Environmental Working Group (EWG) has tested 500 sunscreens, and says that only 39 of them, or 8 percent, do the job. The other 92 percent don't offer strong protection as they claim, and may in fact be dangerous.
Environmental Working Group cautions the consumer to avoid high-SPF sunscreens, because they can give the user a false sense of security. EWG says that though a high-SPF sunscreen like 100-SPF sounds like it should offer strong protection, in reality it would protect like a 3-SP, since people would tend to use less than they need and wouldn't reapply often enough.
Products with little to no protection from UVA rays are also advised against. Protection from UVA rays is important because while they don't cause sunburn, UV rays can add to the risk of skin-cancer and skin-aging.
Each of the recommended sunscreens contain zinc or titanium as active ingredients. None of the top selling sunscreens made it into the recommended group.
"Rather than relying on sunscreens alone for protection from the sun, the EWG recommends avoiding sun exposure entirely during peak hours and wearing protective clothing whenever possible."
http://www.webmd.com/skin-problems-and-treatments/news/20100524/group-calls-some-sunscreens-snake-oil?ecd=wnl_day_052510