What Causes Heat Rash? Why Prickly Heat Powder Can Make It Worse: A Dermatologist Explains the 4 Types and How to Tell It from an Allergic Rash
Thailand's rainy season does not mean cooler weather. It means higher humidity, and sweat evaporates even less easily. This is why heat rash patients at the clinic do not drop off after April ends.
Heat rash (Miliaria) is not a "weather allergy" and it is not acne. It is a condition in which blocked sweat ducts cause sweat to leak out and become trapped within the skin. How deep the sweat leaks determines what the rash looks like, how itchy it is and how it needs to be treated. These are details that most articles on the first page of Google skip.
What exactly causes heat rash?
There are 3 main steps that follow one another.
Blocked sweat ducts from waterlogged surface skin cells after prolonged sweating (maceration), together with a biofilm of Staphylococcus epidermidis bacteria that produces a sticky substance that plugs the duct opening. This matters because it explains why heat rash keeps coming back in the same people.
Sweat leaks out of the duct When the gland keeps producing sweat but the exit is blocked, pressure builds until the duct ruptures and sweat leaks into the surrounding tissue.
Inflammation The salt in sweat directly irritates the tissue, producing red bumps with itching and stinging.
The most common triggers in Thai patients
Hot, humid weather, especially during season changes and the rainy season when relative humidity is high.
Intense exercise followed by letting sweat dry on the body.
Tight clothing or synthetic fabrics that do not breathe.
Heavy creams or lotions, water-resistant sunscreens and repeated applications of powder. All of these block the sweat duct openings further.
Long periods of bed rest, or babies who are tightly swaddled, because their sweat glands are not yet fully developed.
High fever, and some medicines that interfere with sweat gland function, such as clonidine, bethanechol and isotretinoin.
The 4 types of heat rash: how to tell which one you have
The types of heat rash are classified by how deep the sweat duct is blocked. The deeper the block, the more severe the symptoms.
Miliaria crystallina (clear-bump heat rash) The blockage is in the most superficial layer. It appears as tiny clear, fluid-filled bumps on thin skin that break easily, with no redness and no itching. It is common after a high fever or in newborns, and clears by itself within 1-2 days without any medicine.
Miliaria rubra (red-bump heat rash, or prickly heat) The most common type. The blockage is in the middle layer of the epidermis. It causes small red bumps that itch and sting like pinpricks, appearing on the neck, chest, back, skin folds, under the breasts and where clothing edges are tight.
Miliaria pustulosa (pus-bump heat rash) This is miliaria rubra with a secondary bacterial infection, producing pustules on a red base. This group needs antibiotics, not just cooling down.
Miliaria profunda (deep heat rash) The blockage is deep, at the junction between the epidermis and the dermis. It appears as raised, skin-colored bumps resembling goose bumps, without redness and usually with little itching. It often follows repeated episodes of miliaria rubra. This is the most dangerous type, because the area loses its ability to sweat (anhidrosis). If it affects a wide area, the body cannot release heat, which carries a risk of heat exhaustion and heat stroke.
How is heat rash different from an allergic rash?
This is the question people search for most, and it is where misdiagnosis happens most often.
Trigger Heat rash is caused by heat, sweat and humidity. An allergic rash is caused by contact with substances such as soap, fragrance or metals.
Location Heat rash appears under clothing, in skin folds and where clothing is tight. A contact allergic rash appears exactly where the substance touched the skin, with clear borders.
Appearance Heat rash consists of small, uniform bumps of similar size. An allergic rash is a red patch with irregular edges that may ooze fluid and flake.
Timing Heat rash appears during or right after sweating. A contact allergic rash usually appears 24-72 hours after contact.
Response to cooling This is the clearest way to tell them apart. Heat rash improves within a few hours in a cool place. An allergic rash does not improve with cooling alone.
3 other conditions that look like heat rash and are often lumped together with it
Sweat-induced hives (Cholinergic urticaria) Very small bumps surrounded by a wide red halo. They appear quickly, within 5-10 minutes of the body heating up, and fade within 1 hour, unlike heat rash, which lasts several days. This group responds to antihistamines, not just to cooling down.
Inflamed hair follicles (Folliculitis) Pustules with a hair in the center, of uneven size, usually on the back, upper arms and buttocks.
Tinea versicolor (Pityriasis versicolor) Rings of fine scaly rash, lighter or darker than the surrounding skin, not raised bumps. It is a fungal infection and needs antifungal medicine.
Why does powder make it worse?
The most common advice on the Thai internet is to apply prickly heat powder and calamine lotion. Both help in the short term, but they have limitations that few people mention.
Powder gives a temporary cool, dry feeling, but the powder itself blocks the sweat duct openings further, especially when applied over skin that is still sweaty, where it clumps tightly in the skin creases. The more times a day it is reapplied, the longer the rash lasts.
Calamine does relieve itching, but when it dries it leaves a zinc oxide residue on the skin. The effect is the same: it blocks the sweat ducts. It can be used, but should be washed off each time once symptoms settle.
Heavy moisturizers and oils are a common culprit in adult patients who get worse the more they moisturize.
The right principle is to open the sweat ducts, not to seal the skin. So the first things to do are to lower the skin temperature and remove whatever is blocking it.
Treating heat rash effectively, in order
Step 1: Manage the environment
Move to a cool place. Use air conditioning or a fan and let the skin truly dry. This is the main treatment, not an add-on.
Bathe in room-temperature water with a gentle soap. Wash off all sweat and cream residue, then pat completely dry. Do not rub.
Apply a cold compress with a cloth soaked in cold water and wrung out, for 5-10 minutes, 2-3 times a day. It relieves both itching and inflammation.
Switch to loose cotton clothing and change right away when it is soaked with sweat.
Temporarily stop all creams, lotions, oils and powders on the rashy area.
Step 2: Topical medicine if symptoms do not settle within 3-5 days
Low- to medium-potency topical steroids such as hydrocortisone or triamcinolone. Choose a lotion or gel, not a cream or ointment. Use for a short period of 5-7 days to control the inflammation of miliaria rubra.
Zinc as a thin lotion can be used occasionally to relieve itching, but must be washed off.
Oral antihistamines help with itching at night but do not treat the cause.
In chronic, recurrent cases, a doctor may consider a topical medicine that reduces the bacterial biofilm on the skin to break the cycle of blockage.
Step 3: When there are pustules or a secondary infection
This needs topical or oral antibiotics depending on severity. Using a steroid alone at this stage will make it worse, so a doctor should assess first.
How many days does heat rash take to clear?
Miliaria crystallina 1-2 days. It clears by itself without any treatment.
Miliaria rubra 3-7 days, if you stay in a cool place and stop whatever is blocking the skin.
Miliaria profunda A week to a month, and sweating in the area may take even longer to recover.
If there is no improvement after more than 1 week even though you are staying in a cool place, it is probably not heat rash in the first place. See a dermatologist for a fresh diagnosis.
Warning signs that mean you should see a doctor
Pustules, red swelling, pain or fever This suggests a secondary bacterial infection.
No sweating in an area that previously had a rash, together with dizziness, palpitations or fatigue in the heat This suggests miliaria profunda with anhidrosis, which carries a risk of heat exhaustion.
No improvement at all within 1 week even after avoiding heat and stopping creams.
A rash spreading over the whole body with fever or large blisters, mouth sores or red eyes. A severe drug allergy must be ruled out urgently.
A baby with a rash who is also drowsy, not feeding or has a fever
How to prevent it from coming back
Stay in a well-ventilated place when sweating heavily, and shower to wash off sweat within 30 minutes after exercise.
Choose cotton or breathable fabrics and avoid tight clothing in hot weather and the rainy season.
Use a thin, non-comedogenic moisturizer only as needed. Avoid heavy textures and oils on the body in hot weather.
Choose a light gel or fluid sunscreen instead of a heavy water-resistant formula on days when you sweat a lot.
For babies, dress them in a single thin layer, keep the room cool, do not swaddle tightly in hot weather, and there is no need to apply powder to prevent heat rash.
Frequently asked questions
What should I apply for heat rash?
For very itchy miliaria rubra, use a low-potency steroid in lotion or gel form for a short period of 5-7 days, together with cold compresses. Avoid heavy creams, ointments and repeated powder applications, because they block the sweat ducts further. If there are pustules, antibiotics are needed, so a doctor should assess first.
Is heat rash contagious?
No. Heat rash is caused by blockage of your own sweat ducts and is not an infectious disease. However, if there are pustules from a secondary bacterial infection, those bacteria can spread to other areas of skin or to people in close contact.
How is heat rash different from acne?
Acne occurs in hair follicles, with white or black clogged heads, bumps of uneven size, and is linked to hormones and oil. Heat rash occurs in the sweat ducts, with bumps of similar, uniform size, no clogged heads, and is directly related to heat.
What should I do if heat rash appears on the face?
On the face, common causes are heavy sunscreen, makeup and long periods in the sun. Temporarily stop all products, wash your face with room-temperature water, apply a cold compress, and go back to only a thin moisturizer once the rash has calmed.
Is it abnormal to get heat rash every year?
Recurrence in hot weather is common and not dangerous in itself. But if it is getting more frequent, or you start to see raised skin-colored bumps that are neither red nor itchy, see a dermatologist, because it may be developing into miliaria profunda, which affects the body's ability to release heat.
Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed July 22, 2026
If the rash does not improve within 1 week, or if there are pustules with fever, we recommend seeing a dermatologist for a clear differential diagnosis. · Consult the Siam Dermatology medical team on LINE: @siamderm
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