Heat rash shows up reliably in two situations: the first warm spell of British summer, when nobody has recalibrated what the baby should be wearing yet, and in winter when a well-meaning family has wrapped a baby in enough layers to survive a Nordic expedition indoors.
It looks alarming, particularly when the spots are dense or when the child seems uncomfortable, but heat rash itself is harmless. The more important thing it tells you is that the baby has been too warm.
Healthbooq (healthbooq.com) includes guidance on newborn and infant skin care, common skin conditions, and safe temperature management throughout the first years.
What Heat Rash Is
The medical name is miliaria. It happens when sweat ducts become blocked, preventing sweat from reaching the skin surface, so it leaks into the skin instead. The leaked sweat causes inflammation, and the result is the familiar small red bumps or, in milder versions, tiny clear fluid-filled blisters.
Babies are more susceptible than older children and adults because their sweat glands are immature and more easily occluded. The glands themselves work, but the ducts that carry sweat to the surface block more readily. Any situation that causes sustained sweating in a baby, being overdressed, sleeping under too many covers, being in a warm environment, being held against an adult body for extended periods in warm weather, can trigger it.
There are three main types that vary by how deep in the skin the blockage occurs. Miliaria crystallina, the shallowest type, produces tiny clear blisters that rupture easily and look almost like water droplets under the skin. It is the mildest form. Miliaria rubra, the most common type, produces the characteristic red pimply rash with surrounding redness and itching or prickling. Miliaria profunda, the deepest and least common type, produces larger flesh-coloured bumps and is more often seen after repeated episodes of the milder forms.
Where It Appears
Heat rash most commonly appears in areas where skin folds trap heat and prevent sweat from evaporating: the neck, armpits, groin, behind the knees, and any skin-on-skin folds in chubbier babies. In babies wearing nappies, the nappy area is a common site. In overdressed babies, it can appear across the back and chest.
Treatment
The treatment is cooling the baby down. Move them to a cooler environment, remove or loosen clothing, and let the skin breathe. A cool (not cold) bath can provide relief. Pat the skin dry rather than rubbing.
Calamine lotion can help with itching and the mild astringent effect aids drying. Hydrocortisone cream is sometimes suggested but is generally not needed for ordinary heat rash and should not be applied to the face. Avoid thick creams or ointments on areas with heat rash, as occlusive products worsen the blockage.
Most heat rash resolves within a few days once the baby is no longer overheated.
The Overheating Connection
Heat rash is a sign the baby has been too warm. This is clinically significant for sleeping babies. Overheating is an independent risk factor for sudden infant death syndrome (SIDS), and the Lullaby Trust's safer sleep guidance addresses it directly.
Babies sleep most safely at a room temperature of 16 to 20 degrees Celsius. A useful test: if the back of the baby's neck or chest feels sweaty or very warm, they have too many layers on. Hands and feet will often feel cool even in a comfortably warm baby, so these are not the right places to check.
Sleeping bags rated by tog for the room temperature are a practical way to avoid both overheating and under-wrapping. The Lullaby Trust provides a tog-to-temperature guide.
When to Seek Advice
Heat rash does not usually require medical attention. See your GP or seek advice if the rash is not improving after three or four days of appropriate cooling measures, if the baby is also unwell with fever, if there are signs of infection such as pus, crusting, or significant spreading redness around the spots, if the rash is very widespread and the baby seems uncomfortable, or if you are not sure the rash is heat rash rather than something else.
Other rashes can look similar to heat rash. Milia (white spots often present in newborns) are caused by blocked hair follicles rather than sweat glands and are not related to temperature. Erythema toxicum neonatorum is a common newborn rash with red blotches and small central pustules that resolves on its own. Viral rashes often have a different distribution and are accompanied by other symptoms. A GP can assess if you are uncertain.
Key Takeaways
Heat rash (miliaria) occurs when sweat ducts become blocked, causing sweat to leak into the skin and produce small red bumps or clear blisters. It is common in babies because their sweat glands are immature and easily blocked. Treatment is cooling and avoiding further overheating. Heat rash itself is benign, but it indicates the baby has been too warm, which is relevant for safe sleeping.
