Hot air from Morocco has plunged France into suffocating heat for a week. Météo France has placed 17 departments on orange heatwave vigilance for this Thursday, May 28, mainly on the Atlantic coast. Paris and three departments in the inner suburbs are also affected.
Temperatures, which reached 33°C in the capital, led to an episode of ozone pollution in Île-de-France. The region has therefore triggered restriction measures, including differentiated traffic, until Saturday evening. Speed limits are lowered by 20 km/h and only vehicles with a Crit’Air sticker between 0 and 2 are authorized to travel within the perimeter delimited by the A86, in order to limit this episode of pollution which worsens during periods of high heat.
What is ozone pollution?
We speak of ozone pollution when too high a concentration of ozone is found in the air at low altitude, which is abnormal. Ozone is naturally present in the form of a gas at high altitude, in the stratosphere, and protects us from ultraviolet rays (the famous ozone layer). In the lower atmosphere (between 0 and 10 kilometers), however, it constitutes a so-called “secondary” pollutant, which is formed under the effect of a chemical reaction between different pollutants already present in the air.
Under the effect of strong sunlight and high temperatures, ozone is created following a chemical transformation between volatile organic compounds (VOC), methane (CH4) and carbon monoxide (CO), when they are in the presence of nitrogen oxides (NOx). These different pollutants mainly come from road traffic, emissions from certain industries, agricultural activity, heating or even solvents used in painting (VOCs).
Climate change and the increase in heat waves are increasingly creating conditions conducive to episodes of ozone pollution. At the same time, efforts to reduce the emission of the first pollutants are showing their first effects.
What are the harmful effects of ozone pollution?
The World Health Organization (WHO) sets the threshold for ozone in the air not to exceed at 100 micrograms per cubic meter (µg/m3) on average over a period of 8 hours. European regulations have established this limit at 120 µg/m3 on average over the same period and indicate that this threshold should not be exceeded for more than 25 days per year, to preserve human health. Regarding the current ozone pollution alert in France, exceedances could reach 180 µg/m3.
Ozone is considered a corrosive gas that can cause and aggravate cardiovascular or respiratory problems. Ozone pollution, for example, strengthens asthma and can cause coughs, rhinitis, eye and throat irritations as well as chronic illnesses.
According to the European Environment Agency, 70,000 deaths were attributable to ozone pollution in 2022. “These deaths could have been avoided by respecting the WHO indicative values,” notes the agency’s report released in 2024. During periods of peak pollution, vulnerable people (pregnant women, infants, people suffering from cardiorespiratory disease) are recommended to limit their outings.
Furthermore, ozone pollution affects vegetation and crop yields, reducing plant growth rates. According to the European Environment Agency, a third of European agricultural land and 62% of wooded areas were exposed to pollution levels above the threshold set for plant protection.
During periods of peak ozone pollution, immediate measures can be taken. Île-de-France will only authorize, from this Thursday, August 28 from midday, circulation in the city center to cars with Crit’Air stickers 0, 1 and 2. These restrictive measures should last until Saturday inclusive.
To support this measure, the city of Paris is extending free residential parking until this Thursday. The maximum authorized speed has also been lowered by 20 km/h on motorways, expressways, national and departmental roads. Also, the most polluting activities, such as certain maintenance work, are postponed.
To act in the long term, broader plans to reduce pollutants causing the formation of ozone have been put in place in France and in several European countries. They include the latest regulations on motor vehicle emissions, on the composition of fuels, on firewood and even the ban on burning green waste…
According to the National Institute of the Industrial Environment and Risks (Ineris), these measures have made it possible to reduce the intensity of ozone peaks over the last twenty years in France. The average annual levels in the air nevertheless remain high and even increased between 2000 and 2020. The institute therefore calls for the greatest vigilance to limit this average pollution, which could worsen with climate change.
