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?

每個瓶子上都有一個大的SPF數字，我們很多人可能會認為我們知道這些數字意味著什麼，越高的數字等於越多的保護，對嗎？

Well yes, but also no. It's a little more complicated.

嗯，是的，但也不是。這是一個有點複雜的。

The thing is, you probably don't need more than SPF 30 or so.

問題是，你可能不需要超過SPF30左右。

And there are bigger things to worry about like whether you're wearing enough sunscreen to really protect your skin.

還有更重要的事情要擔心，比如你是否塗抹了足夠的防晒霜來真正保護你的皮膚。

SPF stands for sun protection factor.

SPF代表防晒係數。

It's a measure of how much less of the dangerous ultraviolet radiation from the sun hits your skin with sunscreen on than without it.

它是衡量塗上防晒霜後皮膚受到的危險紫外線輻射比不塗防晒霜時少了多少。

Two types of ultraviolet light can reach Earth's surface called UVA and UVB.

有兩種紫外線可以到達地球表面，稱為UVA和UVB。

They have different energy levels and affect our bodies in slightly different ways.

它們的能量水準不同，對我們身體的影響也略有不同。

UVB is the main cause of sunburns and it's directly linked to the development of skin cancers.

UVB是造成晒傷的主要原因，它與皮膚癌的發生有直接關係。

And UVA leads to tans which happens when your body makes compounds to stop UVA from damaging too many skin cells.

而UVA會導致晒黑，當你的身體產生化合物來阻止UVA破壞過多的皮膚細胞時，就會發生這種情況。

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.

美國的SPF只是根據UVB被阻擋的程度來計算的，因為我們在制定標準的時候確實只知道UVB的風險。

So if you wear SPF 30 the general idea is that your skin absorbs about 30 times less UVB than it would without sunscreen.

所以，如果你塗抹SPF30，一般來說，你的皮膚吸收的紫外線比不塗抹防晒霜少30倍左右。

Because chemicals absorb or reflect the radiation to block it from getting into your cells.

因為化學物質會吸收或反射輻射，阻止輻射進入你的細胞。

This roughly translates to SPF 30 blocking 96.7% of UVB.

這大概相當於SPF30可以阻擋96.7%的UVB。

If you wear SPF 40 your skin absorbs 40 times less because it blocks around 97.5% of UVB.

如果你穿上SPF40，你的皮膚吸收率會降低40倍，因為它能阻擋97.5%的UVB。

And even when you get up to SPF 100 that only blocks around to 99% of UVB.

而且即使你達到SPF100，也只能阻擋99%的UVB。

That's not a super huge difference.

這不是一個超級大的區別。

So most people get no added benefit from wearing anything above SPF 50.

所以，大多數人穿上SPF50以上的衣服都不會有額外的好處。

And SPF 30 is usually plenty.

而SPF30通常是足夠的。

But that's only true with the right amount of sunscreen.

但這是要用適量的防晒霜才行。

And people, well we're not very good at wearing it.

而人，好在我們並不擅長佩戴。

For one thing if you remember to wear sunscreen in the first place most of us only wear about a third of what we should.

首先如果你記得第一時間塗抹防晒霜，我們大多數人只塗抹了應該塗抹的三分之一。

Not to mention we're pretty sweaty.

更何況我們的汗水還不少。

Sweat and water makes some sunscreens come off more easily and even the best waterproof stuff still breaks down or rubs off after a couple of hours.

汗水和水讓一些防晒霜更容易脫落，即使是最好的防水東西，幾個小時後還是會壞掉或擦掉。

So experts recommend reapplying sunscreen every two hours regardless of its SPF.

所以專家建議，無論防晒霜的SPF值是多少，每隔兩小時都要重新塗抹一次。

But even with the right amount on some of us still don't get all of the protection we need.

但即使上了合適的量，我們有些人還是沒有得到全部的保護。

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.

自從制定了最初的SPF標準後，我們瞭解到UVA和UVB輻射都會損傷DNA，殺死皮膚細胞，而且兩者都與皮膚老化和癌症密切相關。

So to be safe, you should always get a sunscreen with a broad-spectrum SPF which measures both UVA and UVB Blockage.

所以為了安全起見，一定要買一款具有廣譜SPF值的防晒霜，它能同時衡量UVA和UVB的阻隔效果。

Basically SPF is important.

基本上SPF是很重要的。

But there's a lot more that goes into wearing sunscreen so you can make sure it's actually doing its job.

但是，穿戴防晒霜還有更多的內容，所以你可以確保它真的在做它的工作。

We're uploading summer themed videos all week.

我們本週都在上傳夏季主題的視頻。

So make sure to subscribe if you want to learn more weird facts like this.

所以如果你想了解更多這樣的怪事，一定要訂閱。

And if you want to impress your friends with science while you're swimming, check out these awesome new scishow beach towels available at dftba.com/scishow.

如果您想在游泳時用科學知識給您的朋友留下深刻的印象，請到dftba.com/scishow查看這些令人敬畏的新scishow海灘毛巾。