Phytophotodermatitis (PPD) is an inflammatory, skin reaction that occurs as a result of the combined exposure to UVA and phototoxic agents found in certain citrus plants such as limes. PPD also known as “Margarita Burn” or “Lime Burn” gets it name from a common scenario where someone on a sun holiday is making margaritas. Tequila, orange liqueur and fresh lime juice, an easy, three ingredient cocktail. Unknown are the hidden dangers of a lime. A margarita sunburn. Squeezing a lime is enough for a phototoxic chemical from that lime to land and stay on the skin to cause burns in the shape of splashes. While someone is basking in the sun enjoying their margarita, little do they know their skin is burning at a higher rate than normal. 
The phototoxic agents are naturally occurring and the organic compound class responsible is known as furanocoumarins, specifically the more active psoralen isomers. This type of dermatitis is independent of the immune system and the symptoms include itching and skin blisters that are burning or painful with patches of hyperpigmentation. [1, 2, 3]
The phototoxic reactions range from minor hyper pigmentation of the skin to large and painful bullae that usually appears in a few days to a week. As this isn’t an immune response, it can happen in any individual with or without allergies or hypersensitivity. The phototoxicity and the severity depends on the concentration of furanocoumarins on the skin followed by the exposure to UVA radiation. 
The phototoxicity of furanocoumarins also depends on the linearity of the compound such as the linear furanocoumarin, psoralen, is more phototoxic than the angular furanocoumarin, angelicin.
Figure 1. Angular and linear furanocoumarin
The psoralens when induced by UVA, bind and crosslink keratinocyte DNA leading to cell damage, inhibition of DNA synthesis, increased melanin production and epidermal injury. 
Treatment for PPD includes anti-inflammatory medication to help with pain and topical steroids are used in more severe cases.  The advice given to avoid PPD is to wash hands with soap and any exposed areas after contact with limes or citrus fruits and to also apply sunscreen. 
So when life gives you limes, make margaritas just wash your hands after.
1) Smith LG, Kabhrel C. Phytophotodermatitis. Clin Pract Cases Emerg Med. 2017;1(2):146-147. Published 2017 Mar 16. doi:10.5811/cpcem.2017.1.32739
2) Friedman BT, Harper R, Glucksberg A, Strote J. In the Limelight. J Emerg Med. 2016;50(3):504-505. doi:10.1016/j.jemermed.2015.11.014
3) Abugroun A, Gaznabi S, Natarajan A, Daoud H. Lime-induced phytophotodermatitis. Oxf Med Case Reports. 2019;2019(11):470-472. Published 2019 Dec 9. doi:10.1093/omcr/omz113
4) Kung AC, Stephens MB, Darling T. Phytophotodermatitis: bulla formation and hyperpigmentation during spring break. Mil Med. 2009;174(6):657-661. doi:10.7205/milmed-d-01-7208
5) https://abcnews.go.com/Health/limes-blamed-girls-degree-burns/story?id=20164612 (Accessed 7th January 2022)