Agents that alter the chemical structure of DNA at the risk of causing cancer can be either physical or chemical in nature. In Occupational Health and Safety, the question arises of the consequences of co-exposure to two of these agents: polycyclic aromatic hydrocarbons (PAHs) which are products of the combustion of organic matter, and solar ultraviolet radiation (UV). Many industrial activities use PAH-rich products or emit PAHs as by-products. Workers exposed in these activities are therefore exposed through the skin, and this contamination may be accompanied by exposure to the sun after or even during work (road construction, tar spreading, etc.). The actions of PAHs and UV have been studied separately for many years by IRIG researchers. It was therefore natural for them to look into the co-exposure of people to these agents through two projects of the Cancer Plan carried out in collaboration with teams from Grenoble University Hospital.

The researchers were particularly interested in the metabolism of PAHs by working on human skin explants studied in vitro ^{[1, 2]}. The mechanisms involved in the metabolism of PAHs allow cells to convert these chemically stable molecules into compounds that are more easily eliminated. A small fraction of these metabolites can unfortunately react with DNA and ultimately mutations. A first observation is that irradiation after PAH treatment, but not before, decreases the viability of the skin, presumably by inducing oxidative stress ^[3]. As far as metabolism is concerned, the results are irrefutable. Measurements of metabolic gene expression, metabolites production and the level of damage in DNA converge towards the same conclusion: solar UV reduces the ability of skin cells to metabolize PAHs, whether irradiation takes place before or after contact with chemicals ^{[1, 3, 4]}.
These observations provide important insights in toxicology. First of all, they confirm the risk of photosensitization when exposed to sunlight in the presence of PAHs ^{[1, 5]}. The decrease in the metabolism of PAHs by UV light, which has never been described before, can be seen positively since it induces a delay in the accumulation of DNA damage. On the other hand, this phenomenon can have negative consequences as only metabolized PAHs are eliminated from the skin. Exposure to UV light could therefore lengthen the duration of exposure in tissues.
The role of the aromatic hydrocarbon receptor which controls, among other things, the metabolism of xenobiotics (polluting and sometimes toxic chemical molecules within an organism) and the effect of UV rays on transcription, remains to be determined ^[5].