Heat rash shows up as clusters of small bumps or tiny blisters on skin that’s been overheated or trapped under sweat. The exact appearance depends on which type you have, but the most common form looks like a patch of small, red, inflamed bumps that feel prickly or itchy. It tends to appear in areas where skin folds or where clothing traps moisture: the neck, chest, groin, under the arms, and inside the elbows.
The Three Types Look Different
Heat rash comes in three forms, each with a distinct appearance based on how deep the sweat duct blockage occurs in your skin.
Miliaria crystallina is the mildest type and the most superficial. It looks like tiny, clear, fluid-filled bubbles sitting right on the skin’s surface, almost like small beads of water. These blisters are fragile and break easily. They don’t turn red, and they don’t hurt or itch. This type is especially common in newborns.
Miliaria rubra, commonly called prickly heat, is what most people picture when they think of heat rash. It appears as clusters of small, inflamed, blister-like bumps on a red or irritated base. The bumps feel rough to the touch and produce a characteristic prickling or stinging sensation, along with itching that can be intense. The surrounding skin often looks flushed. This is the most common type in both adults and children.
Miliaria profunda is the least common but most uncomfortable form. It develops when sweat leaks into a deeper layer of skin. The bumps are firm, flesh-colored, and look similar to goose bumps. They can be painful and may break open. This type typically follows repeated episodes of miliaria rubra and is more common in people who live in hot, humid climates for extended periods.
Where It Typically Shows Up
Heat rash gravitates toward areas where sweat gets trapped. On adults, the most common spots are the trunk, chest, back, neck creases, groin, and anywhere clothing sits tight against the body. Skin folds are especially vulnerable because moisture can’t evaporate there.
In babies and infants, heat rash frequently appears on the neck, shoulders, chest, and diaper area. Babies are particularly prone because their sweat ducts are smaller and more easily blocked. Overdressing an infant or swaddling too tightly in warm weather is a common trigger.
How to Tell It Apart From Other Rashes
Several skin conditions can look similar to heat rash at first glance, but a few details help you tell them apart.
Eczema produces dry, flaky, or thickened patches of skin that can appear anywhere on the body, including the hands, face, and inner elbows. The itching with eczema tends to be more intense and persistent than heat rash, and the affected skin often looks shiny from scratching or moisture loss. Heat rash, by contrast, feels rough with distinct tiny bumps and sticks to sweaty, folded areas of skin.
Hives appear as raised, smooth welts that can range from small spots to large patches. They’re typically pale or skin-colored in the center with redness around the edges, and individual welts shift location within hours. Heat rash bumps stay put and don’t migrate.
Folliculitis looks like small pimples centered around hair follicles. Each bump may have a visible white or yellow head, and you can often see a hair at the center. Heat rash bumps are more uniform and don’t follow hair follicle patterns.
What Causes It
Heat rash happens when sweat ducts get blocked and sweat can’t reach the skin’s surface to evaporate. The trapped sweat leaks into surrounding tissue, causing inflammation and those characteristic bumps. Hot, humid weather is the most obvious trigger, but anything that makes you sweat heavily while blocking airflow can do it: heavy exercise, tight synthetic clothing, thick creams or ointments, or even a fever during an illness.
Newborns develop heat rash frequently because their sweat ducts are still maturing and clog more easily. Adults who move from a temperate climate to a tropical one are also at higher risk, since their bodies haven’t adjusted to sustained heat and humidity.
How Long It Lasts
Most heat rash clears up within one to two days once you cool your skin down. Moving to an air-conditioned space, wearing loose and breathable clothing, and letting the affected skin air-dry are usually enough. More severe cases, particularly miliaria rubra with intense itching or widespread coverage, can take a week or longer to fully resolve.
Cool compresses and calamine lotion can ease the prickling sensation while you wait. Avoid heavy moisturizers or ointments on the affected area, since these can further block sweat ducts and make the rash worse.
Signs the Rash Needs Attention
Heat rash occasionally develops a secondary infection, especially if scratching breaks the skin. Watch for bumps that fill with pus instead of clear fluid, increasing pain or swelling, warmth spreading beyond the original rash area, or red streaks radiating outward. A fever that develops alongside a worsening rash is another signal that something beyond simple heat rash may be going on. These signs point to a possible bacterial infection that needs treatment rather than just cooling measures.