Outdoor air pollution originates from natural and anthropogenic sources. While natural sources contribute substantially to local air pollution in arid regions more prone to forest fires and dust storms, the contribution from human activities far exceeds natural sources.

  Human activities that are major sources of outdoor air pollution, include: Fuel combustion from motor vehicles (e.g. cars and heavy duty vehicles); Heat and power generation (e.g. oil and coal power plants and boilers); Industrial facilities (e.g. manufacturing factories, mines, and oil refineries); Municipal and agricultural waste sites and waste incineration/burning; Residential cooking, heating, and lighting with polluting fuels.

  Poor urban planning, which leads to sprawl and over-dependence on private vehicle transport, is also a major factor in accelerated pollution emissions.

  Adverse health consequences to air pollution can occur as a result of short- or long-term exposure. The pollutants with the strongest evidence of health effects are particulate matter (PM), ozone (O3), nitrogen dioxide (NO2) and sulphur dioxide (SO2).

  Although most emissions of ambient air pollution are from local or regional sources, under certain atmospheric conditions air pollution can travel long distances across national borders over time scales of 4-6 days, thereby affecting people far away from its original source. For example, windblown dust from desert regions of Africa, Mongolia, Central Asia and China can carry large concentrations of particulate matter, fungal spores and bacteria that impact health and air quality in remote areas. Therefore, global cooperation is needed to address international flows and sources of air pollutants, complementary to local and regional efforts in air pollution management.

  Particulate matter (PM) are inhalable and respirable particles composed of sulphate, nitrates, ammonia, sodium chloride, black carbon, mineral dust and water. Particles with a diameter of less than 10 microns (PM10), including fine particles less than 2.5 microns (PM2.5) pose the greatest risks to health, as they are capable of penetrating peoples’ lungs and entering their bloodstream. Sources of PM include combustion engines (both diesel and petrol), solid-fuel (coal, lignite, heavy oil and biomass) combustion for energy production in households and industry, as well as other industrial activities (building, mining, manufacture of cement, ceramic and bricks, and smelting).

  Ground-level ozone is one of the major components of photochemical smog and a key health risk linked to breathing problems, asthma, reduced lung function and respiratory diseases. It is a secondary pollutant, meaning that it is not directly emitted. Instead, it is produced when carbon monoxide (CO), methane, or other volatile organic compounds (VOCs) are oxidized in the presence of nitrogen oxides (NOx) and sunlight. In addition to their role as ozone precursors, CO, VOCs and NOx are dangerous air pollutants themselves. Major sources of NOx and VOCs include emissions from motor vehicle exhaust, industrial facilities, and chemical solvents. Major sources of methane include waste and the fossil fuel and agricultural industry. Aside from its health impacts, tropospheric ozone is a short-lived climate pollutant and one of the most important greenhouse gases.

  Nitrogen dioxide, mainly emitted by power generation, industrial and traffic sources, is an important constituent of particulate matter and ozone. There is growing evidence that independently, it can increase symptoms of bronchitis and asthma, as well as lead to respiratory infections and reduced lung function and growth. Evidence also suggests that NO2 may be responsible for a large disease burden, with exposure linked to premature mortality and morbidity from cardiovascular and respiratory diseases.

  Sulfur dioxide ( SO2) is primarily produced from the burning of fossil fuels (coal and oil) and the smelting of mineral ores that contain sulphur. Exposure to SO2 affects the respiratory system and the function of the lungs, and causes irritation of the eyes. Inflammation of the respiratory tract from SO2 can aggravate asthma and chronic bronchitis, as well as increases the risk of infection, leading to increased hospital admissions and visits to emergency rooms. SO2 also combines with water in the air to form sulfuric acid - the main component of acid rain.

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