When you go to buy sunscreen, you might look for one thing: SPF.
當你買防曬霜的時候，你可能會找一個指標：SPF。
It stands for sun protection factor, and based on that name, it seems like it should tell you how well you'll be protected from the sun.
它代表防曬指數，這個名字好像是告訴你產品的防曬效果如何。
Except… that's not totally true.
但這並不完全正確。
Because while SPF might sound like a perfect, rational scientific unit, there are a lot of flaws in the way it's named and the way it's tested.
雖然SPF聽起來像是一個完美合理的科學單位，但是它的命名方式和試驗方式有很多漏洞。
So if you want to be as protected from the sun as possible, you're going to have to know a lot more than that one number.
因此如果你想防曬效果更好，你需要知道更多。
First, as a disclaimer, it's worth noting that countries approach sunscreen regulations differently.
首先，作一個聲明，值得注意的是，各國對防曬霜的規定有所不同。
This episode is a good general overview, but keep in mind that things might be different where you live.
本集只是給大家大概講講，但是要記住不同國家根據的標準有所不同。
Okay, that being said, onward.
好的，說到這裡，我們繼續。
Essentially, SPF measures how much radiation is blocked from reaching your skin.
實際上，SPF 衡量的是被阻隔在你皮膚外的輻射量。
So if it's SPF 30, it will protect you against thirty times more exposure than if you didn't wear sunscreen.
如果 SPF 為 30，那麼與你不塗防曬霜時相比，塗防曬霜能阻擋 30 倍的輻射。
That translates to roughly 97% protection, and it explains why anything higher than SPF 30 isn't that much more effective.
這就表示防曬霜大約能起 97%的防曬作用，這就解釋了為什麼任何 SPF 值高於三十的防曬霜沒有更明顯的效果。
SPF 50, for example, blocks fifty times the radiation, or 98% of it.
再比如說 SPF 是 50 的話，可以阻擋 50 倍的輻射，或者是 98%的輻射。
Which isn't that different.
沒什麼太大差別。
Still, regardless of what that number says, SPF only tells you how protected you'll be from a specific type of radiation.
但是，不管 SPF 值是多少，它只能告訴你能免受多少特定類型的輻射。
The rating you see on sunscreen bottles is only required to show protection against UVB rays, the type of radiation that causes sunburns and leads to skin cancer.
你在防曬霜上看到的數值只顯示了對 UVB 射線的防護效果，這種輻射會導致曬傷甚至引發皮膚癌。
It doesn't have to show protection against UVA radiation, which causes wrinkles, premature skin aging, and again, skin cancer.
但是不代表能夠預防可以導致皮膚老化、皺紋、引發皮膚癌的 UNA 射線。
So even if you're using SPF one thousand, that doesn't mean you're protected against UVA.
因此，即使你用了 SPF1000 的防曬霜，也並不意味著能阻隔 UVA 的輻射。
If you want to avoid both kinds of rays, you'll need to look for broad-spectrum sunscreen.
如果你想要避免這兩種輻射，你需要使用光譜防曬。
Another thing worth knowing about SPF is that the way it's tested isn't bulletproof.
你應該知道另外一件關於 SPF 的事情是它的測試方式並不是完美的。
To figure out how much UVB something blocks, you'd think scientists would use some sort of skin-like compound in a Petri dish.
為了算出有多少 UVB 會被阻擋，你可能認為科學家會在培養皿中用一些類似於皮膚的化合物。
Like, just shine some light on this stuff, and see how much radiation gets through.
直接把它們放在光下，然後看看有多少輻射穿過。
But while that is how they test broad-spectrum sunscreen, it isn't how they test SPF.
但這是他們測試光譜防曬霜的方法，而不是測試 SPF 的方法。
To get an SPF rating, researchers use volunteers.
研究人員通過志願者測試來得到 SPF 的數值。
First, they shine a UV lamp on one spot on participants' bare backs to get a baseline for their tendency to burn.
首先，他們將紫外線燈照在參與者裸露後背的一個點上，得到一個曬傷臨界點的基礎值。
Then, a lab tech applies a gob of sunscreen to a different spot and allows it to dry.
然後，實驗人員會在不同地方塗上防曬霜並且等它變幹。
Finally, the tech turns the UV light back on, and sees how much longer the participants last before turning red again.
最後，技術人員再次打開紫外線燈，觀察參與者的皮膚經過多久會再次變紅。
If they burned in four minutes unprotected, but 60 minutes with sunscreen, that earns an SPF fifteen rating.
志願者在沒塗防曬霜時，四分鐘就會曬傷，但是塗了防曬霜之後，可以堅持 60 分鐘，那麼得到的 SPF 值為 15。
And if this process seems sketchy and full of issues… yes.
這個過程是不是看上去很粗略而且有很多問題啊……是的。
Yes, it does.
你說對了。
For one, people turn red at different amounts of exposure.
首先，人的皮膚在不同程度的暴露下變紅程度會有差別。
So if you used a bunch of pale people in one study and a bunch of people with darker skin in another, you'd get totally different results.
所以如果用一組皮膚相對比較白的人做實驗，再用一組皮膚比較黑的人做實驗，會得到完全不同的結果。
Then there's the amount of sunscreen used in these tests.
此外，這些測試中試用了大量的防曬霜。
While there's tons of variability between testers, protocol calls for no less than two milligrams of sunscreen per square centimeter of skin and that's significantly more than people tend to use in real life.
雖然測試者之間的差別很大，但是實驗方案要求每平方釐米皮膚上，至少要有兩毫克的防曬霜，這比人們在現實生活中使用的要多得多。
In one study, it was more than twice the amount.
在一項研究中，防曬霜的用量超過了日常的兩倍。
Ultimately, these tests just aren't representative of what happens in the real world.
所以這些測試並不能代表真實情況。
They don't take into account things like skin color, or how much sunscreen people actually use.
他們沒有考慮到膚色差異，或者是人們塗抹防曬霜的實際用量。
So when you're buying this stuff, it's probably worth keeping those things in mind.
因此，在你買防曬霜的時候，需要考慮一下以上因素。
For example, maybe this means it's worth putting on sunscreen more often than you currently do.
例如，你可能需要更頻繁地塗抹防曬霜。
If nothing else, though, the good news is that researchers are at least aware that there's a problem here, and they've been working on protocols to hopefully get more consistent results.
別的不說，好消息是研究人員至少意識到了存在的問題，並且一直在改進實驗方案，希望能夠得到更加一致的結果。
Their ideas include using robots that spread sunscreen evenly in all conditions and shifting towards standardized in vitro testing using chemicals in a dish—no human skin involved.
其中包括使用機械設備，在各種情況下均勻塗抹防曬霜，在不使用人類皮膚的情況下，用培養皿中的化學物質進行標準化的體外測試。
This will let scientists get way more data points and will improve accuracy.
這將使科學家獲得更多的數據，也可以提高準確性。
At the end of the day, SPF can be a helpful way to understand what you're buying.
不管怎麼說，SPF 可以幫助你瞭解你要買的防曬霜。
To make an informed decision, you just need to know what it says and where those numbers are coming from.
要做出正確的決定的話，你只需要知道 SPF 是什麼意思，以及這些數字來自於哪裡。
And it's also worth knowing that not all hope is lost, either.
同樣要知道的是，它並不是一無是處。
Even if this is a bit of a flawed system, multiple studies have shown that consistent use of sunscreen, even at low SPF, reduces your risk of skin cancer.
即使這是一個有缺陷的方法，但多項研究表明，持續使用防曬霜，甚至是低 SPF 的防曬霜，也能降低患皮膚癌的風險。
So, it's always worth lathering up!
所以，防曬霜總是值得用一下的！
If you've ever wished you just didn't have to wear sunscreen—well, I've got some sad news for you.
如果你曾希望自己不用塗防曬霜，那麼我有一些壞消息要告訴你。
Almost all amphibians, reptiles, birds, and fish can make their own sunscreen.
幾乎所有的兩棲動物、爬行動物、鳥類和魚類都能自己分泌“防曬霜”。
Mammals are kind of the odd ones out.
哺乳動物是個例外。
If you want to learn more, you can watch our episode about the missing sunscreen gene after this.
如果你想知道更多，可以看我們關於防曬基因消失的視頻。
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