On a chemistry channel like this, you’d probably think this is going to be about a chemist who was exposed to some dangerous substance. And you’re right, except common limes sprayed the chemical at hand onto chemist-turned-science-writer Tien Nguyen while she was making margaritas. Lime peels contain a set of photosensitizing chemicals called furanocoumarins, a category that includes bergapten (5-methoxypsoralen, shown). Members of the carrot family, other citrus peels, and a number of other plants also have photosensitizing furanocoumarins.
The phenomenon, known as phytophotodermatitis, and its symptoms are similar to conventional sunburn, but the mechanism is different. Furanocoumarins can penetrate skin cells. When excited by UV-A light (315–400 nm), they bind to RNA and DNA. More UV excitation of that adduct creates cross-links, leading to cell death. In the presence of oxygen, free radicals can form, causing more damage to skin cells.
The former reaction stimulates tyrosinase to make melanin, darkening the skin. Dermatologists sometimes exploit this chemistry, carefully, to treat pale patches of skin from conditions such as vitiligo.
Limes and carrots can fry your skin? Yes. But you need both the furanocoumarins and UV light to be present at the same time. So wash your hands well after cooking—or bartending—and wear sunblock and you’ll be fine, Nguyen says.
Read more about phytophotodermatitis here: (Mil. Med. 2009, DOI: 10.7205/MILMED-D-01-7208).
Credit: Courtesy of Tien Nguyen
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