Heat rash: Symptoms, appearance, and causes

Heat rash, also known as miliaria rubra, prickly heat, summer rash, or wildfire rash is common and can be uncomfortable.

It happens when a blockage of the sweat glands causes perspiration to be caught in the deeper layers of the skin. Inflammation, redness, and blister-like lesions can result.

People who are overweight, and those who sweat easily, are more likely to get prickly heat. Babies and children are more prone to it because their sweat glands are not fully developed.

Fast facts on heat rash

Blockage of the sweat glands is the primary cause.

Symptoms include red bumps on the surface of the skin.

The rash is often described as ‘prickly’.

Over-the-counter (OTC) topical antibacterials are a common treatment

Symptoms

Symptoms include small red bumps, called papules, which may itch or cause an intense, prickling sensation. They can appear in different parts of the body at the same time.

The most common places for heat rash to occur are the face, neck, under the breasts, and under the scrotum. It can also appear in skin folds and in areas of the body that rub against clothing, such as the back, chest, and stomach. These are all places that tend to sweat more often.

A related condition, which may happen at the same time, is folliculitis. This occurs when hair follicles become plugged with foreign matter, such as dead skin cells and sebum, that can later become infected, also causing inflammation.

Close-up image of heat rash, red bumps (papules) on the skin
This is what heat rash looks like, small red bumps (papules) on the skin.

Heat rash is sometimes broken down into three types:

Miliaria crystalline – the most common form. Includes small clear (or white) bumps filled with fluid (sweat) on the surface of the skin. There is no itching or pain. This is more common in babies than adults.

Miliaria rubra – also called prickly heat, it is associated with red bumps on the skin, inflammation, and a lack of sweat in the affected area. It occurs in deeper layers of the skin and is therefore more uncomfortable. If the pumps progress and become pus-filled, it is referred to as miliaria pustulosa.

Miliaria profunda – least common form of heat rash. Occurs in the dermis – the deepest skin layer. It can recur and become chronic. Miliaria profunda produces relatively large, tough, flesh-colored bumps.

Causes

Woman's face sweating excessively.
Heat rash occurs when sweat glands are blocked.

Heat rash, or miliaria, happens when the sweat gland ducts get plugged due to dead skin cells or bacteria, such as Staphylococcus epidermidis, a common bacterium that occurs on the skin and is also associated with acne.

If the bacteria get inside the plugged sweat glands, it can lead to inflammation that appears as a rash.

Anything that causes a person to sweat more can lead to miliaria. It is common in humid, tropical climates.

Long periods of bed rest, due to immobility, and illness, can make a patient sweat, especially if using an electric blanket and other warm bedding.

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