Summer is the time for outdoor fun. But to enjoy it safely, people should heed some general warnings. Check for ticks. Head indoors at the first sign of lightning. Slather on sunscreen. And if you put up a lemonade stand, squeeze those lemons indoors. Then wash your hands well — at least if you’ll be out in the sun. The reason: Lemons make chemicals that harm the skin.
In the presence of sunlight, these chemicals can lead to painful burns or rashes. Each year, many people — children and adults alike — learn this the hard way. Their burns sometimes will be severe enough to blister. Ouch!
Robin Gehris is a skin specialist in Pennsylvania at Children’s Hospital of Pittsburgh. In summer, she sees these burns in her young patients “at least once a week.” Most cases have been triggered by limes and lemons, she says.
One reasonable explanation: lemonade stands.
The ancient Egyptians first described this special type of sunburn more than 3,000 years ago in the Ebers Papyrus. It’s one of the oldest and most important medical documents (written, yes, on papyrus). Four California doctors wrote about it in a 2016 review paper on this special class of sunburns.
These burns also have a special name: phytophotodermatitis (FY-toh-der-muh-TY-tis). It simply means some plant-based thing has made the skin super-sensitive to sunlight. The topic hits the news every so often. And it just did again in the United States as biologists reported in mid-June that they had discovered giant hogweeds for the first time in Virginia. Former homeowners had planted them in their yard because they liked the plants' exotic look.
The plants look like Queen Anne’s lace on steroids. The “giant” part of their name makes sense. This relative of the carrot can grow to heights of 4.3 meters (14 feet). And this plant makes the same class of toxic compounds as lemons. That’s why biologists tend to approach hogweeds wearing hazmat suits to avoid the chemicals that can cause burns (or, potentially, blindness — although that has not been reported thus far).
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The chemistry of the plants’ defense
The toxic plant chemicals are psoralens (SOR-uh-lenz). Chemists also refer to them as furocoumarins (FOO-roh-KOO-mah-rinz).
It takes skin between 30 minutes and two hours to absorb these chemicals. Later exposure to the sun’s ultraviolet radiation will activate those chemicals, provoking a double whammy. First, those chemicals can bind to — and then damage — DNA. Affected skin cells will die, leaving behind a burn. Secondly, the psoralens can react with any oxygen present to produce a type of molecular fragment known as free radicals. These, too, kill cells.
The kitchen fridge contains plenty of plant-based foods rich in psoralens. Among them: lemons, limes, parsnips, fennel, celery, parsley, dill and members of the mulberry family.
Eating these foods causes no problems. The toxicity occurs only if juice, sap or the leaves from certain of these plants touch the skin. A dribble of citrus juice could leave a streaky red mark. A hand that had been wet with lime juice could leave its likeness where it might have rested on an arm or a leg.
Indeed, some skin doctors have taken to calling phytophotodermatitis “the other lime disease” (a pun on Lyme disease). It’s been seen after people had squeezed lime into Mexican beer that they were drinking outdoors, in the sun. But lemons are another major risk. Ryan Raam of the University of Southern California, in Los Angeles, was part of a team that described a man who came into their hospital emergency room with a large blistering rash. It appeared on the back of both hands and on one foot.
The doctors diagnosed the source of those burns when the man explained that he had just come back from a Caribbean island trip where he had been “hand juicing several hundred lemons.”
In fact, Gehris says, “Often, the [burn] pattern is one of the things that keys us in” to ask about possible skin exposure to foods that make psoralens.
How bad the burn is will depend on how much juice or sap got onto the skin and how long the sun exposure was. A lot can lead to blistering.
This skin damage also can be mistaken as a sign of violence, Raam’s team notes. Reddened skin on a child, they note, “can masquerade as abuse. Many times, the rash will appear as handprints that mimic abuse.” In fact, they cited several instances where this mistake had occurred.
While there’s no reason to handle hogweed, psoralen-making foods pose no risks — as long as you wash exposed skin before heading out into the sun.
activate (in biology) To turn on, as with a gene or chemical reaction.
biologist The study of living things. The scientists who study them are known as biologists.
Caribbean The name of a sea that runs from the Atlantic Ocean in the East to Mexico and Central American nations in the West, and from the southern coasts of Cuba, the Dominican Republic and Puerto Rico down to the northern coasts of Venezuela and Brazil. The term is also used to refer to the culture of nations that border on or are islands in the sea.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances.
citrus A genus of flowering trees that tend to produce fruits with a juicy edible flesh. There are several main categories: the oranges, mandarins, pummelos, grapefruits, lemons, citrons and limes.
compound (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
defense (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)
DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
emergency room Also known as the ER. It's that part of the hospital where doctors initially attend to the immediate medical needs of accident victims and others who need critical care.
exotic An adjective to describe something that is highly unusual, strange or foreign (such as exotic plants).
free radical A charged molecule (typically highly reactive and short-lived) having one or more unpaired outer electrons. It will attempt to steal electrons to make itself whole again through a process known as oxidation. The body periodically generates free radicals as one way to kill off damaged or unwanted cells (such as germs).
hazmat A shortened form of the term hazardous materials.
papyrus A reed native to regions near the Nile river in Egypt. In ancient times, strips from the interior of the plant stalk were cut and laid side by side and pressed down, first in one direction, then in a second layer atop the first (now oriented perpendicular to strips in the first row). When dried, a natural gum in the material glued the strips into a sheet. Papyrus was used as a form of “paper” onto which some people people kept written records.
phytophotodermatitis A poisoning of skin cells by some plant-derived chemical (typically psoralens, also known as furocoumarins). The condition renders the skin especially sensitive to sunlight, leading to sunburn and rashes (which may blister).
psoralens A family of compounds that can absorb ultraviolet light. They occur naturally in the skin of many fruits and vegetables, where they can ultimately poison insect pests. Some therapeutic drugs have also been based on these compounds.
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
review paper (in science publishing) A paper that reviews the data and findings in a broad body of work by many research teams. This may include 50 to 200 different research studies or more. The authors then synthesize the findings, looking for patterns that may emerge from the data. These patterns may strengthen — or weaken — the conclusions that seem reasonable when considering just a single paper or two.
risk The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
ultraviolet A portion of the light spectrum that is close to violet but invisible to the human eye.
News report: M. Weil. Huge, toxic weed found for first time in Virginia, experts say. The Washington Post. June 19, 2018.
Journal: B. Baker, J. Bedford and S. Kanitkar. Keeping pace with the media; giant hogweed burns—a case series and comprehensive review. Burns. Vol. 43, August 2017, p. 933. doi: 10.1016/j.burns.2016.10.018.
Journal: R. Raam et al. Phytophotodermatitis: The other “lime” disease. Annals of Emergency Medicine. Vol. 67, April 2016, p. 554. doi: 10.1016/j.annemergmed.2016.02.023.
Journal: L.A. Marcos and R. Kahler. Phytophotodermatitis. International Journal of Infectious Diseases, Vol. 38, September 2015, Pages 7. doi: 10.1016/j.ijid.2015.07.004.
Journal: J. Moreau, J. English and R. Gehris. Phytophotodermatitis. Dermatology for Pediatric and Adolescent Gynecology. Vol. 27, April 2014, p. 93. doi: 10.1016/j.jpag.2013.11.001.
News report: Patterson Clark. Giant hogweed: The goliath that can leave you blistered and maybe even blind. The Washington Post. June 11, 2013.
Journal: K. Carlsen and K. Weismann. Phytophotodermatitis in 19 children admitted to hospital and their differential diagnoses: Child abuse and herpes simplex virus infection. Journal of the American Academy of Dermatology. Vol. 57, November 2007, p. S88 (supplement 5). doi: 10.1016/j.jaad.2006.08.034.
Journal: M.O. Tucker and C.R. Swan. The mango-poison ivy connection. New England Journal of Medicine. Vol. 339, July 23, 1998, p. 235. doi: 10.1056/NEJM199807233390405.