A heat rash is a common rash on the skin that can show up when you’re hot or you sweat a lot. It can make parts of your skin feel prickly or sting due to overheating. It can itch a lot, but it’s not dangerous.
Some people call a heat rash “prickly heat.” Doctors call it miliaria crystallina or miliaria rubra.
Anyone can get a heat rash, but it's more common in babies and young children. Active people, newborns in incubators, and people on bed rest with fever also are more likely to get it.
What Causes a Heat Rash?
Heat rashes happen most often in humid, hot weather.
When you sweat too much, the ducts from the sweat glands in your skin become blocked.
This causes your sweat to leak into surrounding tissue, which leads to irritation. You may feel the prickly sensation that gives this condition its name.
What Does a Heat Rash Look Like?
On lighter skin, heat rash looks like tiny raised bumps surrounded by red, irritated skin. On brown, darker brown, black, or darker skin tones, the rash may have little to no redness. This might make it less visible and harder to spot.
Instead, the bumps might appear gray, purple, or white. For some people with darker skin tones, the inflammation from the heat rash could cause the affected area to become lighter or darker. Doctors call this postinflammatory hyperpigmentation. Typically, it lasts for a few weeks or months.
Heat rash usually happens on clothed parts of your body, like your:
- Upper chest
Types of Heat Rash
There are three types of heat rash:
Miliara crystallina: This is the mildest type. It's known for little, clear bumps filled with fluid. The bumps can break easily and are on the skin's surface.
Miliara rubra: You may hear this called "prickly heat" because you may have an itchy or prickly feeling along with small, inflamed bumps on the skin. If the bumps become filled with pus, your doctor will call it miliara pustulosa.
Miliara profunda: This type affects the skin's deepest layer. The bumps in this type of heat rash are firm and painful or itchy.
How Long Does a Heat Rash Last?
Heat rash usually lasts about 2 to 3 days. Call your doctor if it doesn't go away after 3 or 4 days or if it seems to be getting worse.
Can a Heat Rash Spread?
Yes, it’s possible for a heat rash to spread to other parts of your body. That happens due to clogged sweat pathways.
Rashes are more likely to spread on parts of your body where your clothes are tight against your skin. That’s extra true when you sweat.
You don’t have to worry about spreading a heat rash to other people, though. It’s not contagious.
Heat Rash Treatment
There’s no standard treatment to get rid of a heat rash. It usually gets better once your skin cools off.
You can do some things at home to get relief:
- Move to a cooler, less humid place.
- Take a cool shower or bath.
- Wear fewer layers of clothes, and loosen your clothes.
- Resist the urge to scratch your skin, or it could become infected.
- Keep your affected skin dry.
- Try calamine lotion, 1% hydrocortisone cream (not hydrocortisone ointment), or a prescription cream to ease your itching.
Don’t use ointments or creams that keep your skin moist. Also skip products that could clog your pores. Avoid using things like:
- Baby powders
- Scented lotions
- Lotions with petroleum or mineral oils
How Can You Prevent Heat Rash?
The key is to try to stay cool and avoid sweating a lot. Some tips that can help are:
- Wear loose, lightweight clothes made of cotton.
- Cool off with fans, cool showers, and air conditioning when you can.
- Try to do things outdoors during times of the day when it’s cooler.
When to Call Your Doctor About a Heat Rash
Call your doctor about a heat rash if:
- The rash is severe or painful or does not go away on its own within a few days.
- You get an infection in an area where you recently had heat rash.
- You have a fever or any other signs of illness.
- The rash is bright red or has streaks.
- The rash starts after you have been taking an antibiotic or new medication.
- The rash leaks pus or forms scabs.