|MadSci Network: Medicine|
SPF stands for Sun Protection Factor and is factor that helps understand how much longer we can be in the sun before burning, relative to our tendancy to burn without sunblock
For example, assume that you are a fair skinned person who normally burns in 20 minutes. Proper use of a sunblock with SPF 15 would mean that, assuming it does not wash off, you could remain in the sun for 20 min x 15 = 300 minutes before burning. Realistically, with any activity, the sunblock will not remain on the skin for the full 300 minutes and it needs to be reapplied regularly.
In answer to your question, SPF does not increase proportionally with a given SPF number. An SPF of 30, for example, blocks 97% of sunburning rays, while an SPF of 15 indicates 93% absorption and an SPF of 2 equals 50% absorption.
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