People who have a physical urticaria
have a physical trigger for their hives, such as pressure, heat, cold
, sunlight, water or exercise. Up to 20 to 30 percent of people with chronic urticaria have a physical cause.
Cholinergic or heat urticaria is a form of chronic hives that is caused by an increase in body temperature. Hives are caused by any increase in body temperature, such as hot showers, exercise, spicy foods, or being under too many covers in bed at night.