The main reasons for the formation of melanin precipitation are ultraviolet radiation, endocrine factors and so on. 1. Ultraviolet radiation: The ultraviolet rays in sunlight are an important reason for the production of melanin. Melanin can block and reflect ultraviolet rays to protect the dermis and deep tissues. Under the stimulation of ultraviolet rays, melanin will accelerate the formation. So in order to avoid melanin deposition, usually need to go out sunscreen. 2. Endocrine factors: endocrine disorders are also the cause of the formation of some pigmented diseases, especially chloasma. Changes in the level of sex hormone during menstruation and pregnancy can affect the production of melanin. There are other reasons for the formation of melanin deposits. Once it occurs, it is recommended to visit a dermatologist to find a suitable solution with the help of a doctor.