Q: How can a margarita burn you?

You may have seen some photos of burns and blisters that were caused by margaritas and sun. (I know medical stuff can make some people squeamish, so I’m not going to post any of the photos directly. You can find them at these links: here and here.)

What these photos show is phytophotodermatitis, an inflammatory rea*ction caused by contact with light-sensitizing plant material and exposure to ultraviolet light. (It’s all in the name– “phyto” refers to plant, “photo” refers to light, and “dermatitis” refers to skin inflammation.) Some plants contain furanocoumarins, a group of toxic organic chemical compounds that may be part of a plant’s defense mechanism against insects and other organisms.



Furanocoumarins don’t mix well with ultraviolet light from the sun. The ultraviolet range is composed of light wavelengths from 10 to 400 nm. This range is broken down into subcategories: UVA rays have wavelengths of 315 to 400 nm, UVB rays range from 280 to 315 nm, UVC rays cover 180 to 280 nm, and wavelengths of 10 to 180 nm are referred to as vacuum UV. Only the UVA rays are problematic with furanocoumarins. Vacuum UV rays are blocked by air particles and UVC rays are stopped by the Earth’s ozone layer, so neither tends to even hit human skin. (That is, if the human is on Earth. Astronauts need to be shielded from UVC and vacuum UV rays.) UVB rays reach the outer layer of human skin, but UVA rays penetrate deeper. If furanocoumarins are absorbed by the skin and then exposed to the penetrating UVA rays, a chemical rea*ction can occur and damage cell membranes. This damage can lead to swelling, blistering, injury, and cell death.

Those suffering from phytophotodermatitis may experience blistering, itching, redness, inflammation, pain, tenderness, or burning sensation. The first signs of a rea*ction generally begin within about 24 hours after exposure of furanocoumarins to UVA rays, with full blown symptoms hitting from 48 to 72 hours after exposure. These symptoms generally stuck around for 7 to 14 days. After that, the skin may go through hyperpigmentation and darken relative to the surrounding skin. This could last for weeks or months.

Furanocoumarins can be found in plants like lemons, celery, fig trees, giant hogweed, wild parsnip, parsley, and carrots. Furanocoumarins can also be found in limes, which is the reason why phytophotodermatitis is sometimes called margarita burn or margarita dermatitis or margarita photodermatitis. There are multiple cases of people using limes to make margaritas before sitting out in the sun and ending up with phytophotodermatitis.



The best way to prevent photodermatitis is to avoid contact with plants that contain furanocoumarins and then standing out in the sun. That said, if you must touch something with furanocoumarins and you can’t wear clothing or gloves to protect you skin, you could wash your skin thoroughly after contact to remove the furanocoumarins.

Also, if you’re going to be out in the sun, wear sunscreen. Even if you’re not exposed to furanocoumarins, regular sunburns can be wicked painful.