Cause of Heat Rash Skin has sweat glands. Normally, they secrete sweat which is a way of expelling heat out of body. Sometimes these ducts of sweat glands block preventing the release of heat from body leading to heat rash. Causes for blocked sweat glands are; • Hot and humid weather • Skin folds in neck, armpit and groin • Tight clothing • Layers of lotions and creams • Synthetic fabric • Prolong bed rest