Skin Allergies:
In the people who already have some sort of skin, allergies are more prone to get heat rashes whenever their surrounding temperature rises. If someone is already suffering from the skin allergy, then there are more chances of getting red itchy hives easily as the sweating increases with increasing temperature in the hot and humid weather. There is a skin condition known as cholinergic urticaria (UC) in which the skin breaks out more rapidly when you get sweaty after a workout or getting nervous. The breakouts result from the impaired function of the nerve fibers present in the sweat glands of the skin. The people having the condition of cholinergic urticaria (UC) have a lot more tendency to react to sweat whenever there is an increase in body temperature.