The rash due to sun exposure is called physical allergy.Cholinergic urticaria is a subcategory of physical urticaria (aka hives) that is a skin rash brought on by a hypersensitive reaction to body heat. Symptoms follow any stimulus to sweat such as exercise (sometimes called exercise-induced urticaria), heat from the sun (which could also indicate solar urticaria), saunas, hot showers .
The visible hives (sometimes called heat bumps) appear as a multitude of small 2-3 mm welts typically surrounded by patches of red skin. The affected area will often feel warm and can be extremely itchy or exhibit a burning sensation. Typically the rash occurs on the upper trunk and the arms but can appear on other parts of the body.
The rash typically develops within a few minutes of a rise in body temperature but can take longer to appear visibly on the skin. The visible rash is often preceded by a general warming of the skin or itchiness. The hives last from a half an hour to several hours.
Cholinergic urticaria can be very difficult to treat.Drug treatment is typically in the form of antihistamines such as loratadine (Claritin), hydroxyzine, cetirizine and other H1 receptor antagonists.