Turns out that most sunscreens don't protect you and may in fact contain harmful chemicals.
Lots of chemicals used today have not been tested for safety, and those in sunscreens are no different. The Environmental Working Group, as part of the Safe Cosmetics Campaign, has put together a study on sunscreens showing that many of the most popular brands have never been tested by the FDA. It also has recommendations as to the safest sunscreens out there.
For more information, check out the study.