Mexican beer dermatitis

From the New York Daily News in 2010:

A substance in lime juice, if left on the skin in the sun, can cause the skin to become discolored, as if by poison ivy or a jellyfish sting -- and the marks can last for months, reports Scott Flugman in the Archives of Dermatology.

Mexican beers, particularly Corona, are typically served with a lime slice wedged in the top of the bottle. The drinker shoves the lime into the bottle and holds his or her thumb over the bottle's mouth while turning the bottle over to mix in the juice.

But if the drinker is not careful, the beer's carbonation can spray lime juice and beer all over his or her skin -- "especially in a patient who is shirtless by a beach or pool," wrote Flugman, a dermatologist in New York...

The resulting reaction is due to a substance called psoralen, used to make the skin more sensitive to a wavelength of ultraviolet light, UV-A, used to treat certain skin conditions. Lemons contain psoralens too, but not as strong.