I have always been weary of spreading chemicals all over my skin and often joked about sun screen being more dangerous than the sun. Well I guess those ideas may be a reality. According to this article, the vitamin A that is commonly used in sun screens to slow the aging of skin actually accelerates the rate of growth of tumors, lesions, and malignant cells in the test animals during lab studies. Not to mention the other chemicals (oxybenzone) that get absorbed into the blood stream and the vitamin D deficiency problem that many people face.
On the following website you can search different kinds of sun screen to see how potentially dangerous they might be as well as see lists of the best and worst sun screens.