Air pollution is caused by gases and particles emitted into the atmosphere by various human activities, such as the inefficient burning of fuel, agriculture and farming.

Dangerously high levels of outdoor air pollution became a problem during the Industrial Revolution, with heavy use of coal causing many serious urban air pollution episodes.

The London smog of 1952 is an extreme case. More than 12,000 people died that year as pollution from residential coal burning, coal for power generation, dirty fuel for transport and industrial pollution interacted with weather phenomena to hang over London. Following public outcry, Britain passed the Clean Air Act (1956).

Air pollution has a significant effect on places near its source, but because it can travel great distances in the atmosphere, air pollution produced in one place can also affect places far away. For example, pollutants that form fine particulate matter (PM2.5) and ozone (O3) can travel hundreds or thousands of kilometers, affecting entire regions and continents.

Although long-distance air pollutants are one of the causes of local air pollution, nearby pollution sources are still a very important determinant of local air quality. Concentration levels of pollutants like nitrogen dioxide (NO2) and sulfur dioxide (SO2) are highest near their sources (transportation, energy production and industry). In a city, pollutant concentrations may be extremely high in areas closest to large sources, while other areas of the same city may be much cleaner.