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When you go to buy sunscreen, you might look for one thing: SPF.
买防晒霜的时候，你应该会看SPF值。
It stands for sun protection factor, and based on that name,
这代表防晒指数，基于此
it seems like it should tell you how well you'll be protected from the sun. Except… that's not totally true.
它似乎应该告诉它保护你防晒的能力有多么好。只是，并非完全如此。
Because while SPF might sound like a perfect, rational scientific unit,
因为虽然SPF可能听起来像一个完美理性的科学单位
there are a lot of flaws in the way it's named and the way it's tested.
但其命名和测量方式存在很多缺陷。
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.
所以如果你想尽可能避免晒太阳，你应该知道的不仅是一个数字而已。
First, as a disclaimer, it's worth noting that countries approach sunscreen regulations differently.
首先免责申明，值得注意的是，各国对防晒霜的规定不尽相同。
This episode is a good general overview, but keep in mind that things might be different where you live.
本期节目是一个很好的概述，但要记住这可能和你所在国家的标准不同。
Okay, that being said, onward. Essentially, SPF measures how much radiation is blocked from reaching your skin.
好了，说到这里，继续往下。从本质上讲，SPF衡量的是有多少辐射被阻挡在皮肤之外。
So if it's SPF 30, it will protect you against thirty times more exposure than if you didn't wear sunscreen.
所以，SPF30将比不涂防晒霜的防晒效果好30倍。
That translates to roughly 97% protection, and it explains why anything higher than SPF 30 isn't that much more effective.
这相当于97%的防晒系数，这也解释了为什么SPF值高于30的防晒霜并没有那么有效。
SPF 50, for example, blocks fifty times the radiation, or 98% of it. Which isn't that different.
例如SPF50阻止了50倍的辐射，或者说98%的辐射。这没什么不同。
Still, regardless of what that number says, SPF only tells you how protected you'll be from a specific type of radiation.
尽管如此，不管SPF值是多少，它只告诉你能保护你免受特定类型辐射的程度。
The rating you see on sunscreen bottles is only required to show protection against UVB rays,
你在防晒霜瓶子上看到的等级只是为了显示对UVB射线的阻止，
the type of radiation that causes sunburns and leads to skin cancer.
这种类型的辐射会造成晒伤并导致皮肤癌。
It doesn't have to show protection against UVA radiation, which causes wrinkles, premature skin aging, and again, skin cancer.
它不需要显示出对UVA辐射的防护，UVA辐射会导致皱纹、皮肤过早老化，以及皮肤癌。
So even if you're using SPF one thousand, that doesn't mean you're protected against UVA.
所以即便你使用SPF1000的防晒霜，这也不意味着你就可以免受UVA的侵害。
If you want to avoid both kinds of rays, you'll need to look for broad-spectrum sunscreen.
如果你想避免两种光线，你需要找到广谱防晒霜。
Another thing worth knowing about SPF is that the way it's tested isn't bulletproof.
关于SPF值的另一个值得了解的地方是，它的测试方法并不是完美的。
To figure out how much UVB something blocks, you'd think scientists would use some sort of skin-like compound in a Petri dish.
为了弄清楚有多少UVB被阻隔，你可能会认为科学家会在培养皿中使用某种类似皮肤的化合物。
Like, just shine some light on this stuff, and see how much radiation gets through.
比如用一些光照射这些东西，看看有多少辐射通过。
But while that is how they test broad-spectrum sunscreen, it isn't how they test SPF.
但他们是这样测试 广谱防晒霜的，但却不是这样测试SPF。
To get an SPF rating, researchers use volunteers.
为了获取SPF等级，研究人员请了自愿者。
First, they shine a UV lamp on one spot on participants' bare backs to get a baseline for their tendency to burn.
首先，他们在受试者赤裸背部的某一块区域照射UV灯，以为他们的晒伤趋势设定一个基准线。
Then, a lab tech applies a gob of sunscreen to a different spot and allows it to dry.
然后，一名实验室技术人员在不同的地方涂了一层防晒霜，然后让它变干。
Finally, the tech turns the UV light back on, and sees how much longer the participants last before turning red again.
最后这名技术人员重新打开紫外光，看看受试者在再次晒伤之前还能坚持多久。
If they burned in four minutes unprotected, but 60 minutes with sunscreen, that earns an SPF fifteen rating.
如果他们在没有保护措施的情况下在四分钟内烧伤，但涂了防晒可坚持60分钟，那么SPF值为15.
And if this process seems sketchy and full of issues… yes. Yes, it does.
如果这个过程看起来很粗略且存在很多问题……是的，确实如此。
For one, people turn red at different amounts of exposure.
首先，造成人们晒伤的暴露程度不同。
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.
如果你在一项研究中使用的是很白的人，在另一项研究中是肤色更深一些的人，就会得到完全不同的结果。
Then there's the amount of sunscreen used in these tests. While there's tons of variability between testers,
然后是这些测试中防晒霜的使用量。尽管受试者之间存在大量的可变性
protocol calls for no less than two milligrams of sunscreen per square centimeter of skin
但按照规定，每平方厘米的皮肤需要至少两毫克的防晒霜
and that's significantly more than people tend to use in real life.
这比人们在现实生活中的使用量要多得多。
In one study, it was more than twice the amount. Ultimately, these tests just aren't representative of what happens in the real world.
在一项研究中，是超过两倍的量。最后，这些测试并不能代表现实世界的情况。
They don't take into account things like skin color, or how much sunscreen people actually use.
他们没有考虑肤色或人们平常使用防晒霜的量。
So when you're buying this stuff, it's probably worth keeping those things in mind.
所以当你在购买防晒霜的时候，或许该考虑这些问题。
For example, maybe this means it's worth putting on sunscreen more often than you currently do
例如，或者这意味着更经常地涂防晒霜
If nothing else, though, the good news is that researchers are at least aware that there's a problem here,
不过，好消息是研究人员至少意识到此处存在一个问题，
and they've been working on protocols to hopefully get more consistent results.
并且他们一直在研究有望得到一致结果的方案。
Their ideas include using robots that spread sunscreen evenly in all conditions
他们的想法包括使用机器人在所有条件下均匀涂抹防晒霜
and shifting towards standardized in vitro testing using chemicals in a dish —no human skin involved.
并转向使用培养皿中的化学物质进行标准化的体外测试—不包含人类皮肤。
This will let scientists get way more data points and will improve accuracy.
这将让科学家得到更多数据点并将提高准确性。
At the end of the day, SPF can be a helpful way to understand what you're buying.
考虑所有情况之后，SPF值可以帮助你了解你要买的东西。
To make an informed decision, you just need to know what it says and where those numbers are coming from.
为了做出合理的决定，你只需要知道SPF指的是什么以及这些数字从何而来。
And it's also worth knowing that not all hope is lost, either.
同样值得了解的是，并非所有的希望都破灭了。
Even if this is a bit of a flawed system, multiple studies have shown that consistent use of sunscreen,
即使这是一个有缺陷的体系，多项研究已经表明坚持使用防晒霜
even at low SPF, reduces your risk of skin cancer. So, it's always worth lathering up!
即便是低SPF值的防晒都能降低患皮肤癌的风险。所以防晒是值得涂抹的！
If you've ever wished you just didn't have to wear sunscreen — well, I've got some sad news for you.
如果你曾经希望自己不需要涂防晒霜——好吧，我有个不幸的消息要告诉你。
Almost all amphibians, reptiles, birds, and fish can make their own sunscreen. Mammals are kind of the odd ones out.
几乎所有两栖动物、爬行动物、鸟类和鱼类都能自制防晒霜。哺乳动物却是异类。
If you want to learn more, you can watch our episode about the missing sunscreen gene after this.
如果你想了解更多，你可以观看我们关于防晒霜基因缺失的视频。
