The Sun Protection Factor "SPF" pertains to the length of protection a sunscreen offers.
An SPF 30 blocks an estimated 97% of UV rays while an SPF 50 blocks 98, only 1% more. The real difference is that laboratory testing has demonstrated the SPF 50 to last for a longer period of time before breaking down or declining in protection, than the 30.
You can calculate the maximum length of protection by multiplying the SPF value by the number of minutes you would start to burn if you were wearing no sunscreen at all. For example, a fair skinned individual might start to burn in 10 minutes x SPF 30= 300 minutes of protection, x SPF 50= 500 minutes of protection and so on. Technically a 30 can deliver 5+ hours of protection and a 50 8+ hours, however, as we are not in controlled lab environments and factors such as swimming, sweating, towel drying, hands and clothes interference etc are present, the official FDA recommendation is to reapply your sunscreen every 2 hours. Therefore, a 30 and 50 perform essentially the same until at least the 2 hour mark.
Higher SPFs of 60, 70, 100 etc, can create a false sense of security. What is most important is to practice safe reapplication as all sunscreens can break down after 2 hours or so under the sun due to external factors.
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