposure to emissions from cooking and heating, environmental tobacco smoke, the inflow of polluted outdoor air and various biological exposures such as house dust mites, fungi and bacteria. The effects of the indoor environment in developed countries have been covered in Chapter 14; we address here the problems associated with the indoor environment in the less economically developed countries (LEDCs) where indoor air quality is dominated by emissions from dirty fuels such as unprocessed biomass, coal and dung cake for cooking and heating, placing women and children in particular at risk of both acute and long-term ill-health.