SPF, or Sun Protection Factor, is the measurement of how well the particular product - sunblock or sunscreen, will protect your skin against sunburn or tanning. SPF can be calculated from the amount of time that it takes to develop mild redness or burning in an unprotected person. For example, if someone develops mild sunburn-related redness in 20 minutes, he or she may develop the same degree of redness in 300 minutes (5 hours) with an SPF 15 sunscreen (15 times 20 minutes) when properly applied.
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An SPF 15 product filters out more than 93% of the UVB in sunlight allowing about 7% penetration. An SPF 30 filters out 97% and allows 3% penetration. So an SPF of 30 is not twice as effective as a 15, but rather it blocks out about twice as much of the penetration. Stated differently it allows only half the UV penetration
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