Present in the atmosphere in gaseous or particulate form, polycyclic aromatic hydrocarbons (PAHs) belong to the family of aromatic hydrocarbons. They are made up of carbon and hydrogen atoms and have a specific structure. PAHs form a family of more than a hundred compounds emitted into the atmosphere by various sources and their lifespan in the environment varies greatly from one compound to another.

They can be classified, according to their number of aromatic rings, into light PAHs (up to three rings) and heavy PAHs (more than three rings). These two categories of PAHs have very different toxicological and physicochemical properties.

What effects on health and the environment?

The toxicity of PAHs varies greatly from one compound to another, each being more or less toxic to health. They have a strong capacity to settle on elementary particles suspended in the air as well as a strong potential for bioconcentration in organisms. Several PAHs are known to be substances CMR, i.e. carcinogenic, mutagenic and toxic for reproduction by the International Agency for Research on Cancer (IARC). Among PAHs, the toxicity of benzo(a)pyrene is well documented: this compound has been classified as carcinogenic for humans by the IARC - group 1 and its capacity to induce lung cancer has been recognized (IARC, 2002). .

The risks associated with these compounds are mainly linked to chronic exposure because PAHs have relatively low environmental concentrations.

PAHs form deposits on plants and also contaminate surface water. They can accumulate in flora and fauna.

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

What are the sources?

Their sources are mainly linked to human activities and particularly to combustion processes. Due to their toxicity as well as the mutagenic and/or carcinogenic properties of some of them, their emissions, production and use are regulated.

In Île-de-France, PAHs are mainly emitted by wood and fuel oil heating, as well as by gasoline and diesel vehicles. They are emitted during the high temperature combustion of coal, gas, oil and wood in an oxygen-poor environment, such as during domestic combustion and that of automobile fuel. Certain industrial processes can also emit PAHs into the atmosphere: refineries, coking plants, foundries, production and use of tar and asphalt, waste incineration plants, production of rubber and tires, etc. In ambient air, some of the PAHs present in the environment come from natural sources such as volcanic eruptions or forest fires.

In indoor air, PAHs are particularly formed during open fireplace fires, but are also emitted by cooking fumes, smoking, etc.

What levels are breathed in Île-de-France? 

A significant drop in benzo(a)pyrene levels has been observed near traffic over the last 20 years in Île-de-France, mainly due to the gradual replacement of old vehicles by recent vehicles, more regulated and less emitting pollutants. the air. On the other hand, no clear trend emerges away from major roads, where concentrations fluctuate from one year to the next. These fluctuations are partly linked to interannual weather variations and a greater or lesser use of heating depending on the temperature, which strongly influences the evolution of PAH emissions associated with the combustion of wood, a fuel that particularly emits PAHs. Wood heating represents almost half of PAH emissions in Île-de-France. Daily maximum levels also fluctuate from year to year. 

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