At very high altitude, in the upper atmosphere, the ozone layer protects living organisms by absorbing part of UV rays. But at low altitude, where we live and breathe, it is a pollutant that irritates the eyes and the respiratory system, and is harmful to vegetation. The high altitude ozone layer and the low altitude ozone communicate little.

What effects on health and the environment?

Reducing ozone concentrations below the thresholds recommended by the WHO would make it possible to avoid around 1 premature deaths each year in Île-de-France (in 700). 

At high concentrations, ozone causes breathing problems, triggering asthma attacks, reduced lung function and the onset of respiratory diseases. In the long term, links are observed with respiratory and cardiovascular mortality, particularly for subjects predisposed by chronic diseases (pulmonary, cardiac, diabetes) and with asthma (incidence or severity).

For more information, see: Effects of air pollution on health.

Low-altitude ozone also has a detrimental effect on vegetation, particularly on the process of photosynthesis, which leads to a reduction in crop yield. It has a necrotizing action on the leaves. Low-level ozone is also one of the rare air pollutants that is also a greenhouse gas, and therefore worsens climate change.

For more information, see: Effects of air pollution on vegetation et Air pollution and climate change.

How is low-level ozone formed? 

Low altitude ozone is a so-called “secondary” pollutant, meaning that it is not released directly into the atmosphere but comes from the chemical transformation of other pollutants. Low-level ozone (O₃) is formed in the atmosphere by chemical transformation of different compounds: volatile organic compounds (VOC), methane (CH₄) And carbon monoxide (CO); in the presence ofnitrogen oxides (NO_x) and under the effect of significant sunshine and high temperatures.

In Île-de-France, nitrogen oxides are mainly emitted by gasoline and diesel vehicles, and volatile organic compounds come from several sources including mainly solvents and paints, certain industries, road traffic (mainly two-wheelers) and plants. Low-altitude ozone is a pollutant that poses a problem mainly in summer, because to produce a lot of ozone, heat and sufficient sunlight are necessary. Climate change therefore favors the increase in low-altitude ozone concentrations. It is also a pollutant that travels and can cross all of Europe.

To learn more, seeinventory of air pollutant emissions in Île-de-France.

What levels are breathed in Île-de-France?

Low-altitude ozone is a secondary pollutant whose levels are greatly influenced by weather conditions, particularly in spring and summer.

Concerning chronic pollution, the impact indicators of low-altitude ozone on health are generally stable over the last 20 years in Île-de-France, with strong variations from one year to the next depending on the weather situation. There is no regulatory limit value for low-altitude ozone, and All Ile-de-France residents are exposed to low-altitude ozone concentrations higher than WHO air quality recommendations. Low-level ozone is the only regulated pollutant for which annual trends do not show improvement. Summer low-level ozone levels in future climate projections are similar to those experienced during the exceptionally hot and dry summer of 2003 in Europe. Simulations suggest that under future climate conditions, summer low-level ozone could pose a serious threat to human health, agriculture and natural ecosystems in Europe.

Low-altitude ozone levels vary depending on the areas of Île-de-France. Peri-urban and rural areas are generally more affected than the heart of the Paris metropolitan area, due to the complex mechanics of the formation of low-altitude ozone.

The number of low-level ozone pollution episodes varies greatly from year to year, depending on weather conditions. In recent years, from a few days to around ten days of episodes of low altitude ozone pollution can be observed over the course of a year.

For more information, see the latest assessment of air quality in Île-de-France.