Fact file

Ozone is a very reactive and unstable compound, composed of three oxygen atoms (the oxygen molecule only has two) and with a characteristic odour. It is naturally present in the atmosphere.

In the upper atmosphere, the stratosphere, it is formed from the photodissociation of a molecule of oxygen (O2) that thus releases an atom of oxygen (O), which combines with another molecule of dioxygen (O2) to yield an ozone molecule (O3). The ozone thus formed constitutes the ozone layer, which protects terrestrial life by filtering the UV rays of the sun.

In the troposphere (in the air we breathe), the mechanism of formation is different. The oxygen atom needed to form ozone this time comes from the photodissociation of a nitrogen dioxide (NO2) molecule. Ozone at low altitude is considered a pollutant because, through its oxidising power, it has a negative impact on health, vegetation and materials.

This pollutant is used for the calculation of the Belgian air quality index, BelAQI


Origin of pollution

Ground-level ozone is a secondary pollutant i.e. it is not emitted directly but it comes from the transformation of other pollutants, called precursors.

It is useful to know that there is competition between the formation of ozone, which is a slow phenomenon (several hours) requiring sunlight, and its destruction, which is rapid (a few minutes). In summer and during the day, it is the formation which predominates, while at night, for lack of solar rays, it is the destruction which prevails.

This balance of formation/destruction is influenced by the presence of:

  • nitric oxide (NO), which promotes the ozone destruction reaction and therefore, in urban areas, ozone concentrations are lower than in rural areas,

  • volatile organic compounds (VOCs), which disturb this balance through a complex series of reactions, the overall effect of which is an increase in the production of ozone.

There is no linear relationship between ozone concentrations and precursor emissions, which makes the fight against ozone pollution very difficult. The only way to sustainably reduce ozone concentrations is through structural measures to reduce emissions of precursors.


Health effects

Ozone is a strong oxidiser that can react with virtually all biological substances. It thus affects human health. It alters cellular functions in the mucous membranes (in particular, the eyes and lungs) leading to decreased lung function, an inflammatory reaction of the respiratory tract, and eye irritation. Certain categories of the population are particularly sensitive, such as children, the elderly, or individuals already suffering from asthma or other chronic respiratory pathologies. During pollution peaks, there may be an increase in hospital admissions for pulmonary problems, and even an increase in mortality.


Environmental impact

Ozone disrupts major physiological processes in plants, such as photosynthesis and respiration. It thus induces a reduction in their growth with, as a consequence, losses of agricultural or forestry yield. It also plays an unfavourable role in the environment by exacerbating the effects of acidifying pollutants. Finally, ozone is a greenhouse gas whose contribution is estimated at 10-20% towards the additional greenhouse effect.

By its oxidising power, ozone can also accelerate the degradation of materials, such as plastics, paints, rubber, nylon, etc.


The situation in Wallonia

In Wallonia, the regulatory values of the European Directive are respected. On the other hand, the long-term objectives of the Directive are still far from being achieved, as are the recommendations of the World Health Organization.

Measurement methods

Ozone is measured continuously and in real time using monitors using the principle of UV absorption.


Find out more: