Some Masks Can Be Worse Than Not Wearing One at All: Physics of How Masks Affect Airflow and COVID-19 Protection

Pressure and Particle Motions Mask

Pressure and particle motions with and without a mask. Credit: Jinxiang Xi

A computer model deepens our understanding of airflow while wearing face masks, where particles land in the respiratory tract and the effectiveness of three-layer surgical masks.

Even though it has been widely known that wearing a face mask will help mitigate the community spread of COVID-19, less is known regarding the specific effectiveness of masks in reducing the viral load in the respiratory tracts of those wearing them.

In Physics of Fluids, by AIP Publishing, researchers from the University of Massachusetts Lowell and California Baptist University examined the effect of wearing a three-layer surgical mask on inspiratory airflows and the mask’s effects on the inhalation and deposition of ambient particles in the upper respiratory airways.

“It is natural to think that wearing a mask, no matter new or old, should always be better than nothing. Our results show that this belief is only true for particles larger than 5 micrometers, but not for fine particles smaller than 2.5 micrometers,” said author Jinxiang Xi. (The SARS-CoV-2 virus that causes COVID-19 is about 0.1 micrometers. — ed.)

The researchers found that wearing a mask with low (less than 30%) filtration efficiency can be worse than without.

They developed a computational face mask model using a physiologically realistic model of a person wearing a surgical mask with pleats and then using numerical methods to track the particles through the mask. They examined the behavior and fates of aerosols passing through the mask, onto the face, into the airway, and, eventually, where they deposit in the nose, pharynx, or deep lung.

The model showed a mask changes the airflow around the face, so that instead of air entering the mouth and nose through specific paths, air enters the mouth and nose through the entire mask surface but at lower speeds.

The lower speed near the face favors the inhalation of aerosols into the nose, so even though masks filter out certain numbers of particles, more particles escaping mask filtration can enter the respiratory tract.

They found the filtration efficiency of the three-layer surgical mask can vary from 65%, if new, to 25%, when used, so wearing a 65% mask properly will provide good protection, but wearing a 25% filtration mask can be worse than not wearing one at all.

“We hope public health authorities strengthen the current preventative measures to curb COVID-19 transmission, like choosing a more effective mask, wearing it properly for the highest protection, and avoid using an excessively used or expired surgical mask,” said Xi.

The researchers found the pleats of a surgical face mask significantly affect airflow patterns, suggesting that mask shape should also be considered as an important factor when estimating mask protection efficiency and designing new masks. Xi said they will further study the effects of mask shapes on human airway protection efficiency.

Reference: “Effects of mask-wearing on the inhalability and deposition of airborne SARS-CoV-2 aerosols in human upper airway” by Jinxiang Xi, Xiuhua April Si and Ramaswamy Nagarajan, 15 December 2020, Physics of Fluids.
DOI: 10.1063/5.0034580

11 Comments on "Some Masks Can Be Worse Than Not Wearing One at All: Physics of How Masks Affect Airflow and COVID-19 Protection"

  1. The face seal seems important, so what happens if the wearer has a beard? Not part of the study in this case, nor in many others. Probably too many variables….

  2. Andrew Dickens | January 5, 2021 at 2:27 pm | Reply

    Masks don’t work. You think thay do because you were told that? Ok, argue with the NEJM about how your talisman (rabbit’s foot, or in this case, your mask) brings you luck.
    New England Journal of Medicine, April 2020:
    https://www.nejm.org/doi/full/10.1056/NEJMp2006372
    “We know that wearing a mask outside health care facilities offers little, if any, protection from infection. ”
    “It is also clear that masks serve symbolic roles. Masks are not only tools, they are also talismans that may help increase health care workers’ perceived sense of safety, well-being, and trust in their hospitals. Although such reactions may not be strictly logical, we are all subject to fear and anxiety, especially during times of crisis.”

    • Suzanne Johnston | January 5, 2021 at 3:00 pm | Reply

      You’re an idiot.

      • I bet you didn’t even the read the article from NEJM did you? Instead you just fired off an ad hominem argument “You’re an idiot”, which is always the last bastion for weak and beggarly arguments.

        • While I haven’t read that particular paper I have read a significant number of the papers published this year regarding masking.

          The short summary of my experience is: if you think making is ineffective, you’re and idiot.

          Dude you replied to just summarized it more succinctly.

          As to why Covid continues to rise with masks… Covid is particularly contagious. I would hope that you already know that different disease are contagious at different levels. Further, to reduce covid cases through masking and distancing alone it is estimated we would need something like 95% population compliance.

          Even though we can get to that level becuse of idiots who still think masks are ineffective, masking and distancing is the best weapon we have until vaccines are distributed.

          • This won’t happen and never will. If you saw the experiment a anesthesiologist did with all the masks on the market it showed the airflow around them. Showed how ineffective masks are and if you go to the 3rd paragraph of this article it summed it all up.

            “ Our results show that this belief is only true for particles larger than 5 micrometers, but not for fine particles smaller than 2.5 micrometers,” said author Jinxiang Xi. (The SARS-CoV-2 virus that causes COVID-19 is about 0.1 micrometers. — ed.)”

            Virus is so small it goes through the masks and will go around face shields. Literally just wash your hands, exercise, and eat healthy.

  3. This is an irresponsible headline- there is nothing about a) the masks lessening the spread outward – which is the whole point of mask requirement. Not to protect the mask wearer, but everyone else. Which they do.
    B) nothing about whether the slower particles that get inhaled actually when wearing a mask are a net increase to not wearing it.

  4. Masks are not to protect the individual wearing them but the public from the individual in the case that s the individual has the virus. To suggest that mask wearing, as indicated for mitigating Covid spread, is for the wearer is completely ludicrous. This is an old argument. And I agree with DW: this is irresponsible and NOT science.

  5. I think after this is all over wearing the mask is more about political control than protecting us from covid. If the masks work so well, why are our numbers still going up?

    • The numbers in the USA are exploding because so many of us aren’t taking precautions. Ask yourself why. (It seems so obvious why: about 40-45% of Americans can’t admit that they’re led by buffoons, and the news they watch is largely propaganda.)

      Other nations took covid seriously. We delayed. They’re masking up & distancing and yes, shutting down when they ought to. We’re still debating and people aren’t complying, including leaders who aren’t leading by example.

      I’m an evangelical, by the way.

      Do the smart thing. Listen to scientists (frequently and constantly, since it’s only natural for their understanding to improve over time, and for there to be disagreements).

      Eat healthy. Supplement with vitamin D3. Many Americans are deficient, and that makes us susceptible to Covid-19.

      Social distance. Quarantine if needed.

      Wear good masks, properly. Wash reusable ones when they’re used. If you can blow through it, or if it’s loose and you can’t keep it tight, toss it.

  6. Famous science guy once said “Never underestimate the stupidity of humanity” trickle down effect, D.C.- inept citizens. Shame.

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