SPF stands for "sun protection factor," which indicates how long you should be able to stay in the sun and how much UV radiation is filtered out. So the higher the better, right? Up to a point. Unfortunately, protection comes down to more factors than just SPF. Protection increases up to an SPF of about 50. After 50, the gains are marginal and mostly nixed by sweating, rubbing or being in the water. An SPF of 30 to 50 should be fine as long as you reapply at least every two hours.