These fuels include coal, petroleum products such as gasoline, and natural gas. The use of these fuels has a number of harmful effects on health and the environment. According to the World Health Organization, outdoor air pollution, mainly due to the burning of fossil fuels, particularly coal, kills at least 800,000 people every year and causes health problems for tens of millions of others. Technology is available to reduce such air pollution, but its use is expensive and leads to increased fuel consumption