Heat Rash aka Prickly Heat: What To Do About It

Heat rash, or prickly heat, consists of clusters of tiny blisters filled with perspiration. It forms when pores become blocked and prevent the sweat glands from releasing perspiration, or when heat and humidity exceed the ability of the sweat glands to cool the body. The rash itches but does not become inflamed. It usually develops in the armpits and groin, and sometimes also on the chest, waist, and back.

Babies are especially vulnerable because their ability to sweat is not fully developed, and because they often wear or lie on waterproof materials, which intensify the effects of heat.

A similar rash, caused by sensitivity to ultraviolet light, may accompany prickly heat or occur independently. Referred to as a polymorphic light eruption, it produces itchy red spots within 24 to 48 hours of exposure to the sun. The rash appears mostly on the body, rarely on the face. Although children sometimes develop the rash, it is more common in young adults, especially Caucasians.

Diagnostic Studies and Procedures

In most cases, a doctor can diagnose heat rash by looking at the affected areas. If the rash looks infected, she may send a skin scraping to be cultured to determine whether a fungus or bacterium is causing the eruption.

Medical Treatments

A doctor may advise applying an over the-counter cortisone lotion or cream two to three times daily (always check with your physician before using these products on a baby). He may prescribe anti-histamines for severe itching or an anti-fungal medication containing cortisone, such as clotrimazole and betamethasone dipropionate (Lolrisone), if a secondary fungal infection has developed. A severe polymorphic light rash may require treatment with an oral steroid drug, such as prednisone.

Alternative Therapies

Remedies that alleviate itching and cool the skin usually work well.

Herbal Medicine. Herbalists often recommend adding thyme tea to bath water. Thyme contains thymol, an antiseptic substance that eases itching.

Hydrotherapy. Soaking in a tepid or cool bath is a soothing remedy. Do not apply soap to the rash. Instead, add 1 cup of oatmeal, 1 cup of baking soda, 2 cups of apple cider vinegar, or 2 cups of laundry starch to the bathwater. A word of caution: Bathe a baby only in plain water. Soaking in a whirlpool bath of cool water is also helpful. Alternatively, take one or two cool showers a day. After bathing, gently pat the skin dry rather than rubbing it, then dust the affected areas lightly with cornstarch or baby powder. Do not apply moisturizing creams and lotions or use bath oil. These products may further clog pores.


You can usually prevent heat rash by staying out of the sun and heat as much as possible, using air conditioning whenever available, and wearing lightweight, loose-fining, cotton garments. If a rash does develop, calamine lotion may ease the itching. Do not scratch. Expose the affected areas to the air when you can. Avoid strenuous exercise or any activity that causes sweating.

When a baby has heat rash, change diapers as soon as they are wet, then pat the skin dry and apply a dusting of cornstarch. Avoid laying an infant on a plastic-covered mattress or pad, instead, use a cotton sheet or towel.

Other Causes of Itchy, Red Rashes

Intertrigo, an inflammatory skin condition that is also aggravated by heat, produces a rash in areas where the skin rubs against itself, such as the groin and under the breasts and arms.

People with lupus often develop a splotchy red rash after exposure to the sun. Certain drugs also increase sun sensitivity and can cause a rash similar to that of a polymorphic light reaction. Other causes include skin allergies, contact dermatitis, and eczema. In babies, a diaper rash can resemble prickly heat.

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