Why does citrus juice cause sunburn?

Charlotte had this burning question for chemist Kate Biberdorf...

Katie - It does. It really does. And it can be horrible. One of my very good girlfriends - she just got married a couple of weeks ago - had a horrible, horrible burn because of this. So she made us margaritas like from scratch, she was squeezing out limes, and then we went out to the pool and we hung out all day, and she ended up getting wicked burns on the palms of her hands that lasted for over a month. And it was horrible. It's a rash that forms, and the rash is called phytophotodermatitis. And essentially what is happening from a chemical perspective is there's a molecule that exists in your lemons, your limes, and your citrus, and it is photosensitive. And so when it receives sunlight or any high energy radiation from the sun - so UV visible light, IR, but usually it's that UV light - when the UV light comes in and hits your hands and hits those molecules, it goes through a chemical reaction and it causes this irritation on your skin, and you can get this horrible rash. So it's called lime disease, but not like Lyme disease, not the bacteria infection. Quite different. So it's like the joking 'lime disease' and it's a horrible rash. So be very careful. So if you're using lemons, limes, eating oranges outside, especially if you live where I live in Texas where we get a lot of heat from that sun, wash your hands and just be careful.

Chris - Is this chemistry similar to the reason that people put lemon juice on their hair to lighten it in the sunshine? Is it capturing energy and sunlight and basically feeding it into things that then make chemical reactions happen? And if it's on your skin, it hurts your hands. If it's on your hair, it makes it bleach.

Katie - Yep. Exactly. It's the exact same type of science. You could think about it in the opposite way for sunscreen. So sunscreen has certain molecules that absorb the sunlight, absorb that UV radiation, and then they break down protecting your skin from the sun. And unfortunately this is the kind of the opposite reaction. So the lemon juice like reacts with the sun and then your skin has a horrible reaction to the molecule that forms.

Chris - So wash your hands, but do enjoy your margarita. I thought you were going to say the error was that she put lemon in your margarita. Because I mean, that's just a cardinal sin, isn't it, doing that?

Katie - It would be a cardinal sin. I'm glad we agree on that. But she did use limes, but it's the same molecule that exists in all these citrus. So you've gotta be really careful with that, but just a little soap and water, 15 seconds, 20 seconds wash your hands. Just like we've been doing all year constantly. So it's no big deal.