Causes of Heat Rash

Heat rash, also called miliaria or prickly heat, appears when sweat cannot leave the skin normally. It is common in hot, humid weather and in babies.

What happens

Sweat glands have narrow ducts that open on the skin surface. When a duct becomes blocked, sweat is trapped under the skin. This causes small blisters or bumps and sometimes itching or a prickling feeling.

What blocks the ducts

• Hot, humid weather, with heavy sweating that overwhelms the ducts
• Tight or non-breathable clothing, such as synthetic fabrics
• Thick creams, ointments and oils that cover the skin and trap sweat
• Heavy bedding or overdressing, which is a common cause in infants
• Physical activity or fever, which increase sweating
• Skin rubbing, for example under straps, in skin folds or under a waistband
• Immature sweat ducts in newborns, which block easily
• Bed rest in hot conditions, where the back stays sweaty
• Some medicines that increase sweating

Who is more likely to get it

Babies and young children, people who are overweight, people who live or work in hot, humid places, and bed-bound people.

Is it infectious?

Heat rash itself is not contagious. Scratching can introduce bacteria and cause infection.