Swedish researchers may have figured out what makes a common sunscreen additive turn into an allergen. 4-tert-Butyl-4'-methoxydibenzoylmethane is used in sunscreens to filter out UV-A light (320–400 nm). It replaced 4-isopropyldibenzoylmethane, which was taken off the market because it caused photocontact allergies. The difference between the two compounds is the substituents in the para positions. Anna Börje and coworkers of the University of Gothenburg investigated the photodegradation products—primarily arylglyoxals and benzils—of four different dibenzoylmethanes to determine how the substitution pattern affects the compounds’ allergenic potential (Chem. Res. Toxicol., DOI: 10.1021/tx900284e). They found the arylglyoxals to be strong skin sensitizers and highly reactive toward arginine. Changing the electronic properties of a para substituent didn’t significantly affect the sensitizing or electrophilic power of the arylglyoxals, the researchers note, so changing the substitution pattern of the parent dibenzoylmethane shouldn’t affect the sensitizing properties either. In vitro assays with benzils show that they are cytotoxic rather than allergenic. Therefore, the allergenic properties of the dibenzoylmethanes are probably caused by the arylglyoxal photodegradation products, the researchers conclude.