What we mean by air is the gas mixture that surrounds the earth. The composition of the chemical air constituents, which are stable under normal conditions, is relatively constant up to an altitude of about 100 km (homosphere) due to the existing convection.

The most common gases in the air are nitrogen with a concentration by volume of about 78 % and oxygen with about 21 %. These two gases make up 99 % of all atmospheric gases.

Apart from the natural constituents of the air, including a number of noble gases and trace gases, the atmosphere contains a big variety of substances which are man-made and have to be considered as potential pollutants. A city's atmosphere may contain up to 1,000 different trace substances.

Human activities have lead to increasing air pollution during the last years. These man-made emissions significantly exceed natural emissions of various substances.

Persistent trace substances (greenhouse gases) can cause considerable climatic changes around the world. As man-made substances do not occur homogeneously in the atmosphere, air pollution in agglomeration areas is worse than in other areas and modifies our breathing air in a way that may lead to negative implications for our health.