Types of outdoor air pollution
There are many different types of outdoor air pollution. Measuring them all would be too expensive so regulators focus on a smaller group of common air pollutants, sometimes known as the criteria pollutants. The following is a summary of the main types of outdoor air pollution.
Nitrogen dioxide (NO2) is a criteria pollutant and contributes to the formation of photochemical smog, with significant impact on human health. Breathing raised levels of NO2 inflames the lining of the lungs and reduces immunity to lung infections. The result is wheezing, coughing, colds, flu and bronchitis, and more frequent and intense asthma attacks.
The major source of NO2 is from combustion of fossil fuels: coal, oil and gas. Most of the nitrogen dioxide in cities comes from motor vehicle exhaust. Other sources of nitrogen dioxide are petrol and metal refining, electricity generation from coal-fired power stations, other manufacturing industries and food processing.
Sulfur dioxide (SO2) is a criteria pollutant. Short-term exposure to SO2, ranging from 5 minutes to 24 hours, is linked with adverse respiratory effects including bronchoconstriction and increased asthma symptoms. SO2 is also a major precursor to fine particulate soot and acid rain.
Primary sources include fossil fuel combustion at power plants and other industrial facilities. Secondary sources include industrial processes such as extracting metal from ore, and the burning of high sulfur-containing fuels by locomotives, large ships, and off-road equipment.
Short-term exposure to carbon monoxide (CO) can reduce the oxygen-carrying capacity of the blood, leading to shortness of breath, dizziness and even death. This is especially dangerous for people with heart disease who already have a reduced capacity for carrying oxygenated blood to the heart.
The majority of CO emissions in urban environments come from mobile sources e.g. cars, trucks, ships and off-road vehicles. Fossil fuel power stations are another major contributor, as well as fires and biogenic sources in rural areas.
In the upper atmosphere ‘good’ ozone (O3) protects life on Earth from the sun's ultraviolet rays. At ground level ‘bad’ ozone is a criteria pollutant that is a significant health risk, especially for people with asthma. It also damages crops, trees and other vegetation and is the main component of smog.
Ground level ozone is not emitted directly; it is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Emissions from industrial facilities and electric utilities, vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC.
Airborne particulate matter (PM) is categorized into different size fractions. Total Suspended Particulate (TSP) includes all particle sizes and is a good measure of nuisance dust. PM10 (particles ≤ 10 microns) is a criteria pollutant and is a serious health risk because PM10 particles can penetrate the lungs. PM2.5 (particles ≤ 2.5 microns) is also a criteria pollutant which has even greater health impact due to risk of penetration deeper into the respiratory system. Research has linked particulate pollution to lung and heart disease, strokes, cancer, and reproductive harm.
Large particles come from natural sources e.g. soil and organic matter stirred up by wind or human activity. Small particles are by-products of combustion e.g. emissions from vehicles and power stations. Particles from these sources react with other gases in the atmosphere to create particles of various chemical compositions. Gas to particle conversion can also produce fine particulate.