Polycyclic Aromatic Hydrocarbons (PAHs) are composed of carbon and hydrogen and include at least two condensed benzene rings. This family includes hundreds of different substances which have two main origins: either they are produced by radical reactions during the incomplete combustion of organic matter (pyrolytic origin) or they are naturally present in crude oil and certain coals (petrogenic origin).
PAHs possess the unique characteristic of being present in the atmosphere in gaseous or particulate form depending on the substance.
Origins of pollution
There are natural sources of PAHs (volcanoes, forest fires) but the emissions are mainly anthropogenic: incomplete combustion (heating, traffic, incineration, power plants), oil refining, coke production, foundries, asphalt production, etc.
In Wallonia, PAHs come mainly from the residential sector and then from energy production.
Due to their high liposolubility, PAHs are found in organs rich in fat. They can be gradually released into the blood. Their lipophilic nature also allows them to easily cross cell membranes.
The toxic potential varies greatly depending on the substance. Some have effects on the immune and reproductive systems. Others are genotoxic and even carcinogenic (skin, lungs). There are few studies on the specific toxicity of a PAH taken in isolation. Most studies focus on the toxicity of a mixture of PAHs. Often, benzo (a) pyrene is used as a carcinogenic risk tracer.
The lifespan of PAHs in the atmosphere varies depending on the substance. Some resist natural degradation processes and persist in the environment for years. They contaminate soils, surface water and groundwater and are found in the food chain, with an amplification of the effects for animals at the top of the food chain.
The situation in Wallonia
European legislation regulates the levels of benzo (a) pyrene. On this basis, no exceedance has been observed in the Walloon Region since 2006.
PAHs are sampled on a foam for the lightest, coupled with a filter for the heaviest, over a period of 14 days. Back in the laboratory, the PAHs are extracted, concentrated and then analysed by gas chromatography with detection by mass spectrometry (GC-MS). The current analysis program includes 17 PAHs, 7 of which are required by European legislation.