The sun is dark as soon as the sun may be too thin stratum corneum, do not pay attention to sunscreen, personal constitution and other reasons. 1. The stratum corneum is too thin: the stratum corneum is too thin so that the skin’s resistance to external stimuli decreases, the skin is prone to dehydration, sensitivity and so on, resulting in red blood, under this stimulus, accelerating melanin deposition, so that the skin becomes black. 2. Do not pay attention to sunscreen: If you usually do not pay attention to skin care, do not pay attention to the skin sunscreen, will also lead to the skin is easy to the sun on the black phenomenon. 3. personal constitution: if belong to the allergic body, the ultraviolet ray is overly sensitive. After being irradiated by ultraviolet rays, it is easier for melanin to accumulate, resulting in the skin being blackened by the sun. The people who are black in the sun should pay attention to avoiding exposure to the sun in their normal life, and go out to do a good job of sun protection. It is recommended that patients should consult a doctor in time to avoid delaying their condition.