The difference between eczema and heat rash, the cause and site of onset. Eczema is a type of dermatitis, usually caused by allergies or certain irritants, such as climate change, ultraviolet radiation, chemicals, household environment, etc. The affected areas are mostly the wrists, the bends of knees, the face, etc., and sometimes it can also spread to the whole body. Heat rash is caused by a viral infection and usually appears as a red rash on the palms of the hands, soles of the feet, and around the anus. symptom