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Air Pollutants

Many different substances and compounds are considered air pollutants. They are generally of anthropogenic origin, and result, in their majority, from combustion processes of fossil fuels. There are also natural pollutants such as gases and dust from volcanos, forest fires, or dust entrained by storms. While these may be of considerable importance on a global scale, it is the anthropogenic pollutants we are most concerned with in the urban environmental context.

From the perspective of spatial analysis, we can discriminate between:

  • fixed or mobile sources;

  • point sources,

  • line sources,

  • diffuse or area sources.

A few categories of pollutants have special significance:

CO2 and CFCs

These are substances with a global effect by contributing to the greenhouse effect and/or stratospheric ozone depletion.

Switzerland is active in promoting a reduction of GHGs (greenhouse gases). The cantons should be associated to this global goal.

CO2 results from any combustion process, but is also part of the natural photosynthetic and respiratory cycles. All vegetation, and forest in particular, fix and store CO2 as part of the photosynthesis and production of organic material, and the role of the oceans in this system may be a determining one.

When emanating from combustion processes, CO2 sources may be well defined point sources such as the stacks of power plants, or more diffuse, such as all the chimneys in a city, but also forest fires or the burning of organic residues.

Sulphur dioxide (SO2)

As one of the main atmospheric acidifiers, sulphur dioxide is perhaps the worst culprit in the gradual destruction of buildings and other objects of historical and cultural heritage exposed to ambient air.

However, in most industrialised countries considerable progress has been made in the reduction of sulphur emission through the use of low-sulfur fuels, and desulfurisation technologies cleaning effluent gas from industrial stacks.

Nitrogen dioxide (NO2)

Nitrogen dioxide is one of two nitrogen oxides emitted by car traffic and the combustion of various fuels such as natural gas, coal, and oil. It causes respiratory illnesses and damage to lung tissue and contributes to acid rain and smog. It also corrodes stone buildings, statues, and monuments including ancient sites of cultural heritage.


Particulates are tiny particles so small that they remain suspended in the air for long periods. Particulates are released by the burning of wood, diesel and other fuels, by industrial plants, and by agriculture: ploughing, which raises dust, and the burning-off of fields, which produces smoke. In cities, road traffic - especially vehicles with diesel engines such as trucks and buses - is one of the main sources. Particulates are also formed by the chemical reaction of gases like SO2 and NO2 with moisture in the air. These are called secondary particulates and are as least as important as primary particulates.

Some classes of particulates are especially detrimental. Particles less then 10 millionth of a meter in diameter are called PM-10s. They have been linked to severe and sometimes fatal respiratory and cardiac ailments, higher rates of infant mortality, and cancer.

Fossil fuel combustion also gives off soot, sometimes known as black smoke. Soot consists of extremely fine carbon-containing particles that, in addition to being a health hazard, are responsible for the blackening of buildings and other outdoor structures in many cities.


Various metals can have adverse effects on health. Cadmium, mercury, arsenic, nickel and lead are emitted from industrial processes, energy production and transport. Mercury's most toxic derivative, methylmercury, enters the food chain and is toxic to the nervous system. Cadmium, arsenic, nickel and lead are considered carcinogenic. Lead also causes digestive problems and damage to the nervous systems, especially in children.


Benzene is emitted mainly by motor vehicles and the chemical industry. Benzene depresses the central nervous system and is a well-known carcinogen.


One well-known form of pollution is summer smog, which is mostly ozone (a form of oxygen with three atoms bound together in a molecule instead of two). Ozone is not emitted as such: it is a secondary pollutant that is formed from Nitrogen oxides and organic compounds through photochemical reactions.

The stratospheric ozone layer which exists at an altitude about 20 km above the earth's surface, is a vital protection against ultraviolet radiation. Yet the gas is severely detrimental to human health when inhaled and should not exist in any significant quantities close to the earth's surface.

Other pollutants, mostly nitrogen oxides and organic vapors, undergo chemical reactions in the air that produce ozone. These reactions increase in warm temperatures and strong sunlight, which is why many cities experience ozone problems in the summer.

Ozone causes breathing problems, reduced lung function, asthma, eye irritation, nasal congestion, and reduced resistance to colds and other infections. Ozone can be especially dangerous for the elderly and the young. It can also damage plants and trees and cause deterioration of rubber and fabrics.


A typical example is dioxine, one of the toxic substances that can be found in the flue gas from waste incinerators if they are not operated correctly, i.e., at too low combustion temperatures.

Many other substances are polluting the atmosphere, but usually in very small amounts that may, at best, be of local significance.

The latest European Directive (Framework Directive on Air Quality Assessment and Management (96/62/EC) lists 13 substances of concern:

  1. Sulphur dioxide (SO2)

  2. Nitrogen dioxide (NO2)

  3. Fine particulate matter such as soot (including mw 10)

  4. Suspended particulate matter

  5. Lead

  6. Ozone (O3)

  7. Benzene

  8. Carbon monoxide (CO)

  9. Poly-aromatic hydrocarbons (PAH)

  10. Cadmium

  11. Arsenic

  12. Nickel

  13. Mercury

See, for example: European Commission, The ambient air quality framework directive - Clean air for Europe's cities. Luxembourg: Office for Official Publications of the

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