Prickly Heat Or Sun Allergy? The Summer Rash Britain Keeps Misdiagnosing In 2026
June gave Britain its hottest June day on record – 37.7C at Lingwood in Norfolk on the 26th – and the Met Office says another heatwave may arrive early next week. Which means chemists across the country are about to sell a lot of products for prickly heat. And a decent chunk of them will be sold to people who don’t have prickly heat at all.
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Here’s the problem. Britain uses “prickly heat” as a catch-all for any itchy summer rash, but there are two completely different conditions hiding under that name – and they have opposite causes, appear on different parts of the body, and respond to different treatment. Get the diagnosis wrong and you can spend the whole summer treating the wrong thing. Plenty of people do, year after year.
Two rashes, one name
The first condition is actual prickly heat – the one doctors call miliaria. It’s caused by heat and sweat, full stop. Sunlight has nothing to do with it.
The second is polymorphic light eruption, or PLE, which is, in plain terms, a sun allergy – an abnormal immune reaction to ultraviolet light. Heat has nothing to do with that one. You can get PLE on a breezy 19C day in May if the sun’s out, and you can get true prickly heat in a stuffy office with the blinds down.
Because both produce itchy red bumps in summer, and because PLE isn’t a household name, almost everyone with PLE spends years believing they get “terrible prickly heat on holiday”. The distinction matters more than it sounds, because the standard advice for one can make the other worse.
Why Britain keeps getting it wrong
Some of this is marketing. Walk into any large chemist in July and you’ll find a seasonal bay – usually somewhere between the travel plugs and the mini toiletries – labelled with “prickly heat” products: cooling sprays, talcs, aftersuns, antihistamines. The branding has been consistent for decades, so the name stuck as the generic term for summer itch, the way “hoover” stuck for vacuum cleaners. Nobody has ever built a point-of-sale display around the words “polymorphic light eruption”, and you can see why.
Some of it’s timing. These rashes appear on holiday or during a heatwave, both situations in which nobody books a GP appointment. The rash fades within a week or two, the holiday ends, and the mystery resets until next summer. A dermatologist would recognise PLE across a waiting room; the trouble is that the people who have it are on a sun lounger in the Algarve when it’s at its worst, self-treating with whatever the nearest farmacia stocks.
And a little of it’s that the names are actively unhelpful. “Heat rash” contains the word heat, so sufferers of the sun-triggered rash rule it out; “sun allergy” sounds dramatic and rare, so people with PLE assume it can’t be them. Fifteen per cent of the population isn’t rare. It’s one person in your average lift.
What prickly heat actually is (and what it isn’t)
Prickly heat happens when sweat ducts get blocked. You sweat, the sweat can’t reach the surface, and it leaks into the surrounding skin instead – producing small raised spots with that characteristic stinging, prickling itch. The NHS page on heat rash is reassuringly blunt about it: it’s uncomfortable, usually harmless, and normally settles within a few days once you cool down.
The giveaway is location. Prickly heat favours skin that’s covered and sweaty – under waistbands and bra straps, on the back where it’s pressed against a car seat or office chair, in elbow creases, anywhere fabric traps moisture. Babies get it on the neck and nappy area for the same reason. If your rash sits precisely where your clothes were, heat is your culprit.
And it isn’t sunburn’s little cousin. You don’t need to have been outside. Some of the worst cases turn up in people who’ve spent a humid week commuting in polyester workwear, or sleeping hot – if June’s nights wrecked you, our guide to what actually cools you at night covers that side of things.
It’s also having a quiet moment among the fit and sweaty. Gym-goers in compression kit, cyclists in bib shorts, anyone doing a summer race in a synthetic top – sustained sweat under tight fabric is exactly the mechanism, which is why sports pharmacists see a spike every marathon season. The fix there isn’t a product either; it’s looser natural fabric and getting out of damp kit quickly instead of driving home in it.
Polymorphic light eruption: the sun allergy nobody told you about
PLE is far more common than its unwieldy name suggests – the NHS estimates it affects around 10-15% of the UK population, and it turns up far more often in women than men, typically starting between the ages of 20 and 40. If those numbers surprise you, that’s rather the point. Millions of Britons have a diagnosable photosensitivity and think they just “come out in a heat rash” every June.
The pattern is distinctive once you know it. The rash appears on skin that’s newly exposed to sun after months under cover – the chest, the forearms, the tops of the feet – and it shows up hours to a couple of days after the exposure, not during it. It’s usually worst in late spring and early summer, then eases as the season goes on, because skin gradually “hardens” to UV with repeated small doses. That’s why the first warm bank holiday claims so many victims while August often passes quietly.
Faces, oddly, are often spared. They’ve been exposed all year, so they’ve kept their tolerance. A rash that skips your face but blooms across your chest after the first garden weekend of the year is telling you something, and it isn’t “buy talc”.
The “polymorphic” part of the name just means it looks different in different people – small bumps in one person, larger raised patches in another, occasionally blisters. What stays consistent is the delay and the distribution. It’s not sunburn, either: sunburn is a dose-dependent burn that anyone can get and that hurts more than it itches, while PLE is an immune reaction that only some people get, at UV doses far below what would burn them. Some sufferers even flare through thin summer clothing or a car window, since the UVA that drives much of it passes through glass. That detail alone explains a fair few “mystery” rashes on drivers’ right forearms.
PLE also runs in families more often than chance would allow, so if your mum “always came out in a rash on the first day of the holiday”, you’ve probably inherited the tendency rather than her luck. It tends to persist for years once it starts, though many people find it slowly softens with age.
The 60-second way to tell them apart
Ask three questions.
Where is it? Covered, sweaty skin points to prickly heat; newly bared skin points to PLE. When did it appear? During or right after getting hot suggests prickly heat; the day after sunbathing suggests PLE. And has the summer pattern changed? Prickly heat strikes whenever you overheat, all season long. PLE is front-loaded – ferocious in May and June, milder by August.
Not a perfect test, but it sorts most cases. Anything that blisters dramatically, spreads fast, or comes with fever or feeling unwell belongs in front of a pharmacist or GP rather than a listicle.
One more clue if you’re still unsure: what happened the last time you went somewhere humid but grey? If Center Parcs in a muggy August set you off just as badly as Majorca did, that’s heat. If a bright, cool week skiing or an April city break in strong sun triggered the same rash, that’s light. Your own holiday history is a better diagnostic record than you’d think – most people just haven’t lined the episodes up.
What actually helps – and the shelf of stuff that doesn’t
For genuine prickly heat, the boring answer is the right one: get cool and stay cool. Loose cotton or linen instead of synthetics, cool showers, a fan at night, and time. Calamine lotion – about £3.50 in any chemist, in the same bottle your gran bought – takes the sting out while you wait. A pharmacist can suggest an antihistamine if the itch is ruining your sleep, though it’s worth knowing the blockage itself isn’t a histamine problem, so tablets blunt the itch rather than clear the rash.
A word on the “cooling” aftersuns and menthol gels that dominate the seasonal shelf: the sensation is real, the treatment isn’t. Menthol tricks the skin’s cold receptors, which feels lovely for twenty minutes and changes nothing underneath. There’s no harm in them – use one if the relief helps you sleep – but don’t mistake the tingle for progress, and don’t pay £9 for what a cool flannel does free.
Babies deserve their own line, since they’re the most frequent sufferers of true prickly heat and can’t tell you what itches. Lukewarm baths, loose cotton layers, and taking one layer off in the heat sort most cases – overdressed babies in July remain a stubborn British tradition. Skip talc, which cakes in the creases and can make blockage worse. And if a baby with a rash is feeding poorly, unusually drowsy or running a temperature, that’s a same-day GP call, not a wait-and-see.
Which brings me to the annual pre-holiday ritual I’d happily retire: the “prickly heat tablets” bought in bulk at the airport chemist and taken for a week as prevention. There’s little logic to it. Prickly heat is plumbing, not allergy – sweat physically trapped under the skin – and no antihistamine prevents a blocked duct. If the tablets seem to work on holiday, it’s usually because what you actually have is PLE, or because the second week of any holiday is when your skin has adjusted anyway.
One more thing to avoid with prickly heat: heavy, occlusive skincare. Thick moisturisers, rich body butters and petroleum layers sit on top of the skin and block sweat ducts further – the same reason slugging can backfire on British skin in summer. Let the skin breathe. This is the one time of year doing less is the treatment.
PLE needs the opposite playbook. Sun protection first: a high-SPF, high-UVA-rating sunscreen on the vulnerable areas, applied properly, before exposure – our round-up of UK-made SPFs covers options that don’t feel like paint. On sunscreen choice, the UVA rating matters more than usual here, since UVA does much of PLE’s dirty work – check for the circled UVA logo or a high star rating on the back of the bottle, not just the big SPF number on the front. Then build up sun exposure gradually in spring rather than going from zero to a full afternoon in the garden; short, regular doses are what builds tolerance. Once a flare has started, a mild steroid cream from the pharmacy usually calms it, and staying covered for a few days stops it extending. If it’s severe or arriving every year, a GP can refer you to dermatology, where supervised light treatment in spring – deliberate, controlled hardening – reliably changes the season for bad sufferers.
But notice what’s happened if you’ve misdiagnosed yourself. The PLE sufferer who thinks they have prickly heat skips the sunscreen, because they’ve been told heat is the enemy – and walks straight into their trigger. The prickly heat sufferer who thinks they’re sun-sensitive slathers on thick SPF and after-sun over sweaty covered skin and clogs their ducts even further. Same shelf, same chemist, opposite mistakes.
When it’s neither
Two lookalikes deserve a mention. Heat and sweat can trigger hives in some people – raised wheals that move around over hours, which neither of the above does. And any rash accompanied by dizziness, headache, intense thirst or confusion in hot weather isn’t a skin problem, it’s a heat exhaustion warning – that’s a get-out-of-the-sun-now situation, not a calamine one.
Rashes that weep, crust, or hurt more than itch can mean infection, and new rashes in anyone on medication merit a quick pharmacist conversation, since several common drugs – some antibiotics, some blood pressure tablets – make skin photosensitive. The summer rash that “came from nowhere” occasionally came from the prescription in April.
If in doubt, use the pharmacist. It’s still the most underused free consultation in Britain: they see these rashes all day every July, they’ll tell you in ninety seconds which of the two you’re most likely dealing with, and they’ll flag the cases that need a GP. That beats an hour of comparing rash photos online, which reliably convinces everyone they have the rarest condition on the page.
So before this next hot spell arrives, look at where last summer’s rash actually showed up. Covered skin or fresh-to-the-sun skin? Answer that one question and you’ll walk past most of the seasonal pharmacy shelf – and treat the fortnight ahead with a £3.50 bottle, a linen shirt, or a decent SPF, whichever your skin is actually asking for. Which one has yours been all along?
