Why does heat rash happen?

What blocks the sweat ducts?

Sweat ducts are the tiny channels that carry sweat from sweat glands to the skin's surface. Skin debris and bacterial films can obstruct them. Sweat then leaks into surrounding skin, causing swelling that can block the channels further. Heat rash is also called miliaria, prickly heat or sweat rash, because apparently one name was not enough for trapped sweat.23

Why does it happen after exercise or during illness?

Weather is not the only trigger. Intense exercise and high fevers increase sweating and can bring on heat rash. Tight clothing and medication patches can cover skin in a way that encourages blockage. Medicines that increase sweating have also been linked to heat rash. If episodes started after a prescription change, ask the prescriber whether the medicine could be contributing rather than changing treatment yourself.2

Babies and young children get heat rash more often than adults. Their sweat ducts are still immature, and babies cannot regulate their temperature as well as older children and adults. Adults can develop it too, particularly with increased sweating or exposure to a hotter, more humid climate.12

What does heat rash look and feel like?

Typical symptoms are small raised spots, an itchy or prickly feeling, and mild swelling. Some spots contain fluid. The rash can appear anywhere and spread, but it is not contagious. Spots often look red on white skin; on brown or black skin, they can look grey or white and be harder to see.1

The depth of the blockage changes the appearance. Very shallow blockage produces fragile, clear blisters, called miliaria crystallina, which can resemble water droplets without redness. Miliaria rubra produces inflamed, itchy bumps. The rarer, deeper form, miliaria profunda, produces skin coloured bumps that are usually not itchy.34

What should you do when it appears?

1. 1Move somewhere cooler and pause activity that makes you sweat. Limit time in the heat.
2. 2Change into loose cotton clothing and take a cool bath or shower. Keep the affected skin cool and dry.
3. 3Use a cold damp cloth, or an ice pack wrapped in a tea towel, for up to 20 minutes to calm itching.
4. 4Pat or tap the rash rather than scratching. Avoid perfumed shower gels and creams, and skip moisturizers on the affected area.

Lightweight bedding helps keep skin cool at night, so the duvet does not need to participate in summer.1

For troublesome itching, a pharmacist can advise on calamine lotion, antihistamine tablets or hydrocortisone cream. Children under 10 need advice from a doctor before using hydrocortisone.1

How long does it last, and when should you get help?

Heat rash usually clears within a few days once sweating settles. Contact a doctor if it is not improving after a few days, or sooner if your baby has a rash that worries you. Children's rashes can look alike, so an uncertain diagnosis deserves a pediatrician's assessment.16

Get medical advice for warm feeling skin or pus drainage, which can indicate infection. Heat rash alone does not necessarily raise body temperature. Dizziness, nausea, vomiting, headache, rapid breathing or a fast, weak pulse during heat exposure need immediate medical attention. Confusion, behaviour changes or slurred speech can signal heat stroke, a medical emergency.57