The common causes of red rashes on hot days are mosquito bites, prickly heat and contact dermatitis. 1. Mosquito bites: hot days are prone to mosquitoes, if the protective measures are not in place, it is easy to be mosquito bites, when the number of mosquito bites is relatively large, there will be a small red bumps on the skin, which looks like a red rash. 2. prickly heat: hot days, people tend to sweat, if the sweat ducts are blocked, resulting in poor sweat discharge, armpits, elbows, neck and other parts of the skin will grow red prickly heat, that is, red rash. 3. Contact dermatitis: hot skin moisture and a lot of sweat stimulation, sensitive people may appear skin allergies, there will be a red rash on the body, accompanied by itching. If the hot days on the body red rash need to timely medical treatment, by professional doctors to clarify the cause of the disease, the development of individualized diagnostic and treatment programs.