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Heat rash—also called prickly heat—causes red, itchy bumps on your skin. It is often a result of exercising in hot, humid weather. Infants, owing to their immature sweat glands, are also susceptible. Heat rash is normally a minor medical condition, but it can be an important warning signal to stop exercising and cool off. If the rash worsens over several days, if you notice swelling, warmth, increased pain or pus, or if you experience fever, chills or swollen lymph nodes, you should see your doctor.


Heat rash, medically known as milaria, is classified as a heat-related illness. It develops as a result of sweat glands becoming clogged, trapping perspiration under your skin, which in turn reacts by forming blisters and bumps and itching, stinging and prickling. Milaria crystallina affects only the most superficial level of skin, and causes clear, painless tiny bumps. With milaria rubra, the most common form, red, itchy blisters and bumps occur deeper in the epidermis. Milaria profunda is less common and afflicts people who have had prior episodes of heat rash. It causes flesh-colored bumps and lack of perspiration, which may trigger heat exhaustion. Heat rash normally goes away on its own after seven to 10 days, but in some cases a secondary bacterial infection can occur.


Being in a hot, humid climate, wearing synthetic fabrics such as polyester and spandex, which don't allow for proper evaporation of sweat, performing strenuous exercise, taking certain medications, and using pore-clogging, heavy creams and ointments can all increase the risk of heat rash. Infants, elderly people, overweight people and people who work outdoors in the heat are most at risk.

Treatment and Prevention

Treatment for heat rash begins with stopping exercise and moving to a cooler, preferably air-conditioned, area. Calamine lotion, cool compresses and anhydrous lanolin can help soothe heat rash. In severe cases, your doctor may prescribe topical steroids. You can use antihistamines to relieve itching. Prevent heat rash by wearing loose, lightweight, light-colored cotton clothing and staying in air-conditioned areas. If you must exercise, limit your workouts to morning or evening hours. Be particularly careful when exercising if you are a visitor to a climate that is hotter than the one to which you are accustomed. The Centers for Disease Control and Prevention recommends staying well hydrated by drinking cool, non-alcoholic beverages. Sports drinks, which replenish salt and minerals lost through perspiration, are also good options.

Heat-Related Illnesses

Ignoring the development of heat rash and continuing with strenuous exercise in hot and humid weather when sweat glands are clogged could lead to a more serious heat-related condition. Heat exhaustion is indicated by profuse sweating, muscle cramps, pale skin, fatigue, dizziness, headache, nausea and fainting. Untreated heat exhaustion can lead to heat stroke, a life-threatening emergency. Cool off immediately by getting to an air-conditioned area. If symptoms of heat exhaustion last longer than an hour, seek medical attention. Heat stroke occurs when the body's cooling mechanisms fail and is indicated by a fever of 103 degrees or higher, red, hot dry skin, headaches, dizziness, nausea and confusion. The CDC advises calling 911 immediately. You can sponge or immerse a person with heat stroke in cool water while waiting for help to arrive.