Air pollution is the introduction of chemicals, particulate matter, or microscopic organisms into the atmosphere; in particular, when concentrations of those substances cause adverse metabolic change to humans or other species. The most common and widespread air pollutants include carbon monoxide, sulfur dioxide, nitrogen oxides and particulate matter.
Presently, the greatest occurrences of air pollution are in China, India, Indonesia, South Africa, Brazil, Mexico and Argentina. Each year air pollution is the cause of millions of human deaths, and even larger numbers of respiratory, circulatory, and cancer-related disease occurrences. Also, indoor air pollution is a significant source of human death and disease—mortality and morbidity—through indoor burning of wood and charcoal (especially in developing countries), tobacco smoking, radon trapping and a host of chemical substances found in paints, printing supplies and cleaning products.
An increase of natural background concentrations to concentrations of a few micrograms per cubic meter of such common pollutants as sulfur dioxide, nitrogen oxides and ammonia may produce, for instance, increased growth of forests; however, higher levels of these chemicals produce such adverse effects on forests as decreased growth, greater susceptibility to diseases and pests, and ultimately to forest die-back.