This article first appeared in Issue 4 of our free digital magazine CURIOUS.
Without the Sun, life on Earth would have had no hope of getting off the ground. A star to warm our little rocky planet helped create conditions that meant life could unfold in this tiny corner of the cosmos. Yet somehow, some humans appear to have developed allergies to sunlight.
Sunlight is made up of an electromagnetic radiation spectrum that includes visible light, infrared (heat), and ultraviolet (UV), the last of which we can’t see or feel but is the culprit of a range of Sun damage that spans from irritating to painful and deadly.
We spoke to leading Sun allergy expert Dr Robert Sarkany, Head of Photodermatology at St John’s Institute of Dermatology at Guy’s and St Thomas’ Hospital, London, about the known and lesser-known allergies to sunlight. You won’t leave the house without sunscreen after reading this.
So, can you really be allergic to the Sun?
RS: The short answer is yes. In lots of different ways.
How does the Sun affect the body?
RS: If you break up sunlight with a prism, do the Isaac Newton thing, you'll find you get a fan of rainbow colors, but just all the visible ones, from the shortest to longest wavelengths. However, if you put your hand in it you won't see anything, but it'll feel hot; that's the infrared. You can't see and you can't feel anything, but if you keep your hand there for long enough, you will sunburn. That’s called ultraviolet (UV) and that's 5 percent of what the Sun sends out to us.
UV can penetrate the skin because it just so happens that the energy in a UV photon matches the energy gap between electron orbitals in the sort of carbon-containing molecules that make us up and by that, I mean proteins, DNA, and, lipids etc. So, that's a very physicsy way of saying that UV chemically changes the biochemicals that make our pathways, and so any bits of the body that are exposed to daylight will change anywhere it gets through. It’ll get through the first millimeter or two of skin, it will get onto the eyes, and what you will have is the very worrying situation that all the time the cells are being exposed, their DNA is being changed, and you get mutations and changes in the DNA.
Any sort of long-term cells, like the stem cells that are millimeters down in the skin, you suddenly find that you've changed the genetic code. You’re going to get all sorts of things suddenly appearing after exposure, which are sort of modified, chemically altered versions of proteins that take different shapes. And your immune system is going to turn around and say, “Oh, don't like the look of that, that's clearly something that shouldn't be there.” So you end up with a load of stuff that looks foreign and your skin starts mounting all these auto-immune reactions against bits of you that have been a bit damaged.
What are the body’s defenses against sunlight and UV light?
RS: Essentially, daylight is a uniquely ubiquitous, potentially very dangerous, environmental thing, which is clearly damaging anything it encounters in the body. As a result of this, we’ve had to develop a very, very sophisticated immune defense system to deal with this major threat that can cause cancer, autoimmune disease, induce mutations in DNA etc.
So, how the hell do you deal with that? You have three major responses: you produce pigment [in the skin] so that less damage happens tomorrow. You tan up quite quickly to protect against UV exposure so if the same thing happens tomorrow it will be absorbed and not get to the DNA.
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You also have an intracellular, complex pathway of DNA repair to look for all those bits of DNA damage, and nip them out before they turn into proper mutations like cancer. The cells whose DNA is irreparably damaged are thrown into cell suicide, also known as apoptosis. They die, and that causes the reaction that we see as sunburn.
How do we avoid an autoimmune disease being triggered by exposure to daylight?
RS: The immune system in skin is profoundly immune suppressed for about three days after exposure to UV and three days is long enough for all the UV damage and altered components like proteins in the skin to be cleared out and got rid of. However, during that three-day period, what you don’t want is to mistake collagen or something normal that has been slightly altered by UV for something which needs to be attacked, inducing an immune response against it. Then, unfortunately, that immune response might start attacking your collagen that has not been UV altered, and you've produced an autoimmune disease.
If anything slips past the defenses of those three responses, you will end up with things that you don't want to happen, pathological things happening when that person's skin is exposed to UV.
What are the different types of Sun allergies, and how we can treat them?
Polymorphic light eruption (prickly heat)
RS: The most common sun allergy in temperate zones, polymorphic light eruption, sometimes called prickly heat, affects one in six people. A few hours after exposure, certain parts of the body (the chest and neck, upper back, shoulders, and thighs) will have an extremely uncomfortable, red bumpy rash, which stays for up to seven to 10 days and then disappears. These people have a slightly more active immune response in that three-day period after exposure.
For really severe attacks, we use immunosuppressing medications, so steroid creams and tablets. For a really bad attack, we can use UVB in a controlled setting to expose the skin to very precise doses and it can actually do what's called hardening of the skin or desensitization of the skin for a few months afterwards, which works very well in many people.
Chronic actinic dermatitis (UV-related eczema)
Less common but can be extremely severe is chronic actinic dermatitis, in which absolutely tiny amounts of UV can induce a really severe dermatitis – a very severe version of eczema in the areas where daylight has been. It can cause dry, inflamed, very itchy skin that can be swollen or weeping on the hands, forearms, face, and neck for up to six months or a year. Most of the normal treatments for eczema are insufficient, so many patients are only treatable with very strong immune-suppressing tablets or injections. These people should avoid the Sun because the triggering dose for another flare is often as low as 120 seconds in daylight. Over a five-year period, for about 50 percent of people, it will disappear.
Erythropoietic protoporphyria (EPP)
Then there are the rarer but very severe genetic diseases that cause photosensitivity. There is a family of inherited diseases called porphyria that all occur due to the accumulation of chemicals internally called porphyrins (essential for the function of hemoglobin).
The one that causes the most severe hypersensitivity is called erythropoietic protoporphyria (EPP) – an inherited lack of an enzyme in the pathway of heme. In people with EEP, in the thinner areas of skin – the face, tops of the feet, and backs of the hands – the porphyrin becomes toxic, it damages whatever it's around as a result of being exposed to daylight. It is caused by visible blue light, not UV, and damages blood vessels in the skin, causing terrible severe pain that can go on for three or four days. These patients have an extremely painful lifelong photosensitivity problem. It was first recognized in 1961, but in the last few years, for the first time, effective treatments appear to be coming through. One drug that has gone past trial stage increases pigmentation in the skin, which should not necessarily be very helpful in EPP, but it appears to have other effects on the skin, which do seem to be rather effective.
Xeroderma pigmentosum (XP)
Rare, but devastating, is xeroderma pigmentosum (XP). I mentioned how, fortunately, we have an effective and sophisticated DNA repair system. But if you happen to have a mutation in one of the genes which encodes the proteins that are part of this DNA repair pathway, then you have an inherited genetic lack of DNA repair in these cells. You can't repair the DNA damage, which means every time these people go outside if it is not dark or nighttime, there will be damage that is not being repaired in all their skin cells, which will turn into mutations. So these patients, if they're not very, very well UV protected by the age of five or six, may develop multiple skin cancers, eye damage, and eye cancers as well. In areas where health service access is poor, the average lifespan is 13 or 14 years old.
In more developed countries where there are better health services but also where there may be less sunshine and sunscreens and other methods of UV protection are more accessible, lifespan can be longer. With specialist services, what we're finding is that many people can live a nearly normal lifespan if they have very extreme and absolute protection.
Hydroa vacciniforme (HV)
Hydroa vacciniforme (HV) tends to come on in children five to 10 years old as a result of severe photosensitivity due to the immune system – it’s a proper allergic reaction. Just 10 to 20 minutes of long wavelength UV radiation (UVA) exposure and you get blisters, big, red, itchy spots that come up on the face and hands, sometimes filled with pus. When they heal after a few weeks, they can leave very deep pock-mark scars. Only in recent years, and we’ve been very involved in this, has the cause of this disease been understood. HV has turned out unexpectedly to be due to chronic active Epstein-Barr Virus (EBV) infection. In other words, once those people are exposed to EBV, which generally causes glandular fever, it appears the immune system is not capable of eliminating it.
The EBV is infecting certain types of T lymphocytes – preventative immune cells – and when UVA goes into the skin of somebody with HV, the infected T lymphocytes seem to be allergic to UVA and dramatically react whenever the skin is exposed. So, HV is a rare and previously not very well recognized way in which chronic active EBV infection in the whole body can cause problems. It can be very life-restricting but in many people, it just goes away. Presumably, the immune system is managing to eliminate EBV and the photosensitivity goes.
There’s a horrible condition called solar urticaria, which is the posh word for hives. Hives is very common, where people get a release of histamine and they get these nettle-like rashes that come up for an hour or three and then go down again. What is rare is the solar version. It comes in a very severe form and is a result of an allergy to UV or visible light in sunshine. You don’t just get nettle-looking rashes, you get half an hour of intensely itchy, swollen red rashes on every single square inch of skin that's not covered by clothes when you go out and it can be from minimal amounts of daylight.
The problem with it is that yes, it goes away within half an hour, it’s not EPP or chronic actinic dermatitis, with bouts going on for days or weeks, but what you've got is this intensely unpleasant thing every time you walk outside the door. People are effectively imprisoned in their houses during daylight hours. And even then they often have to have UV protective window films and things like that. So it can be very unpleasant even though it's not a dangerous disease. It can occur at any age, but young adults are the most commonly affected, and it can go on for decades. We do have improved treatments, injections or tablets, and it can go away on its own.
Finally, there's a thing called photocontact dermatitis, which is not really a sun allergy, but an allergy to certain chemicals in sunscreens. With a few of these chemicals, particularly in sunscreens and in nonsteroidal anti-inflammatory gels and creams like Nurofen or ibuprofen that are put on the skin, the allergy doesn't happen unless you also have UV from sunshine that can trigger that particular chemical allergy in that person.
So there’s a dramatic range of Sun allergies, from the common and irritating “it’s going to mess up my holiday” to the life-restricting and devastatingly life-threatening, which, thankfully, is a bit rarer.