eli5: how does sun exposure cause skin cancer and how does using sunscreen prevent it without causing a different kind of cancer due to its chemicals?


Sunscreen is very commonly encouraged to minimize chances of getting skin cancer. How does skin cancer happen due to the sun’s rays in the first place? Why doesn’t using sunscreen cause cancer if sunscreen is made from chemicals? Are those with a lighter skin tone than those with a darker skin tone more at risk of getting skin cancer?

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The sun produces ultraviolet (UV) light which has a wavelength that can penetrate the skin cells and cause random mutations in the DNA in the nucleus of those cells. Certain mutations can cause the cells to reproduce at a higher rate than normal and/or bypass the natural mechanisms that prevent cells from reproducing beyond the numbers that are healthy. The excess cell growth is called a tumor. Tumors may prevent the tissue from functioning as designed, may prevent other tissues from functioning as designed, and/or spread to other areas where similar negative impacts may be caused.

Darker skin is usually caused by higher melatonin levels. Melatonin and the components in sunscreen both can help to absorb and/or filter UV light, which can help mitigate the risk of UV causing DNA mutations. Lower levels of UV penetration reduce the probability of mutation, thereby reducing the chance of developing skin cancer.

Sunscreens are designed to absorb and/or filter UV light, while being safe for application on the skin. If they contain chemicals that can themselves cause cancer they would be recalled – otherwise the producer may be financially liable for any negative side effects.

All matter in the universe is composed of chemicals, so simply being made of chemicals does not mean that it causes cancer.