an spf 15 blocks about 93% that means 7% of the sun's rays are still getting through to your skin. and that if you are using enough product which nobody ever does. so chances are if you are using an spf 15 the way that most people use it you are probably only getting an spf 8 out of it. so the higher spf the better. using plenty of product applying it liberally or generously will really help you as well and going to the lower end products you are really just not getting enough protection.
why isn't a sunscreen that is below an spf 15 very effective?