Heat wave : the ozone, the gas that makes the air unbreathable

The sun, heat, lack of wind… and lo and behold the return of episodes of ozone pollution, a pollutant that is typical of heat waves in which the concentration can be toxic to humans and the environment. It must not be confused with stratospheric ozone, the famous “ozone layer” which protects the Earth from ultraviolet radiation from the sun, and the ozone that we breathe and that becomes toxic when its concentration increases.

This ozone of the lower layers of the atmosphere is a pollutant that is said to be “secondary” because it is not directly rejected by an activity. It is the product of chemical reactions, under the effect of the sun, involving pollutants in the air, such as nitrogen oxides, emitted mainly by road traffic, and volatile organic compounds (hydrocarbons, solvents…), rejected by the industry. It is largely a problem in the summer, in a period of strong sunlight and if there is no wind to disperse it. It can be transported over long distances.

The number and intensity of peaks of pollution have decreased in recent years, in particular through the reduction of emissions of precursors. “We’re talking about a 10% drop in the amplitude of the peaks since the beginning of the years 2000”, tells the AFP Augustin Colette, a specialist in the modeling of the air quality at the national Institute of industrial environment and risks (Ineris).

This decrease is “not yet satisfactory”, he notes, and it may be cancelled by the warming, called to multiply the heat waves. There is at the same time a relative stagnation of the annual average, or even an increase, ” adds the scientist, who talks about a 5% increase in the annual average over the period 2000-2010.

There is no value legal limit for ozone in Europe

Unlike other air pollutants, there is no value legal limit for ozone in Europe, but a target value of 120 micrograms/m3 averaged over 8 hours. For the protection of the health in the long term, this value should not be exceeded more than 25 days per year, averaged over three years. But over the period 2014-2016, Auvergne-Rhône-Alpes, Grand-Is, Occitania and Provence-Alpes-Côte d’azur are beyond, with sometimes between 40 and 50 days, according to the ministry of the ecological Transition.

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There are other hand in France a procedure of information of the populations in excess of 180 micrograms/m3 hourly mean alerting and beyond 240.

A gas that is irritating to the eyes, nose, and throat

Ozone (O3) is an irritant gas that easily penetrates to the respiratory tract the more fine-grained. It can cause irritation of the eyes, nose and throat, coughing, shortness of breath…, especially among those most vulnerable (elderly, asthmatics, young children).

It is necessary to distinguish the peaks of pollution and chronic exposure to high concentration levels which do not exceed necessarily the target values.

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Thus, concentrations of the daily high ozone are associated with increased asthma attacks, hospital admissions for causes, respiratory and cardiovascular, according to Public Health France. According to a study published in 2016 by this agency, “chronic exposure to ozone would be responsible for nearly 500 deaths to cause respiratory every year” in France. Unless of 48,000 annual deaths attributed to fine particles (PM 2.5) is another pollutant of the air.

During the 2003 heat wave, the ozone responsible for 380 deaths

About this time the peaks of pollution, another study from the same agency involving nine major cities found that during the 2003 heat wave, the ozone would have been responsible for 380 deaths (including 228 in Paris) between 3 and 17 August. At high levels, this gas reduces the ability of plants to photosynthesis.

“High levels of O3 cause damage to the cells of plants, impairing reproduction and growth, reducing the production of agricultural crops, forest growth and biodiversity”, underlines the european environment Agency (EEA).

Unlike ozone, particles, matter, microscopic particles in the air, comes directly from combustion, industrial and household and transport fuel (diesel) in particular. Pollution episodes and the particles are held mainly in the winter. The agencies monitoring the air quality also measures nitrogen oxides (NO2) or sulphur (SO2).

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