Jenny was embarrassed to show me her rash initially, but she desperately needed relief. 

This was a few weeks ago at the height of the record June heatwave and the combination of the heat and this rash was keeping Jenny up at night.

She was exhausted and uncomfortable. ‘It’s probably heat rash, right?’ Jenny said, as she took off her shirt and shoes to reveal lots of tiny red, blotchy spots on her chest and feet.

• Top dermatologist reveals the SPF mistakes millions still make (and the simple fix that could save your skin). Our expert SPF guide FREE in the Your Health newsletter 

This was a reasonable assumption. After all, it was baking hot. But Jenny didn’t have heat rash. 

Instead, she had a similar common condition called polymorphic light eruption, or PLE.

In my years as a GP, I’ve seen people mistake PLE for heat rash time and time again.

And telling the two apart matters, because treatments are radically different. 

And I should know, I suffer from PLE myself.

So, what is PLE – and how can you treat it?

First, it’s important to explain what is heat rash. It’s essentially a plumbing problem. In hot temperatures, sweat ducts become blocked. 

The trapped sweat then leaks into the surrounding skin and irritates it, and tiny spots appear in the places sweat gets caught, such as skin folds and under clothing.

Keep the skin cool and dry, wear loose cotton and it soon settles.

But the same is not true for PLE.

PLE is an abnormal immune reaction to ultraviolet light, the radiation emitted by the sun – not heat.

It typically strikes in spring or early summer when skin that has barely seen the sun all winter is suddenly exposed to strong sunlight, usually appearing within hours or days.

And this is a key difference: although Jenny’s rash appeared when it was extraordinarily hot, PLE has nothing to do with temperature. It’s about being outside in the light.

The location gives it away too. PLE tends to appear on skin that isn’t usually exposed – the upper arms, or in Jenny’s case, the chest and tops of the feet – while skin that sees light all year round, like the face and the backs of the hands, is often spared.

Over time, the skin essentially ‘toughens up’ to the UV exposure as the season goes on, meaning people who spend lots of time outside tend to be less susceptible to PLE.

There’s even a version in youngsters, often boys, who get it on the tops of their ears after a spring haircut suddenly exposes the skin – it’s called juvenile spring eruption.

However PLE is usually more common in women and tends to start between the ages of 20 and 40, for reasons that are unclear to doctors.

The word ‘polymorphic’ simply means many shapes, and the name fits: the rash can be small red bumps, larger raised patches or tiny blisters – but it is almost always intensely itchy.

Now, PLE isn’t dangerous. For many, it typically settles on its own within about a week if you stay out of the sun, and it doesn’t scar.

But patients are often embarrassed by it – red, blotchy patches appearing just as the clothes come off for summer – and as someone who has had this every summer for years myself, I can tell you it can genuinely spoil the first week of a holiday, or the first sunny spell of the year, when you can’t even sleep for the itching.

So what can you do about it?

For most people, PLE doesn’t need active treatment beyond time, cool showers, loose clothing and staying out of the sun.

But antihistamine tablets – available over the counter at most pharmacies – can ease the itch, and emollients help if the skin becomes dry.

If symptoms are really irritating and bothersome, steroid creams work well, and occasionally a short course of steroid tablets is used.

If your PLE is severe, or having a big impact on your life, you may be referred to a dermatologist.

One option is something called desensitisation phototherapy, sometimes called ‘hardening’ – a course of controlled UV exposure in hospital, usually at the end of winter or in early spring, to build your skin’s tolerance before the sunny weather arrives.

Essentially, it mimics in a controlled way what happens naturally to many people’s skin over the course of a normal summer.

As ever, though, prevention is better than cure. You can’t avoid the heat in a heatwave, but you can avoid the sun: seek the shade, cover up, and wear a high-factor, broad-spectrum sunscreen.

And one final word of caution. If your rash doesn’t settle within a week or two out of the sun, if it’s severe, spreading or blistering, or if you’re simply not sure what it is, please seek medical advice. 

Skin conditions can look remarkably similar (including, in rare occasions some forms of skin cancer) so it’s always important to be properly assessed and get the help you may need.

However, in many cases, that red, blotchy patch is likely to be PLE – and, with the right steps, it can be banished.

• Top dermatologist reveals the SPF mistakes millions still make (and the simple fix that could save your skin). Our expert SPF guide FREE in the Your Health newsletter