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. [...] - 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. - Sunscreen doesn’t protect skin from all types of sun damage. The sun’s ultraviolet rays cause damage other than sunburns by generating free radicals that damage skin cells. Sunscreens help, but not completely. Not applying enough sunscreen, not applying frequently, and using products with poor UVA protection can lead to skin damage we cannot see with the naked eye.