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In page Light therapy:

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About 1 percent of the population suffers from vitiligo. Photo therapy is an effective treatment because the skin cells manufacture the pigment when they sense UV light. When they sense UV light the cells manufacture the pigment in order to protect the body from the harming UV light. Treatment is generally required 3 times a week in a clinic and every day at home. It takes about 1 month to achieve re-pigmentation in the face and neck, and 2–4 months in the hands and legs. Narrowband UVB is more suitable to the face and neck and PUVA is more effective at the hands and legs.[1]