Ambient air pollution refers to the contamination of the atmospheric environment as measured by concentrations of air pollutants. In cities, the main sources of air pollution in cities are (a) mobile sources (i.e. transport, such as motor vehicles and marine vessels); (b) stationary sources (power plants and industries); and (c) area sources (e.g. waste burning and dust). Topographical and meteorological conditions are also important factors that affect air quality. The air quality in cities is more than a local issue. For example, a large share of pollution in Manila and Hong Kong is attributed to sources outside the city boundaries. In cities like Ulaanbaatar and Kathmandu, indoor cooking and heating cause significant indoor and urban air pollution.

Criteria pollutants

Air pollutants of most concern are known as “criteria pollutants” and include
• Particulate matter (PM) - originating from a large number of sources, most importantly vehicles, power plants, construction sites, cooking stoves and heaters, and industrial processes. PM types are distinguished by their size as total suspended particulates (TSP, ≤ 30 micron), PM10 (≤ 10 micron) and PM2.5 (≤ 2.5 micron). One important component of PM is black carbon.
• Sulfur dioxide (SO2) - released when burning fossil fuels containing high sulfur levels.
• Nitrogen oxides (NOx) - emitted by vehicles, industrial and power plants.
• Carbon monoxide (CO) - emitted by vehicles and resulting from incomplete combustion of fossil fuels by, for example, cooking stoves and industrial furnaces.
• Ozone (O3) - a toxic gas formed by NOx and VOCs, especially on sunny and windless days, and is a leading component of smog.
• Lead (Pb) - emitted by vehicles burning leaded gasoline and by industrial processes, such as lead smelting. Lead is highly toxic and is no longer used in gasoline in Asia, except for Cambodia, Laos, Mongolia and Myanmar.

Volatile organic compounds (VOCs, including hydrocarbons, alcohols, aldehydes, and ethers), heavy metals other than lead, (e.g mercury), and air toxics (e.g. benzene, toluene, xylene, and ethylbenzene) are released by vehicles, power generation and industrial processes. Because these are not criteria pollutants they are currently rarely monitored and regulated in Asia, but they are still important.