Volatile organic compounds (abbreviated as VOCs) are mainly composed of carbon and hydrogen atoms but can also contain heteroatoms such as oxygen, nitrogen, sulphur or a halogen (most often chlorine). Their vapour pressure is sufficient for them to be found in the air as a gas.
They are grouped into several classes according to their structure and the atoms that they comprise. A distinction is thus made between alkanes, alkenes, aromatic derivatives, halogenated derivatives, etc.
Among these compounds, benzene (C6H6) is the subject of particular attention given its toxicity.
Origins of pollution
VOCs mainly result from combustion phenomena, biological reactions or evaporation of solvents present in paints, inks, glues, cosmetics or stain removers. They can also be emitted when handling petroleum products (refineries, service stations, fuel depots). In Wallonia, the majority of VOC emissions come from agriculture and the use of solvents.
The effects of VOCs on health vary according to the compounds: olfactory discomfort, irritation, respiratory problems, headaches, nausea and, for the most toxic VOCs, mutagenic and carcinogenic effects.
Benzene is haemotoxic, with effects on the immune system and even, in the long term, genotoxic, mutagenic and carcinogenic effects. Other VOCs are also carcinogenic or suspected of being carcinogenic, such as 1,3-butadiene or certain chlorinated derivatives.
VOCs have an indirect impact on health by promoting the production of ozone. Likewise, they can have an impact on particulate pollution. Thus, reactions between ozone and terpenes can produce ultra-fine particles.
VOCs play an important role in the formation process of ground-level ozone and, therefore, have an indirect impact on vegetation and materials.
More stable compounds can contribute to the greenhouse effect and to the depletion of the ozone layer for halogenated compounds.
The situation in Wallonia
The concentrations of benzene, which is currently the only regulated VOC, largely comply with the standard in force at a European level.
The VOCs are sampled on tubes with specific absorption phases (sampling 24 hours but every other day). Back in the laboratory, the VOCs are thermally desorbed and then analysed by gas chromatography coupled with a detector by mass spectrometry. The current analysis programme includes a list of 32 compounds, ranging from chains of 4 to 9 carbon atoms.