It was my third day in St. Martin when I noticed the tiny, itchy bumps on my legs. They were smaller than hives but more rash-like than bug bites. I figured I had just gotten too much sun, given that this was the first time my body had seen prolonged sunlight after an especially harsh winter. But I had been diligent with sunscreen and shade — not even a hint of sunburn. So I lathered back up and spent my last day at the pool.
Several nights later, home in New York City, I noticed the same tiny red bumps crop up on the tops of my arms; the rash now painting an itchy line from my shoulder to the crook of my elbow, down to my forearm. Benadryl and cortisone cream calmed things down a bit, but it flared up every night and morning for a week before finally disappearing.
I assumed it was a contact allergy and blamed all the usual suspects: hotel detergent, a new sunscreen, a hair serum that maybe migrated to my arms while I slept. But it kept coming back. After a few months of trial and error and allergy testing, I finally went to my dermatologist. She listened to the story, zoomed in on pictures of the rash in my camera roll, and asked whether I had ever heard of PMLE: polymorphous light eruption.
It turns out, I’m allergic to the literal sun.
What is polymorphous light eruption?
Polymorphous light eruption (PMLE) is a delayed skin reaction to UV radiation, and it’s actually the most common photosensitive condition, said Hana Ahmed, board-certified dermatologist and dermatopathologist in Florida. It can show up hours or days after exposure, so most people don’t automatically think to blame the sun.
“They call it polymorphous because it can look like a lot of different things, but it’s usually always the same pattern in one individual,” said double board-certified dermatologist Jennifer Tran, who sees this a lot where she practices in Ontario, Canada. That pattern can range from small, raised bumps to larger hives, blisters, eczema-like patches or plaques.
It tends to appear on the outside of the arms, forearms and chest, though it can also occur on the legs, upper back and rarely the face. Redness and itch are pretty standard across the board. (Fair warning before you Google the condition: There’s a wide variety of presentations, and the photos in search results tend to show more severe, widespread reactions.)
It’s not unusual for this condition to show up randomly in adulthood, though family history is a strong predictor, Tran said. PMLE can happen to anyone at any age, but being female between the ages of 20 and 40 and having a history of being sensitive to the sun can increase your risk, too. PMLE is more common in certain geographic areas, with research showing a 10 to 20 percent prevalence in Northern Europe and less common in people who live closer to the equator.
What causes PMLE?
“We don’t know exactly why it happens, but the working theory is that it’s a delayed immune reaction, almost like an allergic reaction to your own skin after it’s been altered from UV damage,” Ahmed said.
Typically, when your skin is exposed to UV rays, it causes both cell damage and an immunosuppressive effect. The latter is meant to prevent your skin from overreacting to the sun, Ahmed explained.
In people with PMLE, that local immunosuppression is faulty; the immune system mistakes molecules that form in the skin as a result of UV exposure as threatening. It then mounts an immune response against the perceived invader, causing an itchy rash — similar to what happens with a contact allergy to a skincare ingredient or poison ivy, Ahmed said. Dermatologists call this a type IV or delayed hypersensitivity, she explained.
PMLE seems to be triggered by UVA radiation more than UVB radiation, so some people even get the reaction through car windows (which don’t reliably block UVA rays).
It usually happens in the early spring with the first significant sun exposure of the year. Apparently my March getaway to the Caribbean was the perfect PMLE trigger. Over the course of spring and summer, flares tend to become less intense as your skin becomes less sensitive, explained Lindsay Boyers, a board-certified dermatologist at Boulder Medical Center in Colorado. Dermatologists call this process “photohardening.”
How to prevent a flare
I was relieved to hear that having a sun allergy doesn’t necessarily mean I have to hide out in my apartment for the rest of my life. But it does mean being more proactive about sun protection. Here are some things you can do to reduce your chances of experiencing a flare.
Phototherapy
For people with severe, recurring flare-ups of PMLE, phototherapy is a safe way to help “harden” the skin and get it used to more UV exposure. Unlike what you would get in a tanning bed or outside, phototherapy exposes you to just narrow-band UVB, which can reduce inflammation and help produce that immunosuppressive effect.
Treatment typically involves going to the dermatologist’s office two to three times a week for several weeks at the start of spring, which can be inconvenient or inaccessible for some. Some people might find a similar benefit from gradually increasing their exposure to natural sunlight in the spring, Tran said, ideally in the morning for 10 to 15 minutes a day.
SPF and UPF
All the experts recommended wearing broad-spectrum sunscreen that blocks both UVA and UVB rays, with at least 30 SPF. But unfortunately, PMLE doesn’t seem to be completely deterred by even the strongest sunscreens, so you’ll need to layer in more sun protection strategies, like hats, shade and UPF clothing.
“I am a huge proponent of UPF clothing, which means it’s been treated to be sun protective,” Boyers said. “Mostly because sunscreen only works as well as you use it.” This lets you stay protected even while you’re in the water.
Polypodium leucotomos supplement
Both Boyers and Ahmed take a supplement to help prevent their own PMLE flare-ups. The extract of Polypodium leucotomos, a type of tropical fern, is sold in various supplements. “It doesn’t fully protect you from the sun, but it does provide a little bit of extra protection because it’s a potent antioxidant,” Tran said.
The research is still emerging but promising: A 2011 Italian study of 57 people (53 with PMLE and 4 with solar urticaria, another sun-induced skin reaction) found that nearly 74 percent had fewer reactions and symptoms after sun exposure when taking the supplement. A 2018 systematic review looked at 18 studies on Polypodium leucotomos (most of which involved Fernblock, the patented formulation used in Heliocare) and found it to have photoprotective effects with no serious adverse effects, but the review noted the need for more research on other formulations. (The review was partially funded by the company that makes Heliocare.) “The studies on Polypodium are very small,” Ahmed added. But it’s widely used for a lot of photo-related rashes, she noted, “things like vitiligo, melasma and anything that could be mitigated by UV exposure.”
How to soothe skin during a flare
If, despite your best efforts, you end up with a rash, a topical corticosteroid — such as over-the-counter hydrocortisone cream or a prescription-strength option from your dermatologist — can help calm things down, Boyers said.
You can also try taking a second-generation antihistamine like cetirizine hydrochloride (Zyrtec) or fexofenadine hydrochloride (Allegra), which are preferable to diphenhydramine hydrochloride (Benadryl) because they last longer and come with fewer side effects, Boyers said. Your doctor may instruct you to take one of these medications more frequently to control the itch, Boyers said. In severe cases, they might prescribe oral steroids.
As much as possible, try to stay out of the sun while your skin heals. “Strict sun avoidance forever is not recommended, as natural photohardening would not occur,” Ahmed explained. “But during an acute flare, sun should be avoided until the lesions have resolved, typically about one week. After the eruption resolves, gradual, controlled exposure can help prevent recurrence through photohardening.”
If I can offer anyone else a little hope in the meantime, I can confirm that my own flares have gotten less intense over the course of the summer and as I’ve implemented the above expert tips. While I’m not thrilled that this might be a yearly occurrence, it was comforting to hear from my dermatologist (and these experts) that I don’t have to let it stop me from booking my next trip. I’ll just try to save any tropical destinations for later in the season and make room in my carry-on for my ever-expanding sun protection toolkit.
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