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Prior to March 2020, there’s a good chance you didn’t know what an N95 mask was,
在2020年3月之前，你很可能不知道N95口罩是什么，
or at least didn’t think about them unless you were doing a home repair project with lots of dust,
或者至少没有想过它们，除非你在做一个有很多灰尘的家庭清理项目，
or live in a part of the world with crazy pollution or wildfire smoke.
或者生活在一个有着严重污染和野火烟雾的地方。
And upon learning about them, you might think (like I did) that an N95 mask is basically a really really fine strainer:
了解N95口罩后，你可能会认为(像我一样)N95口罩基本上是一个非常非常好的过滤器：
a mesh of fibers with gaps too small for dust and other airborne particles to get through.
一种纤维网状物，其空隙非常小，灰尘和其他空气中的颗粒无法通过。
A strainer filters out particles larger than its openings, and not particles smaller than its openings.
过滤器可过滤比它的空隙大的颗粒，不可以比它的空隙小的颗粒。
So with a mask you’d expect that after a certain point, small enough particles will sneak through.
所以用口罩时，你会预期在某个点之后，足够小的粒子会偷偷通过。
But this isn’t how N95 masks work: the particles they filter are generally much smaller than the gaps between fibers in the mask!
但这并不是N95口罩的工作原理：它们过滤的粒子通常比口罩纤维间的空隙要小得多！
What’s more, an N95 mask is actually really really good at filtering both the largest and smallest small particles --
更重要的是，N95口罩实际上非常擅长过滤最大和最小的小颗粒--
it’s medium-sized small particles that are hardest for it to block.
它最难阻挡的是中等大小的小颗粒。
This isn’t at all like a strainer… because N95s are much cleverer than strainers.
它根本不像过滤器...因为N95口罩比过滤器聪明得多。
The overarching goal of an N95 mask is instead to get an airborne particle to touch a fiber in the mask.
N95口罩的首要目标是让空气中的微粒接触到口罩中的纤维。
Regardless of how big an airborne particle is, once it touches a fiber, it stays stuck to it and doesn’t become airborne again.
无论空气中的微粒有多大，一旦它接触到纤维，它就会粘在纤维上，不会再流通。
This isn’t anything special about the fibers, but about the size of the particles.
这与纤维无关，而是与颗粒的大小有关。
At a microscopic scale everything is sticky,
在微观尺度上，一切东西都是黏的，
because the weakly attractive force between molecules is more than strong enough to hold small things in place.
因为分子之间微弱的引力足以把小物体固定在适当的位置。
So you shouldn’t think of N95 masks like a fine window screen that keeps insects of a certain size out;
所以你不应该认为N95口罩像一个很好的纱窗，可以防止某种大小的昆虫进入；
you should think of them more like a sticky spider web that can catch an insect of any size, as long as it touches a strand.
你应该把它们想得更像一张黏糊糊的蜘蛛网，它可以抓住任何大小的昆虫，只要它碰到一根线。
And so N95 masks use a bunch of different clever physics and mechanical tricks to get particles to touch their fibers.
所以N95口罩使用了一系列不同的巧妙的物理和机械技巧来让粒子接触到它们的纤维。
First, many spiderwebs are better than one.
首先，很多蜘蛛网总比一个好。
Unlike strainers, where stacking many identical ones doesn’t improve the filtering at all,
堆积许多相同的过滤器根本无法改善过滤效果，与过滤器不同的是，
more layers of sticky fibers means more chances for particles to get stuck.
N95口罩粘性纤维层数越多，颗粒被粘住的可能性就越大。
And how likely particles are to hit or miss a fiber depends in large part on their size.
粒子碰到或错过纤维的可能性很大程度上取决于它们的大小。
Airborne particles larger than a thousandth of a millimeter basically travel in straight lines, because of their inertia.
由于惯性，大于千分之一毫米的悬浮粒子基本上是直线运动的。
And because there are so many layers of fibers, their straight line paths are essentially guaranteed to hit a fiber and stick.
因为纤维有很多层，所以它们的直线路径基本上会保证它们碰到纤维并粘在一起。
Airborne particles that are really really small are so light that collisions with air molecules literally bounce them around,
而非常非常小，非常轻的悬浮粒子，当它们与空气分子碰撞时，它们就会反弹，
so they move in a random zig-zag pattern known as Brownian motion.
所以它们以一种随机的锯齿状运动模式运动，它被称为布朗运动。
This zig-zagging also makes it super likely that a particle will bump into a fiber and get stuck.
这种锯齿状运动也使得悬浮粒子极有可能撞击到纤维并被粘住。
Particles of in-between sizes are the hardest to filter.
介于两者之间的颗粒是最难过滤的。
That’s because they don’t travel in straight lines, and they also don’t bounce around randomly.
这是因为它们不做直线运动，它们也不会随机反弹运动。
Instead, they’re carried along with the air as it flows around fibers,
相反，它们它们随着空气在纤维周围流通，
meaning they’re likely to get carried past fibers and sneak through even a mask with many layers.
这意味着它们很可能会穿过纤维，甚至穿过多层的口罩。
But N95 masks have a final trick up their sleeve.
但N95口罩还有最后一招。
They can attract particles of all sizes to them using an electric field.
它们可以利用电场吸引各种大小的粒子。
In the presence of an electric field even neutral particles develop an internal electrical imbalance which attracts them to the source of the field.
在电场存在的情况下，即使是中性粒子也会产生内部电不平衡，这会把它们吸引到电场源。
This is why neutrally-charged styrofoam sticks to a cat whose fur has been charged with static electricity.
这就是中性的泡沫聚苯乙烯粘在一只被虐待的猫身上的原因——我的意思是，这只猫的皮毛上带有静电。
But unlike a cat’s fur, an N95 mask’s electric field isn’t just ordinary static electricity.
但与猫的皮毛不同的是，N95口罩的电场不仅仅是普通的静电。
Their fibers are like permanent magnets, but for electricity: electrets!
它们的纤维就像永磁体，只不过是电：驻极体！
Just like you can permanently magnetize a piece of iron by putting it in a strong enough magnetic field,
就像你可以通过把一块铁放在足够强的磁场中永久磁化一样，
you can ‘electretize’ a piece of plastic to give it a permanent electric field.
你可以将一块塑料“驻极体化”，给它一个永久的电场。
By electretizing the fibers in an N95 mask, they gain a long-lasting ability to attract particles,
通过使N95口罩中的纤维成为驻极体，它们就获得了持久的吸引粒子的能力，
which means they capture about 10 times as many particles as regular fibers.
这意味着它们捕获的粒子是普通纤维的10倍。
And this is, after all, the point of an N95 mask: filter out particles from the air.
这就是N95口罩的作用：过滤掉空气中的微粒。
And they do it cleverly. By taking advantage of the molecular scale stickiness of matter,
它们的做法很聪明。利用物质的分子尺度粘性，
using many layers of fibers that catch straight-moving large particles as well as zig-zagging small particles,
使用多层纤维来吸附直线运动的大颗粒和曲折运动的小颗粒，
and having an electric field that attracts all particles, you get a mask - not a strainer -
再加上一个能吸引所有粒子的电场，你就得到了一个口罩，而不是一个过滤器，
that’s really good at trapping both small and large particles, and does a reasonably good job at filtering out middle sized particles.
它能很好地捕获大大小小的粒子，也能很好地过滤掉中等大小的粒子。
Precisely what fraction of those sneaky medium-sized particles get blocked gives you the number of the mask -
准确地说，这些中等大小的微粒中有多少被阻挡了，你就得到了这种口罩的数值，
if at least 95% of those particles are filtered out, then the mask is rated N95.
如果至少有95%的微粒被过滤掉，那么该口罩的等级是N95。
Ok, Caveats. so N95 masks can be very effective.
所以，提醒一下。N95口罩非常有效。
But if you’re a healthcare worker wearing one of them, here are a few important things to look out for.
但如果你是一名医护人员，戴着一个N95口罩，这里有一些重要的注意事项。
The biggest influence on the performance of an N95 mask isn’t actually the mask - it's whether you wear it properly.
影响N95口罩性能的最大因素并不是口罩本身，而是佩戴是否正确。
If a mask isn’t fully sealed on your face, air ( particles you’re trying to filter) can just bypass the filter entirely.
如果口罩没有完全贴合在你的脸上，空气(你试图过滤的粒子)可以完全绕过过滤器。
Dust, smoke, pollen, bacteria, and viruses all have different sizes, and so are filtered by N95 masks to different extents.
灰尘、烟雾、花粉、细菌和病毒大小都不同，所以被N95口罩过滤的程度也不同。
However, germs for airborne illnesses don’t usually travel on their own –
然而，空气传播疾病的细菌通常不会自行传播，
we breathe or cough them out in droplets which have a wide range of sizes.
它们存在于我们呼吸或咳嗽时吐出的大小不一的液滴中。
So the size of the virus or bacteria itself isn’t particularly relevant.
所以病毒或细菌本身的大小并不是特别重要。
N95 masks are intended to be disposable, but the demand from COVID-19 has led to a global shortage of N95 masks
N95口罩本来是一次性的，但新冠病毒带来的需求导致全球N95口罩短缺，
and the reality is that health workers have to reuse them - and thus decontaminate them.
事实是，卫生工作者必须重复使用它们，如此就要净化它们。
It’s important to be aware that certain kinds of decontamination (for example, using alcohol or liquids)
重要的是要意识到某些类型的净化(例如，使用酒精或液体)
can damage the electrostatic properties of a mask and destroy their filtering ability, even if the mask appears visually unaffected.
会破坏口罩的静电特性和过滤能力，即使该口罩在视觉上不受影响。
