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How can ultraviolet light contribute to skin cancer?

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Precancerous growth in a human colon — cancer View through an endoscope of a polyp, a benign precancerous growth projecting from the inner lining of the colon.

Ultraviolet (UV) radiation contributes to skin cancer—including basal-cell carcinoma, squamous-cell carcinoma, and malignant melanoma—primarily by damaging the DNA within skin cells.12

The carcinogenic process occurs when UV rays cause the formation of pyrimidine dimers, which are abnormal structures that form between adjacent nucleotide bases (cytosine and thymine) in DNA. If these dimers occur in a growth-regulatory gene and are not repaired by the body’s natural enzymatic mechanisms, they can lead to uncontrolled tumor development. Additionally, UV light can facilitate photosensitization, where energy is transferred to molecular oxygen, creating reactive "singlet oxygen" that further damages cellular compounds, including DNA.134

The risk of developing cancer depends on the intensity of exposure, the type of UV rays (with UVB considered particularly dangerous), and the amount of protective melanin in the skin. Because melanin absorbs UV radiation and limits its penetration, individuals with fair skin are at a higher risk of experiencing these harmful, carcinogenic changes.12