How Long Does a Heat Rash Last on a Baby?

What is a heat rash?

Heat rash happens when the sweat glands get blocked. Heat rash usually goes away on its own within three or four days so long as you don’t irritate the site further.

Heat rash happens when the sweat glands get blocked. The trapped sweat irritates the skin and leads to small bumps. Babies sweat, too, and in extreme heat, they’re very susceptible to heat rash.

Heat rash is sometimes called “prickly heat,” because that’s the most common type. The name comes from the sensation and the often-reddish color of the bumps.

Heat rash can affect anyone, particularly those who exercise frequently or are unused to hot and humid weather. However, it is particularly common in babies.

Heat rash usually goes away on its own within a few days. 

Signs and symptoms of heat rash on a baby

Heat rash resembles a cluster of small blisters or pimples. In babies, it usually appears in the folds of a baby’s skin or in areas where clothing is snug. Common sites include:

• Chest
• Neck
• Stomach
• Butt
• Crotch

Symptoms of heat rash include :

Raised bumps on the skin

Depending on the type of heat rash, the bumps may range in coloration from flesh-colored to clear to red. They may also be filled with fluid or pus, or feel firm, depending on the type of heat rash.

Red skin

If you’ve been spending time outside and your baby’s skin reddens, you might assume it’s sunburn, but this redness might also be a symptom of heat rash. Keep a close eye on your baby’s skin, and watch for telltale bumps. Even if no bumps appear, it’s a good idea to cool your baby off.

Itchy skin

When you’re experiencing heat rash, sweat is trapped under your skin. Inflamed skin will often feel itchy, or you might feel a burning sensation, even in skin that wasn’t directly exposed to the sun, such as skin covered by your clothing.

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Types of heat rash

There are four types of heat rash. They are classified according to the depth of the blocked sweat ducts:

Miliaria crystallina

This heat rash is the mildest and only affects the ducts in the top layer of skin. The bumps are clear and fluid-filled, and they break easily.

Miliaria rubra (Prickly Heat)

When people talk about heat rash, they are usually talking about prickly heat. In this heat rash, babies develop red bumps that itch or produce a prickling sensation. 

Miliaria pustulosa

If the red bumps of miliaria rubra fill with pus and become inflamed, the rash turns into miliaria pustulosa.

Miliaria profunda

This is the least common type of heat rash. It affects the deeper skin layer, called the dermis. The blocked sweat leaks into the skin, creating firm lesions that are flesh-colored. They may resemble goosebumps.

Causes of heat rash

Heat rash is caused by blocked sweat glands. Common reasons for this include:

• Hot and/or humid weather
• Wearing too much clothing
• Ointment applied to the skin or hair

Breastfeeding mothers should note that moisturizing ointments such as lanolin may also be transferred from the nipple to the area around the baby’s mouth.

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When to see the doctor for heat rash

You should consult a doctor if your baby is experiencing any of the following symptoms:

• Your baby has a mild fever.
• The heat rash lasts more than a week.
• Your baby has a sore throat.
• The rash fills with pus, swells, or spreads.

If your baby has any of the following in addition to the heat rash, seek immediate care:

• Seizures
• Trouble breathing
• A high fever
• Signs of dizziness, nausea, or confusion

These may be symptoms of a more serious condition such as heat exhaustion, which can involve muscle cramps and can lead to heat stroke, an emergency condition in which your body temperature is dangerously elevated.

Diagnosing heat rash

Your doctor should be able to diagnose heat rash by looking at it. There are no tests for the condition. However, your doctor may order tests if they suspect that the rash is not heat rash or that the condition may be more serious, such as an infection.

Treatments for heat rash

Heat rash usually goes away on its own within three or four days so long as you don’t irritate the site further.

To treat and prevent heat rash, it’s important to keep skin cool and dry. To treat and prevent heat rash: 

• Avoid heat and humidity
• Wear loose, cotton clothing
• Make sure to hydrate with plenty of fluids
• Avoid using ointments
• Leave skin open to air
• Keep skin cool with air conditioning or a gentle fan
• Use cool water to wash away sweat and body oil and carefully dry the skin
• For babies especially, pay attention to skin folds that tend to stay wet with urine, sweat, or drool

If the rash is particularly itchy, there are also some over-the-counter treatments you can apply, such as calamine lotion which soothes itchy skin. If the calamine lotion isn't strong enough, contact your doctor who may recommend a mild over-the-counter cream such as hydrocortisone. They may also prescribe a different steroid cream.