Heat rash (miliaria): tiny bumps from blocked sweat

You dressed your baby warmly, and now there’s a sprinkle of tiny red bumps across the neck and chest. That’s very often heat rash. It looks dramatic on delicate skin, but it’s usually harmless, and the fix is mostly about cooling down and taking off a layer.

What heat rash is

Heat rash, called miliaria by doctors, is a rash caused by blocked sweat ducts. The sweat ducts are the tiny tubes that carry sweat from the sweat glands to the surface of the skin. When their openings get plugged, sweat is trapped under the skin, and the skin reacts with small bumps or blisters.

The American Academy of Pediatrics (AAP) says heat rash is very common in the first few weeks of life. DermNet, a dermatology reference from New Zealand, explains why: newborns’ sweat ducts are still immature and block easily. On top of that, the NHS points out that babies can’t control their temperature as well as older children and adults.

It isn’t an infection, and it isn’t contagious.

What it looks like

Heat rash comes in different forms, depending on how deep in the skin the blockage is.

• Tiny clear blisters. When the blockage is right at the surface, you see little clear, fragile blisters, 1 to 2 mm across, that look like drops of water. DermNet says this form affects up to 9% of newborns, typically around one week of age.
• Small red bumps. When the blockage is a bit deeper, you see red bumps a few millimetres across. This is the classic “prickly heat”, and it’s itchy, so your baby may be fussy or wriggly.

The NHS notes that on brown or black skin, the rash may be harder to see, or may look grey or white rather than red.

It shows up where sweat builds up and air doesn’t move much. The AAP lists the upper chest, back, neck folds, hairline and diaper area, though it can appear almost anywhere.

Why babies get it, even in winter

The trigger is heat plus poor airflow. The AAP says heat rash often develops in hot, humid weather, especially when a baby is in many layers, wrapped in blankets or buckled into a seat where air can’t flow around the body. DermNet adds fever and tight, non-breathable synthetic clothing to the list.

That’s why Canadian babies get heat rash in January as well as in July. A snowsuit left on in a warm store, a car seat with a thick blanket over it, or a baby bundled up for a nap in a heated room can all do it.

How to treat heat rash

The treatment is simple: cool the skin and let it breathe. Most heat rash fades within a few days once the skin is no longer overheated.

• Take off extra layers. The Canadian Paediatric Society’s advice for heat rash is to remove excess clothing; it will go away on its own.
• Cool down. The AAP suggests a cool bath or cool, moist cloths to rinse away sweat, then drying the skin completely.
• Let the skin air out. Leave the affected area uncovered when you can.
• Dress in thin, loose cotton. The NHS and the AAP both recommend loose cotton clothing and light bedding.
• Keep the room comfortable. A fan or air conditioning can help in a heat wave.
• Watch the folds. Neck creases, armpits and other folds trap moisture, so pat them dry.

What to avoid: thick, greasy ointments on the rash. The AAP warns they can block the sweat glands further. That goes for heavy baby lotions too. Save your barrier ointments for the diaper area when it needs them, and don’t use medicated creams on a baby without advice from your doctor or pharmacist.

Preventing it

Prevention is mostly about dressing for the temperature, not for the calendar.

The Canadian Paediatric Society’s safe sleep advice applies here too: keep the room at a temperature that feels comfortable to you, dress your baby in a properly fitted one-piece sleeper, and skip blankets in the crib. Overheating isn’t just a skin issue. The CPS says overheating increases a baby’s risk of SIDS (sudden infant death syndrome).

Some practical habits:

• Feel the back of your baby’s neck or chest. Sweaty or hot means too many layers.
• Unzip or remove snowsuits and heavy jackets as soon as you’re indoors or in a warm car.
• Don’t cover a car seat or stroller with a heavy blanket.
• Choose breathable cotton over synthetic fabrics next to the skin in hot weather.

When to call a doctor

Heat rash itself is harmless, but a few signs mean something more is going on. The AAP says to call your doctor if:

• the rash looks infected: tender, with pus or yellow drainage
• your baby has a fever
• your baby is eating less or is less active than usual
• the rash lasts more than three days, or gets worse within 24 hours

DermNet notes that heat rash can become infected with bacteria, usually staphylococci, and that other conditions, like herpes simplex (the cold sore virus), fungal infections and newborn rashes, can look similar. Any rash with blisters in a baby under a few weeks old, or any rash in a baby who has a fever or seems unwell, deserves a prompt call to your doctor or local health line.

The NHS also advises getting medical advice if you’re worried about a rash in a baby.

Heat rash is the skin’s way of saying “too warm”. Take off a layer, cool the skin, keep creams off it, and it usually fades in a few days. If your baby has a fever, seems unwell, or the rash looks infected or won’t go away, call your doctor.