UV radiation is among several types of light emitted by the sun. It's also a carcinogen. Most of the UV light that reaches the earth is UVA, the long-wave ultraviolet rays that penetrate the skin more deeply than UVB (shortwave) rays. These strong rays can reach the skin’s deepest layers, where melanocytes (skin cells that determine the skin’s color) are located. Melanocytes absorb UVA, producing the pigment melanin, which darkens the skin. But too much UVA may damage melanocytes, potentially causing melanoma. UVB rays are not nearly as prevalent or potent, but they may burn the outer layers of skin.