My Advice on Masks for Coronavirus

This is part of a series of articles on Covid-19, please click here to see all of them…

Does wearing a mask actually stop you from getting Covid-19? The short answer is an N95 mask helps healthy people to not get ill from sick people, and the surgical mask helps sick people not make others sick. But even if you had a bunch of N95s, and assuming that you were properly wearing it, almost everyone should not be wearing these all the time. I’d like to explain why, this will take a bit but I have a lot of experience with masks during my ten years in Beijing, and I have written hundreds of articles about N95 masks and air pollution, including for  in China. So in terms of Covid-19, here’s my take.

What Does N95 Mean?

A true N95 mask, technically called a “respirator”, has a strict definition from NIOSH, the , which rates masks in the USA: N95 must filter at least 95% of airborne particles larger than 300 nanometers (0.3 microns, or micrometers). That includes everything floating in the air, including bacteria, carbon soot, pollen, diesel exhaust, viruses, etc. By the way, there are N99 and N100 masks, which do even better (for a price). Here’s a nice image showing particle sizes:

This 0.3 nm size is hard to imagine, but for comparison, the width of a human hair is 70 microns, over 200 times larger than 0.3 micron. In the air pollution field, we talk a lot about PM2.5, meaning particles of 2.5 micrometers, 20 times larger than NIOSH’s 0.3 micron cutoff. A well fitted N95 mask can easily filter out typical PM2.5 particles, since it’s been proven in tests to filter out 95% of particles 20 times smaller. We worry about PM2.5 because they are tiny enough to drift in the air, straight down our airways and deep into lung tissue, instead of getting stuck in our upper airways and coughed out. 

So that’s some groundwork data; what about Covid-19? Do masks work or not?

How large is Covid-19?

A mentions that Covid-19 is 125 nanometers, or 0.125 microns in size. Apparently it’s a bit larger than the influenza virus. So we’ve arrived at an important data point: the Covid-19 virus at .125 microns is smaller than that 0.3 micron cutoff that NIOSH uses in their testing. This means those N95 masks in theory may or may not be as effective for Covid-19 as for PM2.5, because they didn’t test for particles that small. They may work, but it’s not something I think people should stake their life on.

All of this is theory anyway, I write an evidence-based blog and I want real world data, not just tests on a mannequin head. What would be most reassuring would be to see specific results against Covid-19 — but I haven’t seen anything yet. If anyone has, please let us know! Now we must rely on the next best option: research from other viral studies. (I’ll get to those in a minute).

There actually is a special test called , or VFE, where the person wearing the mask speaks for 2 minutes, and the machine checks the percentage of a test virus inside the mask compared to particles outside the mask. (there’s also a bacterial filtration efficiency test). I didn’t have enough time to review VFE for this article, but I would love to see a list of masks approved by VFE.

What About Surgical Masks?

I want to stress first that a surgical mask is much less effective than an N95 against PM2.5 and for viruses. I did my own fit testing for PM2.5 while in China and my surgical mask was only 63% effective! Surgical masks generally only stop droplets, usually clumps of particles larger than 5 microns. Smaller particles like one Covid-19 virus can sneak right through the fabric and into your body. That’s why a surgical mask is mostly for sick patients, not healthy people. But having said all this, at this exact moment, healthcare workers are allowed to use surgical masks if we don’t have N95 in stock. One reason is shortages of N95; another reason is partly from a big study from last year, which I’ll get to in a minute.

What About Published Real World Studies For N95 and Viruses?

What about for masks for Covid-19 prevention in our communities? The best study I’ve seen is from , following healthy college students in petri dishes dorms for 6 weeks during flu season, and giving masks and/or hand sanitizer. The mask-only group did not have less flu than the control group (which had no masks or sanitizer), while the mask+sanitizer group had a very impressive 75% reduction in flu cases! That difference I feel is very important, it suggests that hand washing is much more effective than masks for prevention. 

What about healthcare workers, what’s the best data? The best study was . They tested 2,800 healthcare staff at the front lines of a clinic for four flu seasons, giving them either an N95 or a surgical mask when seeing sick patients. The final data showed no real difference between the groups. This could mean that in real-world use, even though N95 clearly is more efficient in clinical tests, in the real day-to-day world the benefits are a wash. This is partly why right now healthcare workers are allowed to use surgical masks as a backup plan. 

My Own Real World N95 vs. Surgical Mask Results

In all of my public health blogging, I’ve always stressed real data and not theory or anecdotal testimonials, and sometimes you have to do the tests yourself if no one else is. In 2014 I was deeply frustrated with the lack of clarity on whether masks helped against air pollution, so I did my own tests. I visited 3M’s testing lab in Beijing and did my own quantitative fit testing, testing nine popular masks, including one surgical mask.

It’s a pretty cool test, you make a tiny hole in the masks and measure air particles inside and outside the mask, then you spend a few minutes doing real-world tests including reading a poem, turning head left and right, and bending over. I was really impressed at how well an N95 mask did, filtering as much as 97.5% of tiny particles. An N99 mask did even better, exactly as expected with 99.6%. Here’s the data, also showing that poor 63% result with a surgical mask. The worst was Ludun, which horrified me because this brand was hugely popular in China, including for kids, and I was stunned how many millions of Ludun wearers were falsely reassured, spending hours outside thinking they were safe when clearly they were not.

Total particle test results

In an interesting followup, the good folks at Smartair in China also and put together a graph showing all of our results put together. Again, this is real world testing on real faces, and again the N95 masks did great. Don’t forget that all this data is for PM2.5, not for viruses. 


So who should be wearing these? 

Let’s hear directly from :

Wearing medical masks when not indicated may cause unnecessary cost, procurement burden and create a false sense of security that can lead to neglecting other essential measures such as hand hygiene practices. Furthermore, using a mask incorrectly may hamper its effectiveness to reduce the risk of transmission.

As for me, I am not wearing a mask, of any kind, in my daily routine outside of work. I am definitely focused on prevention that has the best evidence: hand washing, social distancing, not touching surfaces and face if possible. I have a box of nitrile gloves in my car which I could use at the market or public places. I still go to my local coffee shop but am more careful in the bathrooms, using the paper towel or toilet paper on the door handle and faucets.

I am also super focused on keeping my immune system in tip top shape, as I detailed in my article last week which I urge you to read if you haven’t, it’s filled with empowering and positive yet effective evidence-based tips.  

As always, I hope I’ve shared credible information that helps you to stay healthy and feel more empowered. There’s a lot we can do to fight this! Please feel free to leave comments below. 

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20 thoughts on “My Advice on Masks for Coronavirus”

  1. thank you for the article, you mean I don’t need to take my badass looking anti-pollution mask to protect myself against the virus ? 🙂 I was happy to take it out of the Beijing Souvenir box…

    By the way, I can’t see any of your images here in France, are they hosted in China ?

    1. My server is in the USA so all the images should be working, sorry, not sure why you can’t see them. Perhaps clear your browser cache?

  2. I am on mac, surfing with safari that gives me broken images symbols, Firefox leaves empty gaps and Chrome does like there is no images at all, so does my browser on my Android phone… bizarre… the only pictures I can see are the first one of you and your mask and helmet and the last one of the boy on profile showing the gap under the mask.

    1. Strange! No one else has mentioned. how about via mobile phone browser?

    1. You are correct, there is a discrepancy, thanks for pointing this out! I will need to revise this post, I wrote that last week before digging into the details more. Thanks for spotting that. Doesn’t change the main issue that masks are in short supply and we need to focus on saving them for healthcare workers.

      1. Thanks!
        I guess many people who are following you since many years like already have some, but indeed.

      2. Thank you for such great articles on remaining safe during this global pandemic. As me and my 13-year-old daughter are at high risk for complications if we get this virus, I am still interested in wearing a mask if we go out (we have not left the house in three weeks and disinfect all deliveries). But we cannot find any. I am thinking about making our own with some non-weave fabric and a 2.5 PM carbon filter… but I can’t find reliable filters online to purchase either! Do you know where one can purchase these materials? Thank you so much for your help.

      3. Sorry I don’t! I hear some people are using HEPA filter material from local hardware store, don’t know how well that would work. I would stick with what the article mentioned, cotton t-shirts and pillowcases. But did you try to buy them from China? I mentioned using to get real masks.

  3. Hello! Your posts are very informative. I was wondering if homemade masks offer any protection against covid 19. I understand that handwashing and social distancing are the frontline defences, but I work in an Amazon warehouse and we are busier than ever with everyone staying home and ordering online. I wouldn’t want to attempt to get a mask that a healthcare worker may need, but I was reading a story about a hospital that was asking for homemade masks and was wondering if it was at all useful.

    1. as a matter of fact there is some evidence Can DIY Masks Protect Us from Coronavirus? – Smart Air Filters has written a fantastic review. Bottom line: yes partially effective at ~50%, surgical masks ~65%, N95 of course awesome at >95%. So ok, if you have no masks at all, these homemade masks are better than nothing. But my gosh people, instead of making these, why aren’t we just ordering real masks right now directly from China at I’ve been pleading for people to do this.

      1. Hi Richard,

        Your info is great! I live in Canada, where it just got so much worse in the past two weeks. I can’t find any masks anywhere, and we don’t even have enough for our medical system. I was looking for reuseable alternatives and found your article about Totobobo mask. I know your opinion has changed over time, but would it be at least a better choice than a regular surgical mask? Thank you very much!

      2. Oh absolutely it would be much better. You can check their website out, they are probably blogging about their masks and COVID-19

      3. Hi there, right now in Canada there is such a huge push to give all our real PPE to healthcare workers. Even if I had something at home that was opened or expired and therefore couldn’t donate it, I wouldn’t wear it because I don’t want people to think I’m hoarding them for myself! I’m sewing my own masks, using articles like the Smart Air one for reference.


        Here is a recent research by the Japanese that will make the argument of using face mask more compelling.

      4. in the study “Do N95 respirators provide 95% protection level against airborne viruses, and how adequate are surgical masks?” , figures 4 5 6 and 7 show the effect of the inhalation flow rate on the fractional penetration of MS2 virus through various masks. Why would small particle sizes (20nm) have less penetration than larger sizes? you would think a smaller sized particle would penetrate the mask easier than a larger sized particle. Am i just reading this wrong?

        In figures 4 and 5 the n95 respirators behave even more strangely as the penetration goes up w particle size to a point then drops? ?? why does this happen?


A family doctor's wellness guide for China