I have some question similar to yours: why is acne vulgaris is so common?

It is a big problem which can not answer in a few words. I can only explain your question base on speculation and hypotheses. There is a keyword related to melasma, it is inflammation. There are too many factors may cause inflammation. For example, sun shine and cold wind. People are living in regions with intense sun and cold wind exposure is particularly susceptible to developing melasma. The inflammation caused by sun shine and cold wind is invisible inflammation just like micro-comedo. When lipid, sugar and protein metabolism is in disorder, endocrine system may also in abnormal, and the pigmentation on skin can not be Subsided

Please search some articles about melasma and inflammation in pubmed may give your more deeply answer.

One of the most important etiologic factor is the race.It was found more common in Asian skin especially the darker skin. Since UV is also very important causative factor ,sun avoidance including sunscreen use may have beneficial effect on the prevention.

Sunshine will make the skin dark in summer, which is a normal physiological phenomenon. The strong sunshine may destroy skin tissue result in micro inflammatory reaction in dermal tissue of local skin. Pigmentation can not subsided with inflammation. It becomes melasma.

I don’t think the most important etiologic factor is the race. Melasma is not only common in Asian skin but also in European especially the skin with deficiency of melanin. Melanin is an excellent photoprotectant although it is make skin dark. It efficiently absorbs harmful UV-radiation (ultraviolet) and transforms the energy into harmless heat. People with fair skin may be more likely to develop melasma.

Melasma is multifactorial. There is no doubt that there are hereditary factors involved, but they have not been delineated. Hormonal stimulational is a common factor - pregnancy, starting or stopping birth control pills are common precipitators of melasma. Excessive sun exposure is often a common historical feature. Any mild or moderate inflammaory condition of the skin may result in pigment stimulation and reveal melasma that previously was not noticed. People with more active pigment cells in the skin ,who naturally have brown or black skin have a higher incidence of melasma ( presumably because their melanocytes are more easily stimulated ). These are the known historical factors, but the actual cause of melasma is unknown. There appears to be a subset of melaocytes that become dysfunctional and incapable of responding appropriately to recurrent environmental stimuli.

I think melanin is related to invisible inflammtion. This inflammation is induced by free radical derived from abnormal oxidation of sugar, fat and protein. A very important vitamin which play a pivotal role in metabolism can prevent this inflammation. It is nicotinamide and niacin. Elderly people and women are frequently in defficiency of niacin can not be diagnosed, because there is no very clearly Diagnostic criteria for diagnose of pellagra. ...

"Pigmentation is the color of 'melanin' which is produced by cells called 'melanocytes' in the epidermis, the superficial layer of the skin. Melanin is produced by a chemical reaction via the enzyme tyrosinase“.

“Skin pigmentation is caused by various physical and chemical factors. It might also be influenced by changes in the physiological function of skin with aging. Nicotinamide adenine dinucleotide (NADH) dehydrogenase is an enzyme related to the mitochondrial electron transport system and plays a key role in cellular energy production. It has been reported that the functional decrease in this system causes Parkinson's disease. Another study reports that the amount of NADH dehydrogenase in heart and skeletal muscle decreases with aging. A similar decrease in the skin would probably affect its physiological function.”

Nakama M, Murakami Y, Tanaka H, Nakata S.Decrease in nicotinamide adenine dinucleotide dehydrogenase is related to skin pigmentation.J Cosmet Dermatol. 2012 Mar;11(1):3-8. doi: 10.1111/j.1473-2165.2011.00592.x.

Thank you Dr Hao, im very much thankfull for your answer. In India I go to places which have an high altitude. There women frequently are found with melasma. The explanation of melanin, inflammation, free radicals, , sun rays, and cold winds suits the findings and common occurrence. Precautions if taken early, may help in the decrease of incidence of melasma.

profylactic use of sunscreens, may have a role in the decrease of melasma.

thank you sir.