What causes chronic urticaria?

Chronic urticaria in clinical definition, refers to skin lesions almost every day attack, and the duration of more than 6 weeks. The reasons are very complex, there are many (about 3/4) can not even be completely found clear reasons. What are the common reasons? First, food or food additives. Such as fish, shrimp, seafood, milk, eggs, beef and mutton, etc., are allergens. Second, inhalants. Like pollen, dust mites, animal fur, etc. Third, infection. Whether viral or bacterial infection, it is possible to cause urticaria. Fourth, drugs. Urticaria is also very common reasons, such as penicillium, sulfa, etc. Fifth, physical factors. Such as mechanical stimulation, environmental stimulation of hot and cold factors, pressure, etc., are possible. Sixth, mental factors and endocrine factors, etc., may be the cause of chronic urticaria.