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.

Two types of heat rash

Heat rash

Miliaria rubra (A), one type of heat rash, appears as clusters of small, inflamed blister-like bumps that can produce intense itching. Miliaria crystallina (B), another type of heat rash, appears as clear, fluid-filled bumps that don't hurt or itch.

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.

Sweat glands

Sweat glands

Eccrine sweat glands occur over most of the body and open directly onto the skin's surface. Apocrine glands open into the hair follicle, leading to the surface of the skin. Apocrine glands develop in areas with many hair follicles, such as on the scalp, armpits and groin. Eccrine sweat glands are involved in hyperhidrosis, though apocrine glands may play a role as well.

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.

May 06, 2022

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  9. Smith CC, et al. Primary focal hyperhidrosis. https://www.uptodate.com/contents/search. Accessed March 2, 2022.

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