宅在家里的你也许不知道，空气净化器可以过滤冠状病毒
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2020年初爆发冠状病毒后，数百万人蜂拥购买口罩来保护自己免受病毒感染。数据显示，口罩可以保护你免受冠状病毒的感染，但是有没有什么方法可以帮助你——空气净化器也可以捕捉冠状病毒颗粒吗?
幸运的是，科学家已经积累了能够回答这些问题的可靠数据。
空气净化器里有什么?
大多数空气净化器由两个部件组成：风扇和滤网。就是这么简单。
点击学习如何自制空气净化器及空气净化器如何工作
我们应该都知道风扇是怎么工作的，但是滤网呢？滤网——通常是HEPA滤网——是所有空气净化器的主力。但其实他们也没什么特别的，它们是在20世纪40年代发明的，只不过是一层合成纤维而已，它可以捕获99%以上的微小空气污染颗粒。
但是冠状病毒有多小呢？HEPA高级过滤滤网能捕获冠状病毒的病毒颗粒吗？让我们分成两个问题来讨论：
1. 冠状病毒颗粒有多小？
2019年新型冠状病毒，或称2019年nCoV，是中国湖北省武汉市首次发现的一种新的呼吸道病毒。科学家们已经拍摄了电子显微镜图像来测量冠状病毒“病毒粒子”（或粒子）的大小。病毒粒子是直径约125纳米(0.125微米)的球形颗粒。最小的颗粒是0.06微米，最大的是0.14微米。
也就是说冠状病毒颗粒比PM2.5颗粒小，但比一些灰尘颗粒和气体大。
2. HEPA滤网能捕获像冠状病毒那么小的微粒吗？
打开维基百科的HEPA滤网词条就能看到这个定义：
“标准要求空气滤网必须移除99.97%大于等于0.3微米的颗粒物。”
这就很了不起了，0.3微米可是很小的！相当于人的头发细200倍，而HEPA滤网可以捕捉99%以上的这么细小的颗粒物！
现在回过头看一下HEPA滤网的定义，好像只提到了大于等于0.3微米的颗粒物。那么和冠状病毒一样大小的小颗粒呢？HEPA滤网能捕获它们吗?
这个问题直接导致多年来大多数人的困惑和很多公司在HEPA滤网上做的虚假宣传，所以我们直接切入主题，追根究底一下HEPA到底怎么回事。
为什么我们关于滤网的直觉是错误的
答案很让人吃惊，我们关于滤网的直觉是不准确的。大多数人包括我在内都觉得HEPA是个网子，如果颗粒物小于网上的洞洞，就会被漏掉。感觉很有道理对吧？
这种直觉对大一点的颗粒物来说没毛病，大的颗粒物飞过HEPA滤网的时候是被卡住的。
但我们像冠状病毒这样非常细小的颗粒物时，情况就不一样了。事实证明，像冠状病毒这样小的颗粒和其他纳米颗粒是通过一种叫做扩散的科学现象被捕获的。
扩散在捕捉病毒大小的小分子方面非常有效。根据美国NASA的说法，HEPA滤网“几乎100%地捕捉到微粒”。
明尼苏达大学用玻璃纤维滤网作为媒介做过一个实验，玻璃纤维的滤网“接近HEPA对小颗粒物的捕捉能力”，也就是说这种滤网和HEPA滤网的表现应该差不多。实验中小至3-20纳米的银粒子被射向滤网媒介（比冠状病毒粒子小3到20倍）。
结果表明，滤网能捕获99.99%的小于5纳米的微粒。HEPA滤网过（甚至是更低级别的过滤器，如熔炉过滤器）在捕捉更小的颗粒，比如冠状病毒方面都非常有效！
这对冠状病毒意味着什么?
如果你住在普通的房子里，或者你在有大量员工的办公室或学校工作，打开你的空气净化器可以帮助过滤掉空气中任何病毒大小的颗粒，潜在地减少传播的机会。通过一次HEPA滤网，就能过滤掉99%的冠状病毒。
结论：空气净化器能捕获冠状病毒吗?
含有HEPA滤网的空气净化器或更低级别滤网的空气净化器可以捕获几乎所有大小的固体颗粒，包括直径0.1微米的颗粒——或与冠状病毒相同大小的颗粒。
如果你住在一个合租的房子里，或者工作在一个封闭的环境里，周围有很多人，打开空气净化器有助于减少冠状病毒的传播。
对于那些有中央空调系统的地方，确保这些系统包含滤网并得到良好的维护。如果你的中央空调系统没有滤网，在每个房间安装空气净化器仍然可以帮助减少病毒的传播。
当心！北京空气污染
北京以及东北地区的空气污染水平远远超过了世界卫生组织安全水平的20倍。
北京市政府因PM2.5含量过高，而在周二宣布了大雾黄色预警。建议大家在屋内打开空气净化器，并限制您在户外的时间！
Can Air Purifiers Protect Us From The Coronavirus (Covid-19)?
With the outbreak of the coronavirus (now called Covid-19) in Wuhan, face masks have been flying off the shelf. Masks are now near impossible to purchase, so is there something closer to home that can help protect us from Covid-19 - can air purifiers also capture viruses?
What's in a Typical Air Purifier?
Most air purifiers are made up of two things: a fan and a filter. They’re as simple as that.
Read how air purifiers work and how to build your own »
Most of us understand how fans work, but how about the filter? The filter – typically a HEPA filter – is the workhorse of any purifier. Turns out they’re nothing fancy either. They were invented back in the 1940s, and they’re just a mat of synthetic fibers. That can capture over 99% of tiny air pollution particles.
Now that we know how simple air purifiers are, can they filter out Covid-19 particles? Let’s break this down into two simpler questions.
1. How Small are Covid-19 Particles?.
Covid-19 is a new respiratory virus first identified in Wuhan in China’s Hubei Province. Scientists have already taken electron microscope images to measure the size of Covid-19 “virions” (or particles). The virions are spherical particles with diameters of approximately 125 nm (0.125 microns). The smallest particles are 0.06 microns, and the largest are 0.14 microns.
That makes the Covid-19 particles smaller than PM2.5 particles, but bigger than some dust particles and gases.
2. Can HEPA Filters Capture Particles As Small As The Covid-19?
If we look to the definition of HEPA filters on Wikipedia, it says HEPA filters must remove: “99.97% of particles that have a size greater than or equal to 0.3 µm”.
It only mentions particles 0.3 microns and above. So what about smaller particles the same size as the Covid-19? Can HEPA filters capture these?
This exact question has led to years of confusion and false advertising claims about HEPA filters, such as this one, so let’s get to the bottom of it.
And this confusion is now leading to more misinformation during the spread of information during the Covid-19 outbreak:
Why Our Intuitions About
Air Purifier Filters Are Wrong
The answer is surprising. It’s surprising because our intuition about filters is wrong. Like most people, I had the intuition that HEPA filters work like a net.
If a particle is smaller than the holes in the net, it gets through. Makes sense! However, it turns out that particles as small as the Covid-19, and other nano-particles get captured using a scientific phenomenon called diffusion.
Learn more about the science behind diffusion and how it captures nano-particles »
Diffusion is surprisingly effective at capturing tiny virus-sized aprticles. According to NASA, HEPA filters capture “virtually 100% of particulates.”
Researchers at the University of Minnesota tested this question with weaker fiberglass furnace filters and higher-grade HEPA filters. In their test, they shot particles of silver from 3 to 20 nanometers at the filters (that’s between 3 to 20 times smaller than Covid-19 particles).
The results showed that filters captured 99.99% of particles smaller than 5 nanometers. Bingo! HEPA filters (and even lower-grade filters like furnace filters) are incredibly effective at capturing smaller sized particles such as the Covid-19.
What Does This Mean for Covid-19?
If you’re living in a common house, or you’re working in an office or school with a large number of employees, turning on your air purifier can help filter out any virus-sized particles in the air, potentially reducing the chance of transmission. HEPA filters, in one pass, can be expected to filter out over 99% of the Covid-19.
Bottom Line: Can Air Purifiers Capture Covid-19?
Air purifiers that contain HEPA filters or even lower grade filters can capture virtually all sizes of solid particles, including particles 0.1 microns in diameter – or the same size as the Covid-19. If you’re living in a shared house, or are working in a closed environment with lots of people, turning on an air purifier could help reduce the spread of Covid-19.
For those in places with central air (HVAC) system, make sure the systems contain filters and are well maintained. If your HVAC contains no filters, installing air purifiers in each room could still help reduce the spread of viruses.
Watch Out - Beijing Air Pollution!
Perhaps another more important reason to turn on your purifier right now is because Beijing and much of Dong Bei's air pollution levels are over 20 times the WHO safe level. Beijing's government announced on Tuesday a yellow pollution alert due to the high levels of PM2.5 in the city. Turn on your air purifier, and limit your time outside!
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