Q&A: What causes heat rash? | Expert Opinion - The Philadelphia Inquirer

As sweat evaporates off the skin, it cools our bodies under hot conditions. When this process goes awry, heat rash can develop. Heat rash occurs when excessive sweat clogs sweat glands, trapping perspiration. As sweat builds up within the sweat glands, small clear blisters start to show on the skin. Sweat then leaks out from the gland into the surrounding skin, causing inflammation that appears as red bumps on the skin's surface.

Heat rash, also referred to as "prickly heat" and miliaria, affects people of all ages and genders, and frequently appears on newborns — often on the neck, chest, and back — due to immature sweat ducts. In adults, heat rash can be widespread. Because sweat cannot easily evaporate from skin folds, the condition frequently arises in the inner elbows, backs of the knees, under the breasts and abdomen and in the groin. The risk of developing heat rash increases with exposure to tropical climates, increased physical activity, overheating, severe sunburn, tight clothing, extended bed rest and certain medications.

Three major types of heat rash vary in appearance and symptoms:

1. Miliaria crystallina is the mildest type. It affects sweat ducts at a superficial level. Clear fluid-filled blisters appear and easily rupture. This type generally resolves quickly.

2. Miliaria rubra is the form commonly described as "prickly heat." In this type, inflammation extends deeper into the skin. Prickly heat is characterized by red bumps that are extremely itchy. Others may describe sensations of burning or stinging. In some cases, bacteria living on the skin can infect the blisters, causing pus formation.

3. Miliaria profunda arises after prolonged sweat retention in the skin. In miliaria profunda, the inflammation extends deeper into the dermal layer of skin. Firm, skin- to white-colored bumps arise and resemble goose bumps. In this type, a large number of sweat glands are clogged and nonfunctional, creating a higher risk for overheating. This form of heat rash tends to occur only in the tropics.

After recovering from miliaria, some people experience a persistent reduction in their ability to sweat. This can last for several weeks and interfere with the ability to exercise or work in a hot environment.

The best treatment for miliaria is to seek a cool environment. Apply cool compresses and take soothing cool baths. Regular use of powders in skin folds can help wick up moisture. Avoid thicker creams and ointments that further moisten the skin. Cases have improved with over-the-counter topical lanolin ointment. Topical and oral prescriptions from your doctor can help. If you have severe symptoms, including fever, chills or swollen lymph nodes, call your doctor. If you've suffered from heat rash in the past, identify the triggers and remember to stay cool.

Elizabeth Jones is an assistant professor in the department of dermatology and cutaneous biology at Jefferson.