A first distinction should be made between primary and secondary pollutants (World Heath Organisation (WHO), 1987). Primary ones are those coming directly from the source of emission. Secondary pollutants are produced as a consequence of transformation and chemical and physical reactions suffered by primary pollutants within the atmosphere.

Suspended particulates (PM).

The term ‘suspended particulates’ refers to finely-divided, non-specific particulates, either liquid or solid, which are small enough to be suspended for hours or days, and carried through considerable distances. They represent a complex mixture of organic and non-organic substances. There are different names to describe these particles depending on the technique used for their determination and their size. Among these names we find: total suspended particulates (TPS), black smoke, breathable particulates, thoracic. In recent years, there is a preference in naming them after more objective features such as their diameter; particles with a diameter inferior to 10 µm (PM10), with diameter inferior to 2,5 m m (PM2,5) (also called fine particulates) or particulates smaller than 0,1m m (ultra fine particulates) .

Sulphur compounds.

The main ones are sulphur dioxide (SO2) produced by combustion in fixed sources (heating, industries) and sulphates, resulting from atmospheric oxidation of SO2. Changes in the type of fuels used in Western Europe have resulted in a considerable reduction of SO2 emissions, although localised high concentrations associated to occasional emissions may still occur.

Nitrogen compounds.

Their main non-natural source of emission is fossil fuels used in transport, heating and energy generation. Most combustion processes produce nitrogen monoxide (NO) which, through oxidation processes result in nitrogen dioxide (NO2). Sometimes data are referred to in terms of NOx, indicating a mixture of nitrogen oxides.

Carbon Oxides.