Smelly Facts: Wrongfully Accused - Does Coumarin Deserve its Bad Rep?

Are our favorite fragrances getting reformulated for no good reason? Ingredients like coumarin and oakmoss are increasingly getting a lot of bad publicity and gaining in notoriety. But are the claims against them substantiated? Some researchers tend to think that it might not all be as black and white as so far presented. Perhaps judgment has been passed too quickly - at least in the case of coumarin.

The main problem with coumarin is that it is suspected to cause skin allergies, namely contact dermatitis. The most commonly accepted way to test for contact dermatitis, is by doing a patch test. But can we actually accept a positive patch test as clear indication of contact allergy to the tested chemical itself? According to some researchers, some confounding variables might be at play. Specifically, as Vocanson M, Goujon C, Chabeau G, et al, state in a 2006 article, "contaminants and derivatives rather than the suspected chemical itself could be responsible for the allergic skin reactions”. They chose coumarin for their experiments, since it had already produced conflicting results in previous research exploring its allergenic potential (for example Frosch et al. observed positive patch tests to coumarin in less than 0.3% of the 1,855 Allergic Contact Dermatitis patients tested). After testing both mice and humans with three different coumarin preparations, Vocanson et al found that “Pure coumarin did not exhibit irritant or sensitizing properties in the local lymph node assay. In contrast, two other commercially available coumarins and three contaminants that were detected in these coumarin preparations were identified as weak and moderate sensitizers, respectively. In humans, pure coumarin was extremely well tolerated since only 1 out of 512 patients exhibited a positive patch test to the chemical.” The researchers concluded “that the coumarin chemical is extremely well tolerated. In contrast, derivatives contaminating some coumarin preparations are responsible for both the irritant and sensitizing properties previously attributed to coumarin.” and further emphasized that “purity of chemicals is mandatory for the assessment of their allergenicity.”

Reference: “The skin allergenic properties of chemicals may depend on contaminants - Evidence from studies on coumarin”, source: International archives of allergy and immunology [1018-2438] Vocanson yr:2006 vol:140 iss:3 pg:231 -238

Image: Chemical Structure of Coumarin, commons.wikimedia.org