Heat Rash: Types, Causes, Treatment, Prevention and More

• Heat rash develops when sweat ducts become blocked and trap perspiration beneath the skin
• Symptoms can range from mild to severe and include red bumps, itchiness and a prickling sensation
• Main causes are hot, humid weather and friction on the skin’s surface 
• Treatments include antihistamines, topical creams, antibiotics and importantly, cooling the affected area

Miliaria, also known as prickly heat, heat rash or sweat rash is a common skin condition that occurs as a result of blocked sweat ducts. Sweat becomes trapped under the skin, leading to mild swelling and the appearance of clusters of small red bumps called papules. These symptoms are accompanied by an uncomfortable sensation of prickling and itching.

Symptoms can be mild, moderate or severe and is a common complaint in hot and humid environments. In most cases, the heat rash will resolve on its own; for moderate–severe cases or when infections develop, over-the-counter (OTC) or prescription medication can bring fast relief.

What Causes Heat Rashes?

The skin has multiple functions, and one vital role is regulating body temperature. When the body’s internal temperature rises, the skin’s sweat glands release water in the form of perspiration to the skin’s surface via ducts, where it evaporates. This action effectively cools the surface and regulates body temperature. 

When sweat ducts become blocked, this perspiration becomes trapped and causes inflammation characterized by fluid-filled papules. These can cause itching, stinging or a prickly sensation. 

These rashes will develop where sweat typically accumulates and is trapped by clothes. These areas include the neck, armpits, under the breasts, elbow creases, lower back and the groin. 

Rashes due to blocked ducts will also develop in areas where skin-to-skin contact chafes skin and causes irritation. The inner thighs and under the arms are particularly susceptible. 

Those who are obese, or overweight, often face greater obstacles in maintaining their health, and skin hygiene. This group is at greater risk of developing heat rash due to moisture trapped in multiple body folds which can also lead to the growth of bacteria and infection.   

Heat rashes can be induced by an increase in body temperature as a result of either physical activity (internal) or a hot, humid environment (external).

Types of Heat Rash

There are several types of heat rash, each classified according to the severity of sweat duct blockage. Symptoms are similar among the three, with some differences. Heat rash usually clears on its own within hours to several days, however severe forms will need medical treatment if infection sets in. 

Miliaria crystallina

As the mildest form of heat rash, miliaria crystallina affects the skin’s topmost layer, the epidermis. 

Instead of a rash, this form is characterized by numerous small blisters that look like beads of sweat, and are filled with a clear liquid. These blisters can rupture easily and typically, have no accompanying symptoms. 

The head, neck and shoulders are the most commonly affected areas for this type. Miliaria crystallin usually resolves on its own within 24 hours. 

Miliaria rubra

Miliaria rubra, also known as prickly heat or heat rash, is the most common form of heat rash. This type affects the deeper layers of the epidermis, the dermis, and is characterized by inflamed, prominent red blisters that cause intense itchiness as well as irritation.

Pain may be present because of this inflammation and symptoms can worsen with further perspiration. 

This form of heat rash develops more often in areas of the body where clothes rub on skin such as the trunk and extremities.

When pustules are present, then this condition is called miliaria pustulosa and is usually indicative of a bacterial infection

Miliaria profunda

Miliaria profunda is the most severe form of the three types and can develop after repeated bouts of prickly heat. It affects the deepest layers of the dermis, and is characterized by deep-seated flesh-colored papules. There is no itchiness associated with this form.

It is common in adults, and typically involves the torso rather than the extremities. 

Heat-related conditions such as heat stroke and heat exhaustion should also be taken into account if miliaria profunda is present.

Similar Conditions

Other skin conditions may be mistaken for miliaria due to similarities in symptoms.  

• Acne papules are inflamed lesions that resemble the red blisters that appear in miliaria rubra, but they are not itchy. They develop as a result of pores being blocked by excess oil, debris and dead skin. 
• Contact dermatitis develops when skin comes in contact with a substance that causes the skin to become inflamed. These include a wide range of triggers including detergents, fragrances, soaps and wool. Symptoms include a sensation of burning and the appearance of tiny blisters.
• Eczema can resemble a heat rash in terms of skin becoming red and inflamed but the similarities end there. With eczema, skin develops dry patches of bacteria-filled blisters and it is a chronic condition. In contrast, miliaria will resolve on its own once the causal factor is eliminated. 
• Folliculitis is an inflammatory skin condition that develops when bacteria forms within hair follicles, resulting in small, red pus-filled blisters. Contrary to heat rashes, this condition is a result of an infection.
• Grover’s disease is a rare skin disorder characterized by the sudden appearance of firm, raised water-filled blisters typically on the chest and back accompanied by moderate to severe itching. These develop due to a malfunction within the skin’s connective tissue. Symptoms can last from weeks to months, and the cause is not known. While women can develop this disease, it is primarily found in men over the age of 40.
• Hives (urticaria) present as raised reddish, itchy plaques that are a result of a skin reaction due to an insect bite or sting, drug or food reaction. In contrast, a heat rash appears in larger areas as smaller, red spots.

Risk Factors

Several risk factors have been identified that would make you more susceptible to heat rash including: 

• Applying heavy moisturizers and creams that block pores 
• Having a high body mass index (BMI) as this increases the risk of developing heat rash due to overlapping folds of skin
• Hot, humid environments 
• Intense exercise or physical activity or being bedridden or immobile for extended periods of time 
• Taking medications that increase sweat gland function, such as beta-blockers and opioids
• Wearing clothes made of synthetic fabrics that don’t allow skin to breathe 

Strategies for Preventing Heat Rash

To prevent heat rash, it is important to wear loose-fitting breathable fabrics to ensure air flow around the skin; cotton clothing is ideal.

After strenuous exercise, take a cool shower and change into clean, lightweight clothing. In hot, humid weather it is important to spend some time in a  well-ventilated or air-conditioned environment to allow your skin to recover.

Lastly, to restore your fluid levels, drink plenty of water throughout the day for both your skin and general health.

Treating Heat Rash

Except for severe forms of heat rash, symptoms should resolve on their own. To ease mild forms of rash, you can apply cool compresses to the affected areas and avoid scratching the rash as this will lead to further irritation and possibly infection. For moderate to severe symptoms, try the following:

• Miliaria rubra: reducing the inflammation is a key concern. Topical steroids such as triamcinolone 0.1% cream are typically recommended to ease swelling, redness and itching
• Miliaria pustulosa: topical antibiotics are prescribed to treat the bacterial infection
• Miliaria profunda: In one study, improvement of symptoms was shown with oral isotretinoin 40 mg per day alongside topical anhydrous lanolin

At-home remedies

You can find soothing relief with simple at-home treatments that can effectively cool down skin, reduce inflammation, itchiness and irritation, and reduce the risk of infection.

• Apply cold compresses using ice packs or a bag of frozen peas to reduce inflammation and relieve itchiness; ensure you wrap the compress in a thin towel to avoid damaging skin
• Apply calamine lotion in a thin layer to the affected area; it is very effective at soothing itchiness
• Colloidal oatmeal, when used within a bath or as a topical paste, has proven anti-inflammatory qualities 
• Aloe vera gel has both anti-inflammatory and antiseptic properties to relieve both swelling and itchiness
• Over-the-counter (OTC) antihistamines can help reduce irritation, itching and inflammation 
• OTC topical hydrocortisone 1% cream can relieve redness, swelling and itchiness 

When to see a doctor

Seek medical attention if heat rash symptoms do not resolve on their own after 1 week of treatment as this could be a sign of infection. Signs of infection also include worsening symptoms such as increased swelling and redness, or pus and pain. Other signs you should see a doctor include flu-like symptoms such as fever, sore throat and muscle aches.

Takeaway

Heat rash develops when sweat ducts become blocked and trap perspiration beneath the skin. This causes a mild to severe rash with red or white papules, with itchiness and a prickling sensation.

Heat rash tends to resolve on its own, and you can quickly relieve the discomfort with at-home treatments, OTC topical ointments or antihistamines. For more severe symptoms, see your doctor as this could be a sign of infection. 

The best way to relieve and manage heat rash symptoms is to keep your skin cool and dry. When possible, avoid activities, restrictive clothing and conditions that can increase perspiration and cause skin inflammation.