口罩真的管用吗？Can masks really stop pollution?
口罩真的管用吗？
(Scroll down for English version)
当全国各地的中国人（以及一大批住在中国的外国人）早晨醒来时都不得不面对严重的空气污染时，现实迫使十几亿中国人民需要在一个原本复杂的科学问题上成为专家——口罩真的管用吗？
从那时开始，我已经给中国各地数以百计的人做过演讲，来分享如何保护自己免受空气污染侵害的知识。在这些演讲中，我听到了一些来自聪明又富有怀疑精神的人们的质疑。在这里我想回答这些疑问，因为很幸运的是，聪明又富有怀疑精神的科学家们（再加一个献身科学的书呆子——我自己）已经用实验数据回答了这些质疑。
怀疑派说：
最危险的颗粒是那些最小的颗粒。但是口罩这么薄，怎么可能阻挡到最小的颗粒？
科学测试：
爱丁堡大学的研究人员测试了不同种类的常用口罩。他们用了柴油发电机来模拟汽车排气装置，向不同口罩排放废气，然后用一个粒子计数器测出有多少颗粒穿透了口罩。下面是我画的实验示意图：
一个重要的细节：他们使用的粒子计数器能够测量到0.007微米的颗粒。我们这里讨论的可是真正的微小颗粒！
首先他们用一个简单的棉质手帕做了测试。有时我在中国看到骑自行车的人会戴着这种手帕。
结果不是很好，过滤了28%的颗粒。
下一个测试的是便宜的医用口罩。
效果不错！这个结果会让人感到意外（在密封性测试中它的表现会差大概20%，参考下面文章，但是已经比大部分人们直觉认为的结果好得多）。
然后他们又测试了一些自行车用口罩：
大部分达到80%了。
接下来测试了一些便宜的3M口罩：
这些口罩的都过滤了95%以上的颗粒。很不错！
结论：即使是非常微小的颗粒，口罩也能过滤掉。
怀疑派说：
理论上口罩是管用的，但是这些测试不是在真人脸上做的。当你真的戴着口罩时，它不能很好地贴合脸部，所以它们还是没用。
科学测试：
这个问题更难回答，因为你必须在实际戴着口罩的时候测试口罩效果。为了做到这一点，你需要一个非常昂贵的密封性测试仪器。幸运的是，在我好几周不断的请求下，3M公司终于同意我用他们在北京的实验室。
蓝色管子采集口罩外的空气，白色的管子从口罩内采集空气（密封测试细节）。
Smart Air创始人Anna和生活在北京的Richard Saint Cyr医生也做了口罩密封测试，所以我把我的数据做了个整合。这是不同口罩在我们脸上的效果：
明确一点很重要：在我脸上做的测试不一定适用于其他人。但是，也有更广泛的实验数据。香港的研究者测试了22位中国人，结果发现3M口罩在他们脸上平均效果达到了99.5%。这个结果相当于Saint Cyr医生和我的测试结果中的最好成绩了。所以有实验数据说明在中国人的脸上，口罩也可以达到很高的密封性。
下面的图是口罩测试结果和价格对比。
好在有效的口罩并不需要花费昂贵的价钱。
例外：关于气体污染
口罩的测试结果很好，但是也要注意：这些测试都是关于颗粒污染的。大部分市面上销售的口罩都不是用来过滤二氧化氮（NO2）或者臭氧（O3）的。所以口罩不是我们100%的保护伞。
这可能是最难回答的一个问题了。然而，还是可以参考两个可靠的研究。研究人员在北京随机分配一组人戴着或者不戴口罩在北京步行，然后测量他们的心率和血压（1，2）。
戴着口罩的参加者血压更低而且心率更有规律。
结论：口罩可以过滤非常小的颗粒，能达到很高的密封性，而且有专业研究结果表明戴着口罩有利于健康。这些应该足够让怀疑派们信服！
Do Masks Really Work?
When a billion people in China (and quite a few expats) woke up to the severe air pollution in almost every city in China, it forced a billion people to become experts in a complicated scientific question: Do masks work?
Since then, I’ve given talks with hundreds of people all around China about how to protect themselves from air pollution. In those talks, I’ve heard doubts from smart, skeptical people. Here I’ll answer those doubts because, fortunately, smart, skeptical scientists (plus one dedicated nerd—yours truly) have empirically tested these questions.
1. “There’s no way they capture the really small particles.”
The skeptic case:
The most dangerous particles are the smallest particles, but masks are so thin. How could they possibly get the smallest particles?
The scientific test:
Researchers from the University of Edinburgh tested different common masks by running a diesel generator (to mimic car exhaust) and piping the exhaust through different masks. They used a particle counter to see how many particles made it through the mask. Here's my super scientific rendering of the setup:
One important detail: the particle counter they used measures down to .007 microns. We’re talking about truly tiny particles here!
First they tried a simple cotton handkerchief. Sometimes I see bikers in China wearing these.
Not great, 28% of particles blocked.
Next they tried a cheap surgical mask.
Surprisingly good! (Fit tests generally show lower results--see below--but still a lot higher than most people's intuition.)
Next they tried several bike masks.
Most were around 80%.
Then they tried several cheap 3M masks.
They all scored over 95%. Pretty good!
Conclusion: Masks capture even very small particles.
2. “OK, they capture the small particles, but when you wear them, all the air just leaks in the side.”
The skeptic case:
Masks work in theory, but those tests aren’t on real faces! When you actually wear them, you can’t get a good enough fit, so they’re basically useless.
The scientific test:
This question is tougher to answer because you have to measure the mask while you'reactually wearing it. For that, you need a really expensive fit test machine. Fortunately, I begged and begged 3M until they let me use their lab in Beijing:
The blue tube is sampling air outside the mask, while the white tube is sampling air from inside the mask (more details on the methods here ).
Smart Air co-founder Anna Guo and Beijing-based Dr. Richard Saint Cyr also tested masks, so I combined all of our data. Here’s how well the masks worked on our faces:
How well do masks work for the broader population?
It’s important to make clear: masks that fit my face well might not fit other people's faces well. However, there is evidence from a broader population that masks fit most people well. A scientific study of 3M masks on 22 Chinese people found a median fit score of 99.5%--essentially the same as the top results from Dr. Saint Cyr and me.
Best yet, effective masks don’t cost a lot of money.
Exception: Gas Pollutants
Masks are great, but note that these are tests on particulate pollution (such as PM2.5). Most commercially available masks don’t target gas pollutants like NO2 and O3, so masks are not giving 100% protection.
3. Is there a documented health benefit of wearing a mask?
This is probably the hardest question to answer. However, there are two solid studies that have randomly assigned people in Beijing to wear masks or not and measured their heart rate and blood pressure (1 , 2).
While wearing masks, people had lower blood pressure and better-regulated heart rates.
Conclusion: Masks capture even the smallest particles—even while people are wearing them—and they have documented health benefits. That should be enough to satisfy even the skeptics!
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