Causes

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.