Heat rash is a skin condition caused by blocked sweat ducts that trap perspiration beneath the surface of your skin. Instead of evaporating, the sweat leaks into surrounding skin layers, producing bumps, blisters, or inflamed patches. It’s one of the most common skin problems in hot, humid environments and affects both newborns and adults, though for different reasons.
Why Sweat Gets Trapped
Your body has millions of eccrine sweat glands distributed across nearly every inch of skin. When you overheat, these glands push sweat to the surface to cool you down. Heat rash develops when the tiny ducts carrying that sweat to the surface become blocked or disrupted, usually because of excessive sweating in hot or humid conditions. The sweat has nowhere to go, so it leaks into the epidermis or deeper layers of skin, triggering irritation or inflammation depending on how deep the blockage occurs.
Several things can cause that blockage: heavy clothing that traps moisture against the skin, thick creams or ointments that physically seal sweat ducts, prolonged bed rest with fever, or simply spending extended time in a humid environment. In newborns, the sweat ducts haven’t fully matured in the first days of life, which is why about 1.3% of neonates develop heat rash within 48 hours of birth.
Three Types, Three Depths
Not all heat rashes look or feel the same. The type you get depends on where in the skin the sweat duct is blocked.
Miliaria Crystallina (Surface Level)
This is the mildest form. The blockage sits at the very top layer of skin, producing tiny, clear, fluid-filled blisters that look almost like beads of sweat sitting on the surface. They don’t itch or hurt, and because there’s almost no inflammation at this depth, they typically resolve on their own within a few days without any treatment. This is the type most commonly seen in newborns.
Miliaria Rubra (Mid-Layer)
This is the classic “prickly heat” that most people picture when they think of heat rash. Sweat leaks into deeper layers of the epidermis, causing clusters of small, inflamed, reddish bumps that itch and sting. It’s the most uncomfortable form and the one most likely to send you searching for relief. The bumps sometimes fill with pus, a variation called miliaria pustulosa, which looks more alarming but follows the same basic process.
Miliaria Profunda (Deep Layer)
The least common and most serious type. Here, sweat escapes into the dermis, the deepest functional layer of the skin. It produces firm, flesh-colored bumps that resemble goose bumps and may break open. Miliaria profunda typically develops after repeated bouts of miliaria rubra. The bumps themselves may not itch much, but the real concern is what happens to your body’s cooling system: with enough sweat ducts blocked at this depth, your body loses its ability to regulate temperature effectively, which can lead to heat exhaustion.
Where It Shows Up
Heat rash clusters in areas where sweat gets trapped. In adults, that means skin folds (under the breasts, in the groin, behind the knees) and wherever clothing presses tightly against the body, like waistbands, bra lines, or snug collars. In infants, the rash tends to appear on the neck, shoulders, and chest, and often spreads to the armpits, elbow creases, and groin. Babies who are swaddled tightly or overdressed are especially prone.
Heat Rash vs. Other Skin Conditions
Heat rash can look like several other conditions at first glance. A few distinguishing features help sort it out. Heat rash is tied directly to heat and sweating, so it appears during or shortly after exposure to warmth and improves once you cool down. Hives, by contrast, produce raised welts that can appear anywhere on the body regardless of temperature and often shift location within hours. Contact dermatitis typically has a clear boundary matching whatever irritant touched the skin (a watchband, a plant leaf) and doesn’t cluster around sweat-prone zones.
Folliculitis, an infection of hair follicles, can look similar to miliaria pustulosa because both produce small pus-filled bumps. The key difference is location: folliculitis centers on individual hair follicles and can appear in cooler weather, while heat rash bumps form around sweat duct openings and track with heat exposure. If a rash persists after you’ve cooled your skin for a day or two, or if it develops spreading redness, warmth, or fever, it may be something else entirely or may have developed a secondary infection.
How to Treat It
The single most effective treatment is removing the trigger. Get to a cooler, drier environment, remove excess clothing, and let your skin air out. For miliaria crystallina, that’s all you need. The blisters resolve in days.
For miliaria rubra, cooling down is still the foundation, but you can layer on a few things for comfort. Calamine lotion soothes itching and helps dry out the bumps. Cool compresses reduce inflammation. Avoid heavy moisturizers or petroleum-based products, which can further block sweat ducts and make things worse. If the itching is severe and not responding to basic measures, a mid-potency steroid cream applied to the affected area a few times a day can bring relief. For particularly stubborn or widespread cases, medications that reduce sweat production may be prescribed.
Most cases of miliaria rubra clear within a week once the skin can breathe and cool down. Miliaria profunda takes longer and sometimes requires medical supervision, particularly if your ability to sweat is noticeably impaired, since that raises your risk of overheating.
Preventing Heat Rash
Prevention comes down to keeping sweat flowing freely off your skin rather than pooling beneath it. Lightweight, loose-fitting clothing made from breathable fabrics (cotton, linen, or moisture-wicking synthetics) makes a significant difference compared to tight polyester or nylon that seals moisture against the body. In humid climates, air conditioning or fans help evaporate sweat before it overwhelms the ducts.
If you exercise in the heat, change out of damp clothing as soon as possible. Showering promptly after sweating removes the salt and debris that can clog duct openings. For infants, the simplest rule is to dress them in about the same number of layers you’d be comfortable wearing. Overdressing a baby in warm weather is one of the most common triggers for neonatal heat rash.
People who’ve had miliaria rubra before should be especially cautious, since repeated episodes increase the risk of developing the deeper, more serious profunda form. If you know you’re prone to heat rash, gradually acclimating to hot environments over several days, rather than jumping into intense heat exposure all at once, gives your sweat glands a better chance of keeping up.