Although atmospheric pollution can have natural sources, for example volcanic eruptions, the term is usually used to refer to the gaseous by-products of man-made processes such as energy production, waste incineration, transport, deforestation and agriculture. During the last 200 years, mankind has begun to significantly alter the composition of the atmosphere through pollution. Although air is still made up mostly of oxygen and nitrogen, mankind, through its pollution, has increased the levels of many trace gases, and in some cases, released completely new gases to the atmosphere. Some of these trace gases, present in elevated concentrations, can be harmful to both humans and the environment.
Air pollution can result in poor air quality, both in cities and the countryside. Some air pollutants make people sick, causing breathing problems and increasing the likelihood of cancer. Others are harmful to plants, animals, and the ecosystems in which they live. Some air pollutants return to Earth in the form of acid rain, which corrodes statues and buildings, damages crops and forests, and makes lakes and streams unsuitable for fish and other plant and animal life.
Man-made air pollution is also changing the Earth's atmosphere so that it lets in more harmful radiation from the Sun. Although we have now banned products which can harm the Earth's ozone layer, ozone holes over Antarctica and the Arctic still form every year. At the same time, mankind is releasing more greenhouse gases to the atmosphere, preventing heat from escaping back into space and leading to a rise in global average temperatures. Global warming will raise sea levels and change climates all over the world. Some places will become hotter and drier, others wetter. The incidence of severe storms and flooding is likely to increase. Global warming will also affect food supply and increase the spread of tropical disease.