[♪♩INTRO] Picture this: you’re on a beach, relaxing, sipping on a beer.

You squeeze a lime into your drink and get hit by some citrus-y mist while you’re sunbathing. No big deal, right?

Well, actually, because of some chemicals in that lime and UV radiation from the sun, your skin is about to get attacked. The symptoms come a day or two after exposure: an itchy, burning rash with fluid-filled blisters — kind of like a bad sunburn. It’s been called lime disease — that’s “lime” with an “i”.

You might also hear it called Mexican beer dermatitis, margarita dermatitis, or Club Med dermatitis, after the tourism company because sometimes people come back with post-vacation rashes. The medical name for this condition is phytophotodermatitis, which tells us that it’s skin damage related to plants and light. Specifically, high-energy ultraviolet light, like the UV rays in sunlight.

This bad skin reaction is caused by certain furanocoumarins, a group of compounds that some plants use to defend themselves and deter hungry herbivores. They’re in some common plants like celery, parsley, parsnip, and some citrus fruits like lemons and limes. Furanocoumarins aren’t toxic cell-damaging chemicals on their own.

They’re activated when they’re exposed to sunlight after a plant’s cells are broken open. With the energy from the UV radiation in sunlight, furanocoumarins can undergo a chemical reaction and bind to certain bits of DNA inside another creature’s cells, messing with the DNA’s structure. When this DNA damage happens, cells can stop working properly and sometimes get killed off.

This reaction is designed to stop animals and insects from eating the plant, since a toxic substance doesn’t lend itself to a great “mouth feel.” Some animals learn to avoid these toxic plants, but certain creatures have evolved ways to get at their tasty nutrients. Some caterpillar species, for instance, get around this defense by changing their behavior. They roll up their leaf of choice and munch on the inside.

That way, they keep their furanocoumarin-riddled food in the dark, so UV light doesn’t activate these toxic compounds. Other caterpillar and butterfly species make specialized proteins in their bodies that react with the compounds and detoxify them, so they can’t react with DNA and cause damage anymore. As humans, our opposable thumbs and culinary arts have helped us get around a lot of plant defense systems, but furanocoumarins can still affect us if we’re not careful.

Limes, for instance, have a cocktail of three furanocoumarins that are concentrated in the juice and peel, ready to be released and activated whenever the fruit is bitten — or squeezed into your favorite drink on a beach day. Furanocoumarins are also found in bergamot oranges, which are a part of the iconic Earl Grey tea smell, and were a popular ingredient in perfumes. Before perfumiers figured out how to make artificial scents to replace bergamot oil, people who wore bergamot perfume outside found themselves with blistery rashes, just like modern-day Club Med partiers And that’s just because if any of these compounds get on your skin while you’re sunbathing, the UV light will cause that DNA-damaging chemical reaction in your skin cells.

Hello, phytophotodermatitis. The reason this skin condition basically looks like a really bad blistery sunburn is because it’s a similar kind of damage. In a sunburn, certain kinds of UV light also damage your DNA, and your immune system starts killing off malfunctioning cells so they don’t become cancerous.

If you do find yourself with this rash, it’s usually treated with drugs like corticosteroids that mimic a hormone and interact with your immune system to reduce inflammation. Symptoms usually get better after a couple of weeks. All it should take to prevent it, though, is to wash off the offending lime juice or other substance right away.

And if you do find yourself on the beach covered in lime juice and can’t get to a shower — hey, I’m not judging — just maybe stay out of the sun like the citrus-scented vampire you are. But the Internet isn’t a doctor, so, y’know, please actually go see one if you have any mysterious rashes. Thanks for watching this episode of SciShow!

If you’d like to learn about another way furanocoumarins can cause medical mishaps, check out our video where Hank explains why you shouldn’t take medicine with grapefruit juice! [♪♩OUTRO]