If a rash occurs, the advice is to wash the area gently and pat dry, rather than rubbing to prevent spread of the infection. It is important not to scratch as it can lead to a serious infection if bacteria breaks through the skin.
Read more: How to keep cool in a heatwave
Polymorphic light eruption
Around one in 10 adults in the UK have a reaction similar to an allergy when they are exposed to the sun or UV light, called polymorphic light eruption (PLE).
The condition causes areas of small, red, bumpy spots – often confused with heat rash – or blisters, which turn into larger, dry, red patches, which can look like eczema. It can last for up to two weeks.
Dr Sturnham, who represents skincare brand Sebamed, says: “PLE is thought to be caused by UV light-altering a substance in the skin. The skin’s immune system becomes hyperactive, and this triggers an inflammatory rash, normally on the arms, chest and lower legs.”
PLE does not usually affect the face, which is one of the key ways doctors can tell the difference between it and heat rash.
The main treatment is avoiding the sun, covering up with loose clothing and wearing a suncream with an SPF of 30 or above. Severe rashes can be treated with antihistamines or steroid creams recommended by a pharmacist or doctor.