The rash develops when the excessive amount of perspiration produced by overheated skin blocks the outlets of sweat glands. Since most sweat glands are situated around the face, neck, shoulders ad chest, heat rash mainly appears in these areas.

Heat rash is most common in babies since their sweat glands do not function as efficiently as those of older children or adults. In babies particularly, heat rash is caused by clothes and bedding that are too warm and not loose enough to allow air circulate.

Young children may also suffer from heat rash if they are too hot in bed, have a fever due an infection, or when they go on holiday to a hotter country. Adults mostly develop prickly heat in tropical climates e.g South Africa, which are hot and humid.