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Formation of ozone

Ozone (O3) is a very reactive gas that is formed in the atmosphere by various photochemical reactions. Ozone is a secondary pollutant and is therefore not emitted directly into the air. Ground-level ozone is formed through the effect of UV light on air pollutants on hot summer days. The ozone precursors are NOx, VOC and CO. In Belgium, about half of the NOx emissions come from transport. Other major sources of NOx are building heating and industry. VOCs are emitted mainly by transport and use of solvents (paints, cleaning products, etc.) in industry but also in households. Apart from the ozone that is produced on hot days, a global background concentration is present at all times.

NOx (=NO + NO2) emissions have a double effect on ozone. On the one hand, NO breaks down O3, thereby forming NO2, and, on the other hand, NO2 aids the formation of O3. In the atmosphere, these pollutants are always in chemical equilibrium. NO has a short lifetime in the atmosphere, so that ozone is mainly broken down in places where much NO is emitted. During this reaction NO2 is formed, which has a longer lifetime in the atmosphere. This ozone-forming substance can be transported over greater distances, and contribute to the formation of ozone in more distant places. This is why ozone concentrations are generally higher in rural areas than in an urban environment with many sources of NOx.


Because of its strong oxidising power, ozone can cause harmful effects to humans, animals, plants and materials. Exposure to high ozone concentrations can cause acute health problems such as irritation to eyes, nose and throat, irritant cough and oversensitivity of the lungs. When high ozone levels are present, everyone (including healthy individuals!) engaging in outdoor physical activity experiences reduced lung function and is at risk of inflammatory reactions in the airways. The effect of ozone varies greatly from one person to another and also people without airways diseases may be extremely sensitive to ozone. This sensitive group, people with lung problems and people engaging in prolonged outdoor physical activity may be most affected, but people with lung diseases are at the greatest risk due to the reduced lung function. Health complaints can be avoided or reduced by refraining from any sporting or heavy physical activity outdoors between noon and 10 p.m. or by staying indoors. Ozone concentrations indoors are on average 50% lower than outdoors.

The effect of long-term exposure to low ozone concentrations is less well known. One study has, however, demonstrated the causal relationship between chronic exposure to ozone pollution and fatality caused by lung disease (Jerret M. et al, 2009).

Ozone also causes damage to plants. Visible effects are mottling or bleaching of the leaves. Invisible effects are reduced resistance and damage to the cells. As a result, more energy goes towards the restoring plant tissue, at the expense of growth. For crops this leads to reduced yields, for forests to less biomass production and reduction in biodiversity. Chronic exposure to lower ozone concentrations has a greater impact on vegetation than acute exposure to high concentrations. Long-term exposure to ozone also results in erosion of materials such as rubber and plastics.

The impact of ozone is not limited to harmful effects to public health, vegetation or materials. Ozone is, after carbon dioxide and methane, the third major anthropogenic greenhouse gas. Indirectly, ozone contributes even more to the greenhouse effect since damage to vegetation will lead to reduced absorption of CO2.

How to tackle ozone pollution?

The ozone issue is a global issue that requires a global approach. Short-term measures such as speed restrictions during periods of ozone smog have no effect in our region since a drop in NOx emissions will initially lead to a decrease in ozone breakdown, which will actually result in higher ozone concentrations. An effective decrease in ozone concentrations can only be achieved by sustainable measures at European and even global level which drastically reduce VOC, NOx and methane emissions (more stringent emissions standards, cleaner fuels, less traffic, etc.).

EU and WHO standards

standardprotection objectiveaveraging periodvaluemax number of exceedancesDate by which value is to be met
EU information threshold human health 1 hour 180 µg/m³
EU alert threshold human health 1 hour 240 µg/m³
EU target value human health max. daily 8-hour 120 µg/m³ 25 (average over 3 years) 1 january 2010
EU long term objective human health max. daily 8-hour 120 µg/m³ 0
EU target value vegetation May to July (between 8:00 and 20:00 CET) 18000 (µg/m³).hours (average over 5 years) 1 january 2010
EU long term objective vegetation May to July (between 8:00 and 20:00 CET) 6000 (µg/m³).hours
WHO guideline human health max. daily 8-hour 100 µg/m³ 0