Air pollution is the accumulation in the atmosphere of substances that in high enough concentrations, endanger human health and produce harmful effects on living organisms and other materials. Smog has seriously affected more people than any other type of air pollution. It is caused by water condensing on smoke particles, usually from burning coal. As a coal economy has gradually been replaced by a petroleum economy, photochemical smog has become a problem in many cities. It results from the irradiation by sunlight of hydrocarbons and other pollutants in the air. Oxides of sulfur and hydrogen, carried long distances by the atmosphere and then precipitated in solution as acid rain, can cause very serious damage for vegetation, waterways, and buildings. Pollutants also come from other sources such as decomposing garbage in landfills and solid waste disposal sites which emit methane gas, and many household products. Some of the pollutants also come from natural sources. Many emit particulates and volatile organic chemicals(VOC's) into the atmosphere. Ultrafine dust particles, dislodged by soil erosion when water and weather loosen layers of soil, increase airborne particulate levels. .
One very serious effect of the pollution of our atmosphere is the greenhouse effect, which can cause dangerous warming of the Earth. The greenhouse effect can be defined as the warming of the Earth due to atmospheric accumulation of carbon dioxide, which absorbs infrared radiation and slows its escape from the irradiated Earth. Certain gases such as carbon dioxide and methane along with water keep ground temperatures at a global average. The gases keep the environment warm because as incoming solar radiation strikes the surface, the surface gives off heat that the gases trap and keep near ground level. The pollution that humans create in the Earth's atmosphere could cause the surface to warm up to a dangerous degree.