Since we were little kids with our parents slathering sunscreen on us before heading out in the sun, we've noticed SPF with some number after printed really big on the labels. As kids we likely didn't know, or care, what that meant. Once you get older and have to buy your own, many of us probably just picked the one with the highest number.
Although the 'bigger the number the better' mentality still prevails, there's definitely a general misunderstanding as to how those numbers should be interpreted.
Sun Protection Factor SPF stands for Sun Protection Factor and what this number quantifies is the actual amount of time you can spend in direct sun before you start to get a sunburn.
Now you're thinking, "but wait a second, skin color and sensitivity to sun varies from person to person." That's right, and that is one reason why SPF numbers shouldn't necessarily be the number one factor in your choice of sunscreens.
Another tricky thing about SPF that many people may be unaware of is how the increase in the number actually relates to the amount of protection provided. For example, a sunscreen with an SPF of 15 blocks 93% of UVB rays. In contrast, a sunscreen with an SPF of 40 blocks 97.5% of UVB. So even though the SPF number increases by 25, the gain in percentage of protection is only 4.5%.
Why is it so? Well, that's because, as the name implies, the Sun Protection Factor is just that – a number that factors into an equation. Without getting too technical, the SPF number is means this: if your skin normally starts to burn after 10 minutes in the sun, you can multiply 10 minutes by the SPF number and that in theory should be how long you can be in the sun before you start to burn.
Example:
(10 minutes before burning) X (SPF 15 sunscreen) = 150 minutes before burning
One more thing to note about SPF numbers is that they can be wildly inaccurate. Talking with the owner of a certain cosmetics factory, she said they tested a chemical-based SPF 50 sunscreen from one of the most well-known brands and it ended up being SPF 11. However, SPF 11 would block around 90% of UVB so unless someone is lying out in the sun with a stopwatch, they would probably never realize it.
We're not saying SPF numbers don't matter, after all we do need some kind of relative benchmark to compare different sunscreens to. However, the idea that an SPF 50 or 100 sunscreen is going to provide exponentially better protection than, say, SPF 30 is simply not the case.