Stratospheric ozone protects life on Earth whilst tropospheric ozone, more prevalent in summer because of high temperatures and pollution, is a harmful gas. What causes ozone? What are its components? How can we measure and monitor the developing situation?

The European Union has enacted several directives to prevent Member States exceeding certain pollution levels. See 'International and European actions' (HTML)

In Belgium both the Regions and the Federal state are concerned. The DG Environment of the FPS Health, Food chain safety and Environment in particular is involved in improving the environmental performance of heating equipment, limiting the organic component content of certain products (paints, varnishes) etc. See 'What does Belgium do?' (HTML)

Stratospheric ozone, protecting life on Earth

Ozone is an organic compound incorporating 3 pale blue oxygen atoms (O3), which give the sky its colour. The human sense of smell can detect ozone. It is an irritant gas with a highly characteristic and slightly pungent odour.

It is naturally present in large quantities in the upper layers of the atmosphere, between 25 and 40 kilometres in altitude, which is why it is known as the "stratospheric ozone layer". At this height, the ozone layer acts as a screen that filters out a large proportion of the sun's harmful ultraviolet rays. In particular, ultraviolet light is responsible for some skin cancers. If the ozone layer did not exist, the quantity of harmful ultraviolet would be so high that it would make life on Earth impossible.

This protective layer is threatened by pollution and especially by CFC gases (chlorofluorocarbons), which rise into the upper atmosphere and destroy the ozone there (by catalysis), causing the hole in the ozone layer. The gases which were used in the past are no longer permitted under the Montreal Protocol (WEB).

Tropospheric ozone, a harmful gas

Ozone is also present in the lower layers of the atmosphere. It increases in strength on sunny summer days as the result of a complex photochemical process involving volatile organic compounds and nitrogen oxides caused by humans. See 'What causes tropospheric ozone?' (HTML)

Its effect on human health varies depending on its concentration in the air. If inhaled in strong concentrations, it irritates the respiratory tract and the eyes, may cause temporary changes in lung function and aggravates the symptoms of cardiorespiratory patients. Furthermore, the effects are more noticeable in children and the elderly. We still have little information about the effects of long-term exposure to ozone. See 'What can I do?' (HTML)

Its harmful effects on the environment are already evident with average and even weak ozone concentrations slowing plant growth. Lower agricultural produce and forestry yields, which vary in size from species to species, have also been noted.

Finally, we must not forget that other pollutants are also present during an ozone peak (notably the precursor gases that help to form ozone, as well as fine particles emitted by diesel vehicles, or formed by secondary reactions) and that their harmful effects are likely to reinforce each another.