Health Information Library

Overview

Heat rash — also known as prickly heat and miliaria — isn't just for babies. It affects adults, too, especially in hot, humid conditions.

Heat rash occurs when sweat is trapped in the skin. Symptoms can range from small blisters to deep, inflamed lumps. Some forms of heat rash are very itchy.

Heat rash usually goes away once the skin cools down. Severe forms of the condition might need treatment from a health care provider.

Symptoms

Adults usually develop heat rash in skin folds and where clothing rubs against the skin. In infants, the rash is mainly found on the neck, shoulders and chest. It can also show up in the armpits, elbow creases and groin.

Types of heat rash

The types of heat rash are classified according to how deep the sweat is trapped in the skin. Signs and symptoms for each type vary.

• The mildest form of heat rash is called miliaria crystallina. It occurs when the opening of the sweat duct on the surface of the skin (sweat pore) is blocked. This form is marked by tiny, clear, fluid-filled bumps that break easily.
• A type that occurs deeper in the skin is called miliaria rubra. It is sometimes called prickly heat. Signs and symptoms include small, inflamed blister-like bumps and itching or prickling in the affected area.
• Occasionally, the inflamed bumps of miliaria rubra fill with pus. This form is called miliaria pustulosa.
• A less common form of heat rash is called miliaria profunda. It affects the deepest layer of the skin (dermis). It causes firm, painful or itchy inflamed bumps that look like goose bumps and may break open.

When to see a doctor

Heat rash usually heals by cooling the skin and avoiding exposure to the heat that caused it. See your health care provider if you or your child has symptoms that last longer than a few days or the rash seems to be getting worse.

Causes

Heat rash develops when a duct that leads from a sweat gland to the surface of the skin is blocked or inflamed. This then blocks the opening of the sweat duct on the surface of the skin (sweat pore). Instead of evaporating, sweat is trapped beneath the skin, causing irritation and bumps on the skin.

Risk factors

Factors that increase the risk of heat rash include:

• Being a newborn, as newborns have immature sweat ducts
• Living in a hot, humid climate
• Being physically active
• Being on bedrest for a long time and having a fever

Complications

Heat rash usually heals without scarring. People with brown or Black skin are at risk of spots of skin that get lighter or darker in response to inflammatory skin conditions (postinflammatory hypopigmentation or hyperpigmentation). These changes usually go away within weeks or months.

A common complication is infection with bacteria, causing inflamed and itchy pustules.

Prevention

To help protect yourself or your child from heat rash:

• In hot weather, dress in loose, lightweight clothing that wicks moisture away from the skin. Don't wrap newborns in too many layers.
• In hot weather, limit physical activity. Stay in the shade or in an air-conditioned building. Or use a fan to circulate the air.
• Keep your sleeping area cool and well ventilated.
• Avoid creams and ointments that can block pores.
• Avoid drugs that cause sweating, such as clonidine, beta blockers and opioids.

Diagnosis

You don't need tests to diagnose heat rash. Your health care provider is usually able to diagnose it by examining the skin. A condition that looks like heat rash is transient neonatal pustular melanosis (TNPM). TNPM mainly affects newborns with brown or Black skin. It's harmless and clears up in a couple days without treatment.

Treatment

Treatment for mild heat rash is cooling the skin and avoiding exposure to the heat that caused the condition. Once the skin is cool, mild heat rash tends to clear quickly.

Self care

Tips to help your heat rash heal and to be more comfortable include the following:

• Press a cool cloth on your skin or take a cool shower or bath. It may help to let your skin air-dry.
• Avoid using oily or greasy moisturizers, cosmetics, sunscreens and other products that can block pores further. Instead use a moisturizer with wool fat (anhydrous lanolin), which helps prevent sweat ducts from getting clogged.

Preparing for your appointment

A visit with a health care provider usually isn't necessary for heat rash. If your rash is more severe, you may want to see your primary care provider or a doctor who specializes in skin disorders (dermatologist) to be sure it's heat rash and not another skin disorder.

Before you go, it's a good idea to list questions you have about your condition. For heat rash, questions to ask your health care provider include:

• What could have caused this rash?
• How can I treat it?
• Do I need to limit physical activity until the rash clears?
• How can I prevent it in the future?