evidence: But the evidence of how well sunscreen protects against skin cancer is mixed. For starters, it’s difficult to study the links between sun exposure, sunscreen use, and skin cancer because sun exposure across a person’s entire lifetime contributes to skin damage. By the time a participant enrolls in a study, the damage may have already occurred. In addition, it can be hard to assess how well and often study participants apply sunscreen. Researchers categorize skin cancers into two main types: melanoma and non-melanoma. Some data show that sunscreen is more useful in preventing non-melanoma skin cancers. There is also limited evidence that sun exposure and sunburns increase one’s risk for developing melanoma, the deadliest form or skin cancer. But there are also many studies that find sunscreen does not reduce one’s risk for developing either type of skin cancer. [...] - Many sunscreens include a vitamin A additive that may speed the development of skin cancer. The sunscreen industry adds a form of vitamin A called retinyl palmitate to some sunscreen products to help prevent aging. But studies show it may trigger development of skin tumors and lesions when used on skin in the presence of sunlight.