Sunlight
Another form of physical urticaria can be caused by the sunlight, known as solar urticaria. People with this condition develop distinct red welts within minutes of exposure to the sun, regardless of the outside temperature. This is in contrast to heat rash, a commonly misattributed condition caused by sweat ducts that have become clogged. Other kinds of physical urticaria tend to involve an overreaction by certain immune cells to a stimulus like cold or vibration, but solar urticaria might be caused when photosensitive chemicals in the body are created in response to UV radiation, which then somehow trigger an unwarranted antibody response.
