What Is Heat Rash

Overview

Heat rash, also known as prickly heat and miliaria, is a rash that appears when sweat is trapped underneath the skin. Symptoms can range from small, raised bumps that can be itchy to large, inflamed lumps.¹

Heat rash can affect both babies and adults and is especially common in hot, humid conditions. It is usually harmless and improves after a few days but severe cases may lead to further complications and require treatment from a healthcare provider.¹

Causes of heat rash

Heat rash occurs due toexcessive sweating which results in blocked sweat glands. This blockage causes the sweat to backflow into the dermis or epidermis.Instead of evaporating, sweat is trapped underneath the skin which causes irritation and bumps on the skin.¹

The general causes of heat rash include:¹

• Hot weather 
• Intense exercise
• Ointment - certain ointments can block sweat glands, especially in young children
• High fever
• Tight clothing

There are three different types of heat rash: miliaria crystallina, miliaria rubra and miliaria profunda.¹

Miliaria crystallina

This is the mildest form of heat rash which occurs when the opening of the sweat duct on the surface of the skin is blocked. It affects both adults and children (usually younger than 2 weeks old). The rash consists of small, clear, fluid-filled bumps that easily break and the most commonly affected sites are the head and neck. The rash will usually clear away within a day after the surface layer of skin rubs off.¹

Miliaria rubra

This is the most prevalent form of heat rash and is often called prickly heat. It involves blockage of the sweat glands in the deeper layer of the skin (epidermis) and triggersan inflammatory response. This results in tiny, inflamed bumps and may also cause itching or prickling in the affected area. The inflammatory response may cause painful symptoms which worsen during sweating and cause further irritation. In babies between the ages of 1 to 3 weeks, the most commonly affected areas are the groin, neck and armpit. In adults, the most common sites are are the neck, scalp and skin fold, especially where clothes rub on the skin.¹

If the inflamed bumps of miliaria rubra are filled with pus then this form is termedmiliaria pustulosa and may indicate a bacterial infection.¹

Miliaria profunda

This is a less common form of heat rash that affects the deepest layer of the skin called the dermis. It results in firm, large, inflamed bumps that can be itchy and painful and may break open. This type of heat rash is usually caused by people who have had multiple previous cases of miliaria rubra. In adults, the main affected area is the neck but it can also appear on the legs and arms. The rash usually appears within minutes to hours of sweating and usually clears up after sweating has stopped.¹

In both miliaria rubra and miliaria profunda, blockage of sweat glands in the affected areas may cause anhidrosis. This is where the sweat glands make little to no sweat which can lead to heat exhaustion due to the ineffective thermoregulation of the body.

Signs and symptoms of heat rash

The symptoms of heat rash are:

• Small bumps on the skin - normally on the neck, chest and upper back
• Itchy, prickly skin
• Mild swelling
• Tiny water blisters (in babies)

The rash is often red, althoughthis may not be as evidenton brown or black skin. It can appear anywhere on the body but usually manifestsin skin folds and where clothing rubs against the skin. In young children, the rash oftenappears on the shoulders, neck and chest but can also appear on the armpit, groin and elbow creases.¹

Management and treatment for heat rash

The main way to treat heat rash is to cool the skin and avoid exposure to the heat that caused the condition. This prevents you from sweating and irritating the rash.¹

To keep your skin cool you can:

• Wear loose clothing made of breathable fabrics
• Avoid tight and excessive clothing
• Take cool baths or showers
• Stay hydrated to avoid dehydration
• Switch to lightweight bedding
• Sleep in a well-ventilated room

Home remedies to ease the itching and prickly sensations include:

• Press something cold on your skin for up to 20 minutes e.g. a damp cloth or an ice pack wrapped in a tea towel
• Avoid perfumed shower gels or creams
• Pat or tap the rash. Do not scratch it

You can also speak to a pharmacist about your heat rash and they might recommend:

• Calamine lotion - to relieve discomfort
• Antihistamine tablets - to relieve symptoms
• hydrocortisone cream - children under 10 and pregnant women need to get advice from their doctor before using this

Although a visit to your doctor is not necessary for a heat rash, if your rash is severe it may be best to see a doctor that specialises in skin disorders (dermatologist) to make sure it is not another skin disorder.

Diagnosis

To diagnose heat rash, a healthcare provider will examine your skin. However, in severe cases, a punch biopsy or a tzanck smear may be needed to distinguish heat rash from other skin conditions such as herpes simplex.

Other skin disorders that resemble heat rash are:¹

• Herpes simplex
• Neonatal acne
• Fungal skin infections
• Grover disease
• Bacterial folliculitis
• Drug rashes

Risk factors

Heat rash can affect anyone from any race and age group. However, the risk factors that increase your likelihood of developing heat rashinclude:

• Being a newborn - newborn babies have immature sweat ducts
• Hot, humid conditions/countries
• Being physically active and doing vigorous exercise
• If you have a fever
• If you are on bed rest

Complications

Complications of heat rash include:¹

• Hypopigmentation and hyperpigmentation - spots of skin can get lighter or darker in response to an inflammatory skin condition 
• Bacterial infection - this can lead to inflamed itchy bumps filled with pus
• Anhidrosis - this can lead to poor thermoregulation and heat exhaustion
• Hyperhidrosis - excessive sweating in non-affected areas

FAQs

How can I prevent heat rash

To help prevent heat rash, you can:

• Wear loose, lightweight clothing in hot weather
• In hot weather, do not wrap babies in multiple layers
• Limit physical activity in hot weather 
• Keep your sleeping area well-ventilated
• Avoid ointments and creams that can block sweat glands

How common is heat rash

Heat rash is one of the most common skin rashes but mainly affects babies and young children.¹

Are heat rashes itchy?

A heat rash can sometimes be very itchy and uncomfortable. The rash can appear as small, inflamed blister-like bumps that can cause intense itching.¹

When should I see a doctor?

Heat rash usually clears up without much treatment by cooling the skin but you should see a doctor if:

• Your symptoms have not improved after a few days
• Your rash looks infected
• Your child has a rash and you are worried

Summary

In summary, a heat rash, also known as prickly heat or miliaria, is a skin condition that ariseswhen sweat is trapped underneath the skin. This results in small, red bumps that can be itchy and uncomfortable or even larger, inflamed lumps. Heat rash is commonly caused by hot, humid conditions and intense exercise which causes excessive sweating and results in blocked sweat glands. The rash usually clears up after a few days by cooling the skin but general treatment includes taking cool baths and showers, wearing loose clothing, pressing a cold cloth or ice pack to the affected area and sleeping in a well-ventilated room. You can also speak to a pharmacist who can recommend calamine lotion, antihistamine tablets or hydrocortisone cream. 

If left untreated, heat rash can lead to further complications such as hypopigmentation and hyperpigmentation, bacterial infection, poor thermoregulation and excessive sweating in non-affected areas. If your symptoms have not improved after a few days, your rash looks infected or your child has a rash, you should contact a doctor as soon as possible.