Heat Rash (Miliaria) in Babies: Causes, Treatment and Prevention

Heat rash — known medically as miliaria — is extremely common in babies and young children, particularly in warm weather or when they have been overdressed. It is almost always harmless and resolves quickly once the baby is cooled, but it can look alarming and cause parents significant worry. Understanding why it happens and what to do about it makes it much easier to manage.

Why Do Babies Get Heat Rash?

Babies have sweat glands that are structurally immature. When they overheat, the sweat glands work hard but the ducts through which sweat reaches the skin surface can become blocked — particularly in areas where skin rubs against skin or clothing. Trapped sweat leads to inflammation just under the skin, producing the characteristic rash.

Babies are particularly prone to heat rash because:

• Their sweat glands are not fully developed
• They have a relatively large surface area compared to their body mass
• They cannot remove their own clothing or tell you they are too hot
• They are often held close to a caregiver's warm body
• They are frequently dressed in multiple layers, particularly in the early months when parents are cautious about cold

What Does Heat Rash Look Like?

There are several types of miliaria, depending on how deep the blocked ducts are:

Miliaria Crystallina (Superficial)

The mildest and most common form. Appears as tiny, clear or white blisters (vesicles) filled with fluid, which look like water droplets on the skin. They are not itchy and cause no discomfort. They burst very easily with gentle touch. Most common on the forehead and neck.

Miliaria Rubra (Prickly Heat)

The most commonly recognised form. Red, small papules and pustules in the skin folds and on areas covered by clothing — neck, armpits, elbow creases, and the trunk. The skin feels prickly or irritated (hence "prickly heat"), and your baby may scratch or seem uncomfortable. More common in older babies and in humid, hot conditions.

Miliaria Profunda (Rare)

Affects deeper layers of the skin and appears as flesh-coloured or reddish papules. This type is uncommon in infants.

Where Does It Appear?

Heat rash typically appears in areas where skin touches skin or where clothing fits snugly: the neck, armpits, elbow creases, behind the knees, and across the upper back and chest. In young babies, the nappy area (even without a nappy rash cause) can be affected. The face — particularly the cheeks and forehead — is also common.

Treatment

Heat rash in babies does not need medical treatment in most cases. The steps to resolve it are straightforward:

Cool the baby: Move your baby to a cooler environment. Remove excess clothing and let them wear just a nappy or vest in hot weather.

Air the skin: Allow the affected areas to be exposed to air rather than covered. Avoid tight clothing that traps heat in the areas affected.

Cool bath: A lukewarm (not cold) bath can soothe irritated skin and help bring down your baby's temperature. Do not use soaps or products on the affected areas.

Avoid creams and lotions: Thick creams or emollients applied over heat rash can worsen it by trapping more heat and blocking the sweat ducts further. Keep the skin clean and bare.

Hydration: Ensure your baby is well-fed and hydrated. In hot weather, breastfed babies may need to feed more frequently; formula-fed babies over six months can have small amounts of cooled boiled water.

Heat rash typically begins to fade within 24–48 hours of cooling the baby down, and most cases resolve completely within three to four days.

When to See a GP

Heat rash almost always resolves without medical input. However, contact your GP or call NHS 111 if:

• The rash is not improving after three to four days of cooling measures
• Your baby has a high temperature (above 38°C in babies under three months, or above 39°C in older babies)
• The rash appears to be spreading and is accompanied by your baby being unwell
• There are signs of infection: the spots become filled with yellow pus, increasingly red and tender, or crusted
• Your baby seems extremely uncomfortable, is not feeding, or is unusually lethargic

Preventing Heat Rash

Prevention centres on avoiding overheating:

• Dress in light layers: In warm weather, a vest and light cotton sleepsuit is often enough. Remove layers as temperatures rise.
• Choose natural fabrics: Cotton and bamboo allow the skin to breathe better than synthetic materials.
• Do not overdress for bed: Follow safe sleep temperature guidelines (see our overheating guide). The room temperature for sleeping should ideally be 16–20°C.
• Avoid prolonged skin-to-skin in very hot weather: Whilst skin-to-skin is wonderful for bonding and feeding, a hot adult body can raise a baby's skin temperature. Take breaks and ensure air circulation.
• Keep the home cool: Use fans, open windows in the morning before the day heats up, and use window blinds or curtains to reduce solar gain.

Heat rash is one of those conditions where the treatment is simply creating better conditions for your baby. It is a useful signal that your baby has been too warm — take it as useful information and adjust accordingly.