Adults usually develop heat rash in skin folds and where clothing causes friction. In infants, the rash is mainly found on the neck, shoulders and chest. It can also show up in the armpits, elbow creases and groin.

Types of heat rash

The types of miliaria are classified according to how deep the blocked sweat ducts are. Signs and symptoms for each type vary.

• The mildest form of heat rash (miliaria crystallina) affects the sweat ducts in the top layer of skin. This form is marked by clear, fluid-filled blisters and bumps (papules) that break easily.
• A type that occurs deeper in the skin (miliaria rubra) is sometimes called prickly heat. Signs and symptoms include red bumps and itching or prickling in the affected area.
• Occasionally, the fluid-containing sacs (vesicles) of miliaria rubra become inflamed and pus-filled (pustular). This form is called miliaria pustulosa.
• A less common form of heat rash (miliaria profunda) affects the dermis, a deeper layer of skin. Retained sweat leaks out of the sweat gland into the skin, causing firm, flesh-colored lesions that resemble goose bumps.

Heat rash develops when some of your sweat ducts clog. Instead of evaporating, perspiration gets trapped beneath the skin, causing inflammation and rash.

It's not always clear why the sweat ducts become blocked, but certain factors seem to play a role, including:

• Immature sweat ducts. A newborn's sweat ducts aren't fully developed. They can rupture more easily, trapping perspiration beneath the skin. Heat rash can develop in the first week of life, especially if the infant is being warmed in an incubator, is dressed too warmly or has a fever.
• Tropical climates. Hot, humid weather can cause heat rash.
• Physical activity. Intense exercise, hard work or any activity that causes you to sweat heavily can lead to heat rash.
• Overheating. Overheating in general — dressing too warmly or sleeping under an electric blanket — can lead to heat rash.
• Prolonged bed rest. Heat rash can also occur in people who are confined to bed for long periods, especially if they have a fever.

Factors that make you more prone to heat rash include:

• Age. Newborns are most susceptible.
• Tropical climates. People living in the tropics are far more likely to have heat rash than are people in temperate climates.
• Physical activity. Anything that makes you sweat heavily, especially if you're not wearing clothing that allows the sweat to evaporate, can trigger heat rash.

Heat rash usually heals without problems, but it can lead to infection with bacteria, causing inflamed and itchy pustules.