An itchy rash after being in the sun isn't always prickly heat. Dr Mark Porter explains why some people develop a sun allergy, what triggers it and how to prevent it.
How do I prevent prickly heat? It started during the heatwave in May. After being out in the garden, I developed an itchy red rash on the tops of my feet and my chest where my shirt was open.
I have never had a problem before, but now it is happening regularly if I spend too much time in the sun. Friends have told me to avoid sunscreens because they can block the pores, but this hasn’t helped, so I just cover up.
Classic prickly heat (miliaria rubra) is caused by blocked sweat glands and more common in hot, humid conditions rather than just strong sunlight. Your story is more suggestive of another condition calledpolymorphic light eruption (PLE). It is often confused with prickly heat, which might explain why your friends’ advice hasn’t helped and is probably exacerbating the problem.
PLE is a sensitivity to sunlight, mainly the highly penetrative UVA component that can pass through clouds, water, glass and even some clothing. It causes an itchy, red rash that develops anywhere from a few hours to a few days after exposure, and the classic story is someone who notices it on the second or third day of their holiday in the sun.
It is normally symmetrical, and commonly affected sites include the chest, tops of the feet and arms. The key to prevention is to reduce your exposure to UVA by using a decent sunscreen. Go for a sunscreen that has an SPF of 50 and a five-star UVA rating and reapply every two hours as UVA filters break down quickly, and try to avoid direct sun exposure during peak UVA radiation (11am-3pm).
Some dermatologists also advocate beta carotene supplements, but the science is shaky. You’d need to take at least 15mg daily from a week before sunny trips abroad up until the end of the holiday, or during warmer spells at home. PLE typically develops in young adulthood, particularly in women in their twenties and thirties, and it’s unusual to suddenly be troubled when you are older.
So, in new cases like yours, I check for medicines that can trigger photosensitivity. Examples that may – and I stress may – worsen or trigger PLE include some statins (like atorvastatin), tetracycline antibiotics such as doxycycline, thiazide diuretics (water tablets) like bendroflumethiazide, and the anti-inflammatory naproxen. If you have started any of these recently you should mention your rash to your GP to see if they think one of these may be responsible.
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