In cases of chronic urticaria, the cause is much more difficult to determine. Very often, it is never identified. Chronic urticaria caused by foods is more common in children than in adults. Tartrazine is thought to be a cause of urticaria. Environmental or physical factors, such as exposure to heat, sunlight, or the cold can cause hives in some people.
Hives occur when fluid leaks through the cells surrounding tiny blood vessels and enters the skin.
This happens when a substance called histamine is released from a specialized type of cells, called mast cells, which lie along the blood vessels in the skin. Allergic reactions to foods or medications can cause the release of histamine by mast cells. It is usually not difficult to tell when a person has hives. The hives that appear cause considerable itching and can make a person extremely uncomfortable. The discomfort caused by hives can increase distress by making it difficult to sleep.