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Home > The ABC of air > Pollutants
Ozone (O3)

In the stratosphere (between 10 and 60 km in altitude), ozone (O3) is a natural filter that protects life on earth from the harmful (ultraviolet) rays of the sun . The "ozone hole" is a partial disappearance of this filter, linked to the ozone destroying effects of certain pollutants emitted into the troposphere and that move slowly into the stratosphere.

The sources

In the troposphere (between the ground and 10 km in altitude) O3 concentrations should naturally be low. This ozone is a pollutant termed "secondary". It is usually the result of the chemical transformation in the atmosphere of certain pollutants known as "primary" (in particular NOx and VOCs), under the influence of solar radiation. The reaction mechanisms are complex and the highest concentrations of O3 appear in summer, on the outskirts of areas emitting primary pollutants, and the ozone can then be transported long distances.

Health effects 

O3 is an aggressive gas that easily penetrates to the smallest respiratory airways. It causes coughing, impaired lung function as well as irritation of the eyes. Its effects vary depending on the individual.

Effects on the environment

O3 has a negative effect on vegetation (on crop yields, for example) and on some materials (rubber. ..). It contributes to the greenhouse effect and to acid rain.

The reference standards

Information and recommendation threshold for sensitive individuals Alert warning threshold for the general population
180 µg/m3 hourly average 240 µg/m3 hourly average over 3 consecutive hours
300 µg/mhourly average over 3 consecutive hours
360 /µgm3 hourly average

In Upper Normandy, these thresholds are governed by the Prefectural by-law of 9th March 2015

Air quality objective for the protection of human health Air quality objective for the protection of vegetation
120 µg/m3 for the maximum daily rolling 8-hour average 6 000 µg/m3.h  AOT40
concentrations (hour average) accumulated over the growing season (defined as the summer months May to July)
Target value for the protection of human health Target value for the protection of the protection of vegetation
120 µg/m3 for the maximum daily rolling 8-hour average not to be exceeded more than 25 days per year 3-year average 18 000 µg/m3.h en AOT40
 concentrations (hour average) accumulated over the growing season (defined as the summer months May to July) during five years