Chris Birchby, founder of sun-care Coola argues that:

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Text 2 - How to Tell if Your Sunscreen Protects You From the Sun - Here’s what you need to know.

    Don’t go overboard with the SPF. The American Academy of Dermatology recommends using an SPF of at least 30, but most experts agree to not go over 50. It’s not that a higher SPF doesn’t provide any more protection, but once you get above 50, that increase is negligible. Case in point: SPF 50 blocks about 97% of UVB rays, while SPF 100 blocks about 99%.

    But most sunscreen users don’t think about that; rather, they see a number that’s twice as high and assume they’ll get twice as much protection or that the protection will last twice as long, which cultivates a false sense of security that could lead to a bad burn. “SPF values above 50 are really misleading,” Lunder says. “They offer a very small increase in sunburn [UVB] protection, and they don't offer better UVA protection.” She says that the FDA is considering a rule to cap SPF values at 50, but nothing has been finalized.

    And then there’s the fact that, although the increase in SPF doesn’t add much protection, it could increase your chances of negative side effects from the ingredients. “We do not recommend SPF of 50 or higher, as the minimal added protection does not outweigh the exponentially more active ingredients required to do so,” Chris Birchby, the founder of the sun-care line Coola, tells Teen Vogue. “More active ingredients increase the chances of skin irritation.”

http://www.teenvogue.com/story/how-to-tell-if-sunscreen-protects-you-from-the-sun