The pathogenesis of melasma is very complex. At present, it is believed that it may be related to the following factors: ultraviolet radiation, cosmetics, endocrine disorders, genetics, drugs, oxygen free radicals, local microecology, systemic lesions of the organism, pregnancy, mood swings, etc. According to the source they are divided into two categories: exogenous etiology and endogenous etiology. I. Exogenous causes 1. UV radiation Sunlight exposure is considered to be the most important factor. Even if the skin is good, it will be tanned after long time exposure to strong sunlight. Not to mention that the original pigmented skin is more sensitive to UV rays. The ozone layer at high altitude is thinning year by year, and the role of blocking UV rays is getting weaker and weaker. UV damage to human body is also increasing day by day, which is closely related to the increase of pigmentation spots. Shen Dawei and others surveyed 5606 healthy people over 10 years old in Urumqi, Xinjiang, and found that the prevalence of melasma was 9.7%. The prevalence of melasma was 10.9% in male patients and 89.1% in female patients, of which 41.8% were unmarried. The authors concluded that the high prevalence of melasma in Urumqi may be related to the fact that Urumqi is located on the northwest plateau, with an average altitude of 653.5 m and strong sunshine in spring and summer. Sanchez investigated 76 cases of melasma patients and found that 100% of them had aggravation of pigmentation after sun exposure. In addition, for those patients who are cured after treatment, if they are often exposed to sunlight, chloasma will often recur, and avoiding sunlight can make it reduce or even disappear. 2.Cosmetics and certain topical drugs Cosmetics cause skin pigmentation abnormalities and lead to the occurrence of melasma, which may be related to certain ingredients in cosmetics such as oxidized pressed oleic acid, citric acid, water yangate, heavy metal elements, preservatives, fragrances, dyes and even beeswax matrix, especially poor-quality cosmetics are more harmful. Wang Changjin et al. analyzed the etiology of 680 cases of melasma patients, and more than 50% were obviously related to the use of cosmetics. When certain ointments are used topically to treat skin diseases clinically, although the skin diseases are cured, the pigment spots left behind often cannot subside. In addition to dermatophyllin, skin relaxation and enzymatic cream, which are most likely to cause pigmentation, topical retinoic acid preparations can both treat melasma and induce it. The American Safety Medication Committee has collected 8 cases of such patients; in addition, the topical use of silver and mercury preparations can cause melasma. 3.Inflammatory factors When inflammation occurs in the skin, no matter it is treated or not, after the inflammation subsides, it often leaves pigment spots of different severity. For example, contact dermatitis, sunburn, lupus erythematosus, eczema, psoriasis, rose congenital rash, flat warts, seborrheic dermatitis, solar dermatitis, acne, etc. 4.Physical factors There are many physical factors that cause pigmentation spots. In addition to the ultraviolet rays mentioned earlier, other factors such as heat-assisted radiation, frostbite, sunburn, burns, scratching and friction can cause pigmentation spots. Especially in peeling therapy, some peeling drugs use some chemical corrosive agents and keratin exfoliating agents, and the skin is forcibly peeled off by some chemical substances, although the pigmentation spots or scars are temporarily removed, but long-term peeling, the epidermal protective layer is repeatedly damaged and the skin barrier is destroyed. In this way, even breeze blowing Buddha and light light light will cause pigmentation spots on the peeling area. The principle of pigmentation spots caused by grinding is the same as this. 5, skin micro-ecological imbalance In recent years, it is found that the transient bacteria in the melasma lesion area have obviously changed, such as the appearance of pigment-producing micrococcus, rods and aerobic gram-negative bacilli. There are symbiotic and antagonistic effects between the flora, which participate in the formation of epidermal lipid film and constitute the skin biological barrier, and can nourish the skin and participate in skin cell metabolism. The skin itself has the ability to maintain ecological homeostasis, and if the host skin, environment and flora are in a state of disharmony i.e. microecological imbalance, pathological damage to the skin can result. Some authors applied localization, quantitative and qualitative methods to conduct microecological studies on 51 cases of melasma and found that Micrococcus, Rhodococcus and aerobic gram-negative bacilli producing brown pigment and orange pigment increased significantly, while resident Propionibacterium acnes decreased significantly, suggesting that the alteration of skin flora and the absorption and deposition of bacteria producing pigment are involved in the formation of melasma. Endogenous factors The so-called endogenous factors refer to certain diseases in the human body, such as endocrine disorders, pregnancy, oral or injected certain drugs, genetics, lack of trace elements, mental factors and other factors, etc. 1.Endocrine disorder Endocrine disorder is currently recognized as the main cause of chloasma. For example, during menstruation, the pigmentation of melasma is sometimes dark and sometimes light, and changes with the change of menstruation. In the process of taking or injecting contraceptive pills, facial pigmentation can be seen in about 80% of people, and even if they stop using contraceptive pills, the facial pigmentation does not fade immediately and often lasts for years. It has also been reported that chloasma appears on the face of women during or around the time of pregnancy, and its incidence varies from 30% to 70%. After delivery, the pigmentation can be reduced or even disappeared in about 87% of patients, but it can recur after taking oral contraceptives and can be relieved after stopping the pill. Melasma can occur in other patients with hyper- or hypoadrenocorticism, hyper- or hypothyroidism, hyperpituitary gland function and gonadal diseases. Especially, the incidence of hyperthyroidism and thyroidectomy syndrome is 4 times higher than that of normal people. Genetic factors In the past, it was thought that chloasma was an acquired disease and had nothing to do with genetics. However, in recent years, there is information that it is related to heredity. Melasma is more likely to occur in patients with congenital stupidity and certain races or populations, such as Latin America, where the incidence rate can reach 60%-70%, much higher than other regions. Several cases of familial melasma have been reported. During the treatment of 300 melasma patients, it has been found that those with family history should account for more than 30%. There are also reports of 28 cases of male melasma patients, 70% of which have family history, and it is believed that genetic factors are the main cause of male melasma. 3.Uterus, ovaries and other gynecological diseases Chloasma patients are mostly found in women, and the ratio of male to female sex is 1:6.5 or higher. It is related to certain gynecological chronic diseases, such as menstrual disorders, dysmenorrhea, uterine adnexitis, infertility, breast lobular hyperplasia, uterine fibroids, ovarian cysts, cervicitis and other female reproductive system diseases, all of which can lead to facial pigmentation spots. 4.Hepatobiliary diseases Certain liver diseases such as cirrhosis, chronic alcoholism, especially biliary cirrhosis, are more obvious. The pigmentation is brownish red, widely distributed, and more obvious in exposed areas. The etiology of this kind of pigmentation is still unclear. 5.Emotional factors Melasma is related to emotional changes, and mental depression often leads to deepening of pigmentation. The common emotional changes of melasma patients are mainly irritability, depression and neurasthenia. Because the patients are depressed, painful and anxious, withdrawn and lonely, eager to achieve, disappointed and thankful for the bad psychology after the onset of the disease, and these emotions can further aggravate the condition of melasma. Some people reported two cases of melasma caused by emotional depression, both of them were induced by extreme grief and emotional depression after losing their loved ones for two months. 6.Abnormal trace elements Trace elements also have an impact on the development of melasma. It has been reported that the concentration of trace elements such as copper, zinc, iron and magnesium in serum of melasma patients is not normal. Some people found that: the level of serum zinc of pregnant melasma patients is higher than that of pregnant non-melasma patients, the level of serum copper of pregnant and non-pregnant patients is higher, the level of serum iron of pregnant melasma patients is higher than that of non-pregnant melasma patients, and the level of serum magnesium is lower than that of non-pregnant patients with or without melasma during pregnancy. The above results suggest that there may be a relationship between the alteration of certain trace elements and the occurrence of melasma. It has been shown that the ability of tyrosinase to catalyze the formation of melanin from tyrosine is directly proportional to the amount of copper ions. Elevated serum copper level can increase skin tyrosinase activity, increase pigmentation and occur chloasma. 7.Oxygen free radicals There are endogenous free radicals in living organisms, which are metabolites in the human biochemical cycle, and their accumulation can cause various damages to biological membranes, nucleic acids, collagen and biological enzymes. There are many oxygen free radical scavengers in human body, and melasma patients may have some disorders in this regulatory system. There are many types of free radical scavengers, besides the enzymatic antioxidants mentioned above, there are also some non-enzymatic antioxidants such as carotenoids, vitamin C, vitamin E, etc. It is difficult for any single scavenger to remove all the excess free radicals in the body, and the scavengers need to combine with each other to synergistically remove free radicals in order to inhibit the occurrence of melasma. 8. Other patients with tuberculosis, psoriasis, neurosis and other diseases also often suffer from this disease. In addition, chloasma is the most common pigmentation disease among brain tumor patients. In addition to the face, it can also occur in other parts of the body, with an incidence of 7%. Chloasma can be induced by regular use of certain drugs such as dormantine, methantoin, phenytoinamide, dibenzoylecgonine, and antiseptic. The mechanism is unclear. Vitamin C, A, B12 and other deficiencies can easily lead to brown spots.