Natural air pollution problems on the Earth are as old as the planet itself. Volcanos, fumaroles, natural fires, and desert dust have all contributed to natural air pollution. Humans first emitted air pollutants when they burned wood and cleared land (increasing wind-blown dust). More recently, the burning of coal, chemicals, oil, gasoline, kerosene, diesel, jet and alcohol fuel, natural gas, biomass, and waste, as well as the release of chemicals into the environment, have contributed to several major air pollution problems on a range of spatial scales. These problems include outdoor urban smog, indoor air pollution, acid deposition, the Antarctic ozone hole, global stratospheric ozone reduction, and global warming.
Urban smog is characterized by the outdoor buildup of gases and particles that are either emitted from vehicles, homes, industrial facilities, power plants, incinerators, or land-clearing and natural fires or formed chemically in the air from emitted pollutants. Smog affects human and animal health, structures, and vegetation. Urban smog occurs over scales of meters to hundreds of kilometers.