Sun Protection Factor ratings are tricky, seemingly purposefully so, and the numbers are misleading. SPF numbers represent UVB ray blocking power in the amount of rays that it would take to create the same effect if no sunscreen was used. In other words, SPF 30 means that you would have to have 30 times as much UVB radiation to create the same exposure level. As the numbers get higher the ability of the sunblock to stop the rays is only slightly more and measurements begin to lose accuracy. For instance, SPF 50 blocks 98% of UVB rays and SPF 100 may block 99% of UVB rays. In the EU the highest SPF is 30+, in Australia SPF 50+ is the highest and the FDA is considering banning labeling that over SPF 50. As it turns out, other sunblock issues are more important to sun protection than any SPF numbers over 50.
When SPF is tested, the official application amount is 2 mg per square cm of the skin surface or approximately one ounce (or shot glass) of sunblock for a medium size body (with one heaping teaspoon for the face). What? That seems like you would use an entire bottle of sunscreen in one day with a family of four. Most people apply 20% to 50% of his amount, significantly decreasing their SPF by about a fourth root or square root. If you apply half a shot glass of SPF 50 sunscreen to your entire body, the real value you are getting from the sunscreen is about SPF 7.
Another issue is that nearly all sunblocks start degrading in one hour and after two hours they are gone. SPF is only tested up to 80 minutes. Some of the newer sunblocks claim to last longer, but there is no evidence supporting these claims. Sunblock should be applied three to four times during a day in the sun. A sunscreen container of three or four ounces, about the size accepted by airport security, is barely enough sunscreen for one day on a Costa Rican beach. Make sure to bring at least 25 fluid ounces to your next surf vacation week in Hawaii!