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But what's with those numbers? The sun protection factor, or the SPF, ranges from a low of 2 to a high of 50 and sometimes higher. Most of us understand that the SPF has something to do with providing protection from the sun's damaging ultraviolet rays. But beyond that, the number is shrouded in mystery. What does SPF mean? The SPF is a comparison of the time it takes before the skin will start to redden with and without sun protection. In addition, SPF pertains only to UVB rays, the ones that cause tanning but also sunburns. For example, if it would take 20 minutes for an individual's skin to burn without sun protection, then a SPF of 15 means that when applied, that person can remain in the sun 15 times longer, or up to 5 hours, without worrying about burning. The greater the SPF, the longer the protection it offers, but only up to a certain point. SPFs greater than 15 offer only small increments of additional protection. And that's the theory behind the SPF. In reality though, sun protection regardless of SPF becomes less effective when it's not evenly applied, a person goes into the water and a person sweats. Both water and sweat diminish the product's effectiveness that's why it's extremely important to reapply sun protection frequently.