This distinct boundary occurs when sun exposure stimulates melanin?the pigment that colors your skin?on uncovered areas while protected skin remains lighter. It is very common and completely harmless. To help even out your skin tone, the most important step is protecting your skin from further sun damage by wearing a broad-spectrum sunscreen daily. Over time, your skin will naturally exfoliate and the line will fade. You can also try gentle over-the-counter creams containing ingredients like niacinamide or vitamin C to help speed up the process.
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What causes a distinct line of dark and light skin on my arms, and how can I treat this uneven skin tone?