Skin rash caused by heat: a complete guide to causes, types, and treatment

When temperatures rise and humidity intensifies, many people begin to experience skin discomfort. One of the most common problems during hot periods is skin rash, a reaction that mainly affects the neck, shoulders, chest, and other areas where the skin folds over itself. Although it is a frequent and usually harmless condition, it is important to understand what causes it and how to manage it effectively.

Understanding miliaria: what happens when a skin rash develops

Heat rash, technically known as miliaria, occurs due to a specific process in the dermis. According to Dr. Angela Lamb, board-certified dermatologist at Mount Sinai, “when sweat glands and skin ducts become blocked due to high heat and humidity, sweat gets trapped beneath the epidermis, resulting in small bumps or blisters.”

Although “heat rash” is the common term people use, it is not exactly how medical professionals officially diagnose this problem. The mechanism is simple but uncomfortable: when the sweat system cannot function properly under extreme heat and humidity, sweat buildup causes that characteristic skin irritation.

This type of skin rash can appear in different forms, depending on where the obstruction occurs in the skin layers. The common factor in all cases is the sensation of itching, although the severity varies depending on the specific type.

Three types of heat-induced skin rash: how to identify each one

There are three distinct classifications of miliaria, each with particular features that differentiate them.

Crystalline miliaria: the mildest form

Crystalline miliaria is the mildest presentation of skin rash and occurs most frequently in newborns. It forms when obstructions appear at the superficial openings of sweat ducts. Visually, it looks like small sweat drops trapped under raised skin. A distinctive feature is that it is generally not inflamed, does not cause significant itching, and is less red than other types of skin rash. In some cases, it may not even show notable redness.

Miliaria rubra: the most common type

Miliaria rubra is the most common form of skin rash and can affect newborns and up to 30 percent of adults living in warm, humid regions. Dr. Rajani Katta, board-certified dermatologist practicing in Houston, describes this variant as “very red, with intense itching and filled with bumps.”

This type of rash occurs when sweat is blocked in the middle of the epidermis, a deeper layer than the previous type. The obstruction at this level causes a more pronounced inflammatory reaction, explaining why it causes greater discomfort and irritation.

Deep miliaria: the least common but most bothersome

Deep miliaria is the least frequent presentation of skin rash. It occurs when sweat glands are obstructed in the dermis, the deepest layer where this condition can manifest. Although it does not cause as much itching as miliaria rubra, the resulting bumps tend to be firmer, more red, and potentially more painful.

All these variants of skin rash tend to develop in specific areas of the body: armpits, groin, neck, abdomen, and under the breasts. Babies often show symptoms on the neck, shoulders, and chest. Tight clothing increases the likelihood of developing this problem, as it restricts sweating and creates microenvironments of moisture against the skin.

Newborns are particularly susceptible to skin rash because they have less developed sweat ducts that clog easily, along with multiple skin folds. Dr. Katta warns that the risk increases “especially if they are overdressed when it’s hot outside.”

Differentiating skin rash from other dermatological conditions

Skin rash is often confused with other skin conditions, which can lead to inappropriate treatments. Correctly identifying the problem is essential for proper management.

Skin rash versus eczema

Eczema is a chronic inflammatory condition that can worsen significantly with extreme weather and summer humidity. It causes itching, dry skin, eruptions, blisters, and potential infections. According to Dr. Lamb, people with mild eczema might not notice it during mild weather, but when temperatures rise, they may experience flare-ups that are easily mistaken for skin rash.

“Both conditions can be red and itchy, and sometimes occur in the same areas,” explains Dr. Katta. However, there is an important visual difference: “eczema tends to look more irregular and scaly.” Affected eczema areas have poorly defined borders and a flatter texture, whereas skin rash typically appears as distinct, more defined bumps.

Skin rash versus polymorphic light eruption

Another condition often confused with skin rash is polymorphic light eruption. It produces an itchy or burning rash characterized by small inflamed bumps or slightly raised patches of skin. Its distinctive feature is its association with sun exposure. As Dr. Lamb explains, it is “a unique condition that people notice when they take a sunny vacation for the first time in a long while.”

Although it can occur at any time, it typically appears when people travel in spring or early summer and are exposed to much more solar radiation than their bodies are used to, especially after winter months. This causes “rashes usually confined to sun-exposed areas.”

The fundamental difference is that “unlike skin rash, polymorphic light eruption is not caused by temperature or humidity, but specifically by solar radiation,” clarified Dr. Lamb.

Effective strategies to treat and prevent skin rash

Experts agree that anyone can develop skin rash under the right conditions. If you have already developed it, the initial strategy is crucial: move to a cooler environment.

Immediate treatment

The first step is to get out of the heat and seek shade, avoiding peak hours of high temperature and humidity. Remove tight clothing that impedes sweat evaporation. It is equally important to rinse off thick lotions like sunscreens or dense moisturizers, as these products can clog pores and prevent the skin from breathing properly.

According to Dr. Nadine Kaskas, a dermatologist at Mount Sinai, the next step is “taking a cold shower or applying cold compresses with a clean cloth.” To relieve discomfort, over-the-counter ointments like calamine lotion can be used. If irritation becomes especially intense, consulting a doctor is recommended, as a prescription topical steroid cream might be necessary.

Without specific treatment, skin rash generally subsides on its own once away from the hot, humid environment. However, there is a small risk of developing an infection if blisters open and the skin is not kept clean and protected.

Preventive measures

If you must stay in warm environments, Dr. Kaskas recommends taking proactive measures to prevent overheating, which increases susceptibility to skin rash. Strategies include seeking shade regularly, taking breaks from physical activity, avoiding excessive exertion, and staying well-hydrated.

Dr. Lamb favors portable fans that can be hung around the neck, as they provide direct ventilation. Wearing breathable clothing is critical: loose, lightweight garments that allow sweat to evaporate from the skin, according to Dr. Katta. This type of clothing keeps the body cool and facilitates air circulation.

The importance of early recognition

A key point emphasized by Dr. Katta is that “skin rash acts as a warning sign” that sweat glands or ducts are not functioning properly. This can make one more vulnerable to more serious conditions like heat exhaustion or heat stroke. If you notice any skin irritation while exposed to heat, it is essential to take immediate measures to cool down and assess whether medical intervention is needed.

Effective management of skin rash involves early recognition, prompt response to initial symptoms, and implementing preventive measures for future heat exposures.