The etiology of melasma is still inconclusive, and there exist a variety of influencing factors, most of which are not determined. For example, pregnant women are prone to facial pigmentation, and some of the pigmentation produced during pregnancy will remain for a long time after delivery to form stubborn melasma. It has been reported that female melasma patients have increased levels of luteinizing hormone and decreased estrogen, while male melasma patients have increased levels of luteinizing hormone and decreased testosterone. Many melasma shades vary with the female physiological cycle. It has also been reported that oral contraceptive pills can trigger melasma. 2.Ultraviolet light has an important role in the formation of melasma, among which ultraviolet B is more important to the formation of melasma, which can increase the synthesis of melanin by stimulating the activity of tyrosinase. In almost all treatments, there is a strong emphasis on avoiding sunlight while treating melasma. Applying sunscreen can effectively prevent melasma. 3. Microbial imbalance The formation of melasma may be related to the micro-ecological imbalance on the skin surface. There is a significant increase in Propionibacterium acnes, Micrococcus and Gram-negative bacilli in the melasma area compared to normal people, and these bacteria are able to produce brown pigment and inulin. The reduction in the number of live bacteria and isolation rate of resident anaerobic bacteria reduces skin colonization resistance, and the competitive inhibition and interference between flora is weakened, leading to the proliferation of other pigment-producing micrococci, resulting in skin pigmentation. 4.Free radical damage Free radicals can damage various tissues of human body, and the formation of melasma may also be related to free radicals. Excessive production of oxygen free radicals in the body and reduced activity of antioxidant enzymes may trigger melasma. It is even thought that the change of nitric oxide in vascular endothelial cells may also be related to melasma. 5.Destruction of skin barrier Improper makeup and face washing habits and long-term over-stimulation of skin make the skin barrier weakened, thus producing chloasma. 6.Heredity Some people believe that melasma has hereditary tendency, and about 30% of melasma patients have family history[. Although there are many related factors, it is still difficult to confirm the real cause, and most patients do not have a clear cause. The top 2 related factors are sun exposure (51.6%) and excessive skin rubbing (50.1%), and the former is generally recognized as a cause of melasma formation, while the latter may cause the destruction of skin tissue structure, thus causing a series of causal effects and eventually leading to The former is a commonly recognized cause of melasma formation, while the latter may cause damage to the skin tissue structure, resulting in a series of causal effects that eventually lead to melasma.