Does Your Face Mask Fit Right to Protect You? Probably Not…

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Does your face mask fit right

This week we introduced N-95 masks in our Colorado office as well as the HEPA filtration masks we had already 3D printed. Given the almost mythical status granted to the N-95 mask during the pandemic, I expected to be utterly impressed. However, it soon became clear that my mine didn’t work. My first clue on why it wasn’t working was my fogged procedure glasses. So let’s dive into how masks should fit and how you’ll know if your expensive mask is doing its job or is merely a fashion accessory.

The Data on Masks

When I see some science reporter writing on the latest university experiment that uses some device to see what gets through a mask, I chuckle. Why? That says little about how the mask works in the real world. In fact, the only studies I pay attention to are those that have randomized people to wear different types of masks and track to see if they got sick. Why? That’s a real-world test of the technology being used.

This is the conclusion from a recent Lancet article on this topic, “Evidence that face masks can provide effective protection against respiratory infections in the community is scarce, as acknowledged in recommendations from the UK and Germany.” (1) A research study in 2009 that tested surgical masks to prevent others in a household from contracting a respiratory virus found that this method didn’t work (8). There’s also a review paper that looked at 17 different studies on whether wearing a surgical mask would slow the spread of respiratory viruses, however, the findings were mixed (3). The study authors concluded, “None of the studies established a conclusive relationship between mask/respirator use and protection against influenza infection.” 

How Some Masks Can Help-Fit and Filter

There is research showing that better masks can reduce your risk of catching an aerosolized virus like the coronavirus. How are those masks different? They obey the “Two F Rule”.

The two Fs of mask quality are fit and filter. The fit is how much the mask lets in or out around the edges. A good mask lets very little in or out in those areas. A big problem spot is for surgical masks and N-95 masks is the bridge of the nose.

“Filter” means that the mask is made of a material that exchanges air that’s capable of blocking at least 95% of airborne particles. That’s where the “95” moniker of an N-95 mask originates.

Cloth Surgical Masks

We’re not going to spend too much time on cloth masks here. They haven’t been proven to work in any clinical study. In addition, the fit is usually atrocious. However, they are starting to make them in all sorts of fabrics and themes, so they look cool.

Surgical Mask

The main difference between a cloth mask and a good surgical mask is that the latter expands around the face and has a good metal insert around the nose that can be molded to the face.

The other day I forgot my surgical mask on my Whole Foods lunch run. Hence, I got one from their staff on the way into the store. Regrettably, this was the budget version of a surgical mask with a plastic insert near the nose that never molded to fit. Hence, this thing was letting air in and out near the top.

How bad are surgical masks at the “fit” part? A recent study of 5 different “good” surgical masks concluded that 80–100% of the masks failed an OSHA-accepted fit test with 12–25% leakage around the sides of the mask (3). The worst N-95 masks that were tested had 1% leakage.

N-95 Mask/Respirator

For a picture, see my mug above on the left. The idea here is two-fold:

  • Better fit to the face-see above.
  • Better filter material

KN-95 Mask

K95 mask

The KN-95 is the Chinese version of the N-95 mask. The good news is that it’s built to have a better face seal and filter material. The bad news is that counterfeits are common. How can you see if the KN-95 mask you have works? There is a three-part test:

  • With the mask on, try to blow out the flame of a match or lighter. If the mask is real, you won’t be able to do this.
  • With the mask on, empty the contents of a pack of sweet and low into a spoon and try to smell it. A real KN-95 will block much of the odor.
  • Fill the mask with water like a cup. A real mask won’t leak.

Fit Testing

In healthcare and industrial settings, there are OSHA fit tests that are conducted. Why? If your mask doesn’t fit, even the best filter in the world won’t help.

My Sunglass Test

This is what was happening to my mask. I instantly knew that if my mask was fogging my procedure glasses that my breath was escaping around the bridge of the nose. If you’re wearing your mask outdoors and it keeps fogging your sunglasses, it’s not working to help you or the people around you.

Smoke Test

This uses a chemical called Stannic Chloride (shown to the left here) which is a respiratory irritant. My N-95 mask failed this test because of a poor fit. My 3D printed mask passed because I had custom fit it to my face.

You can replicate this test by using the match above and creating smoke and trying to smell it with your mask on. A properly fit N-95 mask or better should filter out the smoke particles.

The upshot? The first sign that your mask isn’t working is the sunglass test. As you can see, there are other ways to test masks that are simple and easy to do at home! The single biggest issue is fit, so make sure you test what you wear!



(1) Feng S, Shen C, Xia N, Song W, Fan M, Cowling BJ. Rational use of face masks in the COVID-19 pandemic. Lancet Respir Med. 2020 Mar 20. pii: S2213-2600(20)30134-X. doi: 10.1016/S2213-2600(20)30134-X.

(2) MacIntyre CR, Cauchemez S, Dwyer DE, et al. Face mask use and control of respiratory virus transmission in households. Emerg Infect Dis. 2009;15(2):233–241. doi:10.3201/eid1502.081167

(3) Bin-Reza F, Lopez Chavarrias V, Nicoll A, Chamberland ME. The use of masks and respirators to prevent transmission of influenza: a systematic review of the scientific evidence. Influenza Other Respir Viruses. 2012;6(4):257–267. doi:10.1111/j.1750-2659.2011.00307.x

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6 thoughts on “Does Your Face Mask Fit Right to Protect You? Probably Not…

  1. David Wieland

    Until seeing that you referred to the pictured mask as an N-95 Mask/Respirator, I thought that “respirator” referred to masks with an exhale valve. That’s the kind of Moldex N-95 mask I bought years ago from Lee Valley Tools to use in checking my attic insulation and avoiding fogged up glasses that usually resulted from using a simple filter mask. If some good comes from this crisis, I hope it includes the development of medical masks that are more effective and practical.

  2. Tom McAuliffe

    Dr. Centeno,
    To your point about cloth masks not all that helpful, just yesterday I heard of a zinc infused fabric that has been developed in Israel and found this link:
    “Sonovia’s ultrasonic fabric-finishing technology, invented by two Bar-Ilan University chemistry professors, mechanically infuses antiviral, antimicrobial zinc and copper oxide nanoparticles into textiles for facemasks and other protective products.”
    Proper fit and proper (antiviral) fabric, sounds like a winning combination!
    Thanks for your postings. I find them very helpful no matter the subject, and am sharing with my friends.

    1. Chris Centeno, MD Post author

      Tom, the issue is that the pore size and fit in a cloth mask is not helpful in filtering out the 1-3 micron particles that hang in the air. So while some zinc may help, that doesn’t deal with the pore size and fit issues.

  3. Shari

    I have thoroughly enjoyed reading your blog over these last few months. I especially appreciate that you provide direct links to the data and case studies in which you reference, many of which I go on to read the actual findings. Great job breaking it down for the general public that does not understand the intricacies of the science. My question is about the new media talking point of wearing masks “to protect others from YOU (assuming YOU is an asymptomatic carrier)” Have you written about this subject in your blog? What is your stance on this and can you please provide me with any specific research that supports said stance? Thank you for sharing your thoughts and insight and I look forward to continuing to read your blog.

    1. Chris Centeno, MD Post author

      There is no hardcore research supporting mask used to protect others. I wrote this blog a few weeks ago:

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