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It's summer here in the Northern Hemisphere, which means we're seeing a whole lot more sunscreen.  Every bottle has a big SPF number on it, and a lot of us probably assume we know what those numbers mean.  Higher numbers equal more protection, right?

Well, yes, but also no.  It's a little more complicated.  The thing is, you probably don't need more than SPF 30 or so, and there are bigger things to worry about, like whether you're wearing enough sunscreen to really protect your skin.

SPF stands for Sun Protection Factor.  It's a measure of how much less of the dangerous ultraviolet radiation from the Sun hits your skin with the sunscreen on than without it.  Two types of ultraviolet light can reach Earth's surface, called UVA and UVB.  They have different energy levels and affect our bodies in slightly different ways.   UVB is the main cause of sunburns and it's directly linked to the development of skin cancers, and UVA leads to tans which happens when your body makes compounds to stop UVA from damaging too many skin cells.

SPF in the United States is only calculated based on how much UVB gets blocked.  Since we really only knew about UVB's risks when making the standards.  So if you wear SPF 30, the general idea is that your skin absorbs about 30 times less UVB than it would without sunscreen, because chemicals absorb or reflect the radiation to block it from getting into your cells.  This roughly translates to SPF 30 blocking 96.7% of UVB.  If you wear SPF 40, your skin absorbs 40 times less, because it blocks around 97.5% of UVB and even when you get up to SPF 100, that only blocks around 99% of UVB.  That's not a super huge difference.

So most people get no added benefit from wearing anything above SPF 50 and SPF 30 is usually plenty, but that's only true with the right amount of sunscreen and people, well, we're not very good at wearing it.  For one thing, if you remember to wear sunscreen in the first place, most of us only wear about 1/3 of what we should, not to mention we're pretty sweaty.  Sweat and water make some sunscreens come off more easily and even the best waterproof stuff still breaks down or rubs off after a couple of hours.  So experts recommend reapplying sunscreen every two hours regardless of its SPF, but even with the right amount on, some of us still don't get all of the protection we need.

Since making those initial SPF standards, we've learned that both UVA and UVB radiation can damage DNA and kill skin cells, and both are strongly linked to skin aging and cancer.  So, to be safe, you should always get a sunscreen with a broad spectrum SPF, which measures both UVA and UVB blockage.  Basically, SPF is important, but there's a lot more that goes into wearing sunscreen so you can make sure it's actually doing its job.

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