January 4th, 2023

Medical Masks vs. N95 Respirators for Preventing COVID-19 Among Healthcare Workers

As promised, the end of 2022 saw a trio of controversial COVID-19–related publications.

First up is something that always causes a stir — a study on masks! Reviewing a study on masks in the COVID-19 era is like poking a hornet’s nest with a stick, and this one is no exception.

But let’s poke away! Aside from receiving a lot of attention when it first appeared online, it gives us a chance to review some interesting clinical research principles.

Here’s the study question — does using a medical mask while caring for a person with confirmed or suspected COVID-19 provide healthcare workers “noninferior” protection to an N95 mask?

That “noninferior” term is often confusing, so best to translate it into plain English, which is what Harvard biostatistician Michael Hughes kindly did for me many years ago. Noninferior simply means “not too much worse than.” Noninferiority studies are great when the thing you’re testing has other advantages to the standard of care — it might be simpler, or cheaper, or both. As a result, a noninferiority design is quite appropriate for comparing surgical masks (cheaper, easier) to N95s.

Now one thing the statisticians ask us clinicians is to define the noninferiority margin — in other words, if not-too-much-worse is a key determinant, how much worse would we tolerate? In this study, if the upper bound of the 95% confidence interval of the hazard ratio of surgical to N95 masks was less than 2, then they’d be declared noninferior.

That sounds like a lot — would we really tolerate something that gives us only half the protection of an N95 mask? One thing to remember about noninferiority margins is that the smaller the margin, the bigger the required sample size. I suspect that anything smaller would have made the study impractically large.

The study was conducted in 29 healthcare facilities in Canada, Israel, Pakistan, and Egypt from May 2020 to March 2022, with 1009 participants. It’s important to scrutinize the dates of all COVID-19 studies because the vaccines, prior COVID-19 (with residual immunity), and variants have greatly changed the nature of SARS-CoV-2’s transmissibility, severity, and our response to it. As I’ve noted previously, the post-Omicron era includes vastly more people who had COVID, and vastly more people who stopped preventive measures while out and about in society.

So finally, let’s get to the primary results. RT-PCR–confirmed COVID-19 occurred in 52 of 497 (10.46%) participants in the medical mask group versus 47 of 507 (9.27%) in the N95 respirator group. You don’t have to be a statistician to conclude that these numbers are pretty darn close.

This yields a hazard ratio of 1.14, with a 95% confidence interval of 0.77 to 1.69. That means the surgical masks could be as much as 69% worse (but less than twofold worse, the non-inferiority margin), or even 23% better, at protecting healthcare workers.

In short (drumroll), the strategy of wearing surgical masks was noninferior to N95s among people caring for people with confirmed or suspected COVID-19.

Perspectives on this study:

The critical view:  The study was sloppy and, frankly, unethical. It’s already been proven in several models that filtration of respiratory viruses is more effective with a well-fitted N95 than regular masks — and COVID-19 is clearly transmitted by an airborne respiratory virus.

A twofold noninferiority margin is way too high. Even if they’re only 69% worse, why should healthcare workers take the chance?

The study didn’t even test the efficacy of the masks, since undoubtedly many of the participants got COVID while not even caring for COVID-19 patients and not wearing the N95s — either elsewhere in the hospital or (even more likely) in the community. This is especially true in Egypt, which accounted for many of the cases in the study during the post-Omicron period. How can we say the masks didn’t work when infections were occurring outside the patient room?

Let’s look at another country, how about Canada? There, surgical masks were more than twofold worse than N95s. Shouldn’t that be our model?

Finally, the study took a long time to enroll and had several modifications before completion. Doesn’t that alone make the results unreliable?

The supportive view: This study proves that a policy of recommending uncomfortable, expensive N95 over surgical masks is pointless. The highest form of evidence — the randomized clinical trial — shows they’re noninferior to cheap surgical ones.

Lots of clinicians hate N95s. When the study was first posted online, one of the smartest doctors I know asked me flat out — “So can I finally ditch these things? By the end of the day, I feel like my face has been in a vice.”

Yes, there are differences between countries, but importantly this was a post-hoc analysis. We should ignore these analyses because if you measure something frequently enough with smaller and smaller sample sizes, you’re bound to find something that’s statistically significant that supports your hypothesis.

And if you’re going to focus on a country, isn’t the current landscape of COVID much more like Egypt (during Omicron and high community transmission) than Canada (early in the study, very low event rates)?

As for those filtration studies? Remember, clinical trials enroll human beings — not mannequins or robots wearing masks.

My take: Both sides have excellent points. I learned a lot from reading insightful commentaries taking both sides of this debate — both praising the study (here and here) and criticizing it. Plus, there’s an excellent accompanying editorial.

As for what I think?

I believe that if the study were large enough, if the N95 masks were properly worn and correctly fit tested, if in-hospital COVID-19 transmission in break rooms during snacks and lunch could be excluded, and (an even bigger task) if transmission at restaurants and weddings and concerts and gyms (meaning in the community) could be excluded, then this study would have shown that surgical masks are not as good as N95 masks in protecting healthcare workers.

In other words, they’d be too much worse to make up for the lower cost and greater comfort. More than twofold worse, as defined by the study.

But that’s a lot of ifs, and is not the real world. The real world is messy, and such stipulations would be impossible. In the real world, the surgical masks in this randomized clinical trial were noninferior for the primary endpoint of PCR-diagnosed COVID-19 in the healthcare workers.

Using N95 masks in the post-Omicron era is like giving someone an excellent umbrella during a rainstorm, but only during the brief downpours when dashing from the car to the front door — the times of highest direct exposure. The rest of the day, with steady and frequent rain, they use either a broken umbrella or none at all. Plenty of chances to get wet.

No wonder the study showed surgical masks to be noninferior. COVID-19 is now everywhere, and patient-to-healthcare provider transmission of the virus is a small fraction of the exposures happening globally.

Do I still wear an N95 when caring for patients with confirmed or suspected COVID-19? Yes. After all, I still use an umbrella when dashing from the car to the door during a downpour.

Do I also believe that getting COVID-19 is much more likely at a restaurant or party or medical meeting than in the patient’s room?

Also yes. That’s just the world we live in now.

12 Responses to “Medical Masks vs. N95 Respirators for Preventing COVID-19 Among Healthcare Workers”

  1. Wissam Atrouny says:

    In early 2020 when N95 were in shortage we were using surgical masks in most COVID rooms. It took me more than a 100 COVID rooms visits for at least 20 min each (I was consenting for plasma) before I was infected myself (likely from the hospital then).

  2. Thira Woratanarat says:

    Thanks for very insightful article.
    Agree with you…

  3. Patrick says:

    Great article Paul. I agree the non-inferiority design is appropriate, and as is always the case I’m sure most of the people objecting to it wouldn’t have minded if the result had fallen in their preferred direction. Kudos to the authors for at least trying to address what looks like an important question for the rest of our working careers.

    In the post-pandemic world there are other harms that need to be factored in as well.

    My grandmother is in the hospital at the moment (I am a bit younger than you!) with new back pain which needs a workup. She has no cognitive decline but does have poor hearing, and because everyone caring for her is N95 masked she can’t hear a thing they are saying. As a result she’s been chalked up as confused/delirious, had the requisite course of antibiotics for the phantom UTI and otherwise hasn’t really been helped by her stay in the hospital. A surgical mask might well be enough – given the choice she would probably choose that, and as a former research nurse she would certainly be happy to be part of a proper trial.

    My conflict of interest is that I hate wearing the damn N95, but know that a non-zero number of patients and staff do acquire COVID and other respiratory viruses in the hospital. I would like better studies to tell me how much good these masks actually do in the real world.

  4. Bob Sherwood says:

    First, leave the choice to the worker. Stop mandating anything. This isn’t measles or TB, where your only exposure is going to be in a hospital setting in most cases. This is now a common cold, exposure happens everywhere and anywhere. We need to stop masking for COVID except in very specific situations.

  5. Nancy Rosen says:

    In the past two weeks I spoke to an ID doctor in NYC who said that he believes there is the possibility of fomite transmission of the latest variant, because many who wear quality masks are getting their first Covid infections possibly by touching germs hands to eyes and nose.

  6. Allison says:

    Thanks for the great article. Thoughts on the need for protective eyewear?

  7. Mark VonHolle says:

    Thank you for so boldly poking the hornets’ nest and for the excellent article!

    I am glad you mentioned the importance of proper donning and fit testing in your comment: “if the N95 masks were properly worn and correctly fit tested”.

    As I suspect you and many of your readers know, OSHA has two different fit test standards (qualitative and quantitative).

    1. QUALITATIVE FIT TEST (QLFT): relies on the respirator wearer’s senses to determine if there is a gap in the seal of the respirator to the wearer’s face.

    2. QUANTITATIVE FIT TEST (QNFT): uses a machine to measure the filtration efficiency towards the inside of.

    The qualitative fit test is very subjective, whereas the quantitative standard is scientifically measurable.

    There is no federal standard for the actual performance of an N95 as it relates to fit.

    From my own testing experience (which is admittedly anecdotal) it appears as though the majority of N95s will not pass the quantitative standard, even though they may pass a qualitative standard for the same individual being tested.

    The maximum quantitative standard test score possible is 200. The minimum passing score is 100. MANY of the N95s I have personally tested on multiple individuals scored lower than 20. Only two of the more than twelve N95s I tested on multiple individuals passed the quantitative test, with one in the range of 150 and another closer to the 200 point maximum score. So, it is no small wonder to me that the efficacy for many N95s does not seem to be much different that an ear loop surgical mask.

    I have found that N95s that are designed to provide the best seal and fit are those with good nose foam and a nose wire that conforms to the users face and maintains its shape and seal. There are also other design considerations related to leakage around the nose, cheeks and chin. None of these elements are required to receive the N95 certification. Many N95s have no nose foam. Many have surface applied nose wires that do not conform or maintain the seal against the users face. Some of these surface applied wires had already fallen off during shipment of the product in the boxes I opened.

    So… in all fairness, studies that are conducted comparing surgical masks to N95s in general are not going to produce the same results compared to a study that is based upon an N95 that is properly designed to ensure the best fit and seal. If an N95 is not designed to such a standard, and the user cannot pass a quantitative fit test, it seems like there could be a false sense of security associated with its usage.

    I believe if the quantitative fit test standard was more widely used, the real-world protection benefits of N95s would be greater. Of course, breathability, comfort, durability, and peripheral field of vision visibility and fogging of lenses are additional details to consider, especially as we seem to be entering into a new normal regarding pandemics and respiratory protection.

    Yes, we understand the real world is messy, and the article is based upon a randomized clinical trial. I appreciate this opportunity to add my experience and perspective to this great conversation.

  8. Lazarus Long says:

    Unfortunately, you missed the most crucial criticism of this study. Not really your fault as the authors hid it in the StudyProtocol PDF, instead of it being in the Methods section, where it should be – and where it was in Loeb’s previous N95 RCT.

    3 feet – nurses were to put on, and take off the N95 at the 3 feet.

    “2.3.2 N95 respiratory: Nurses randomized to the N95 group will already have been fit-tested with an N95 respirator and will be required to use this when providing care to or when within three feet of a patient with any febrile respiratory illness.”

    Febrile: “febrile respiratory illness, defined as symptoms of a fever ≥ 38 degrees Celsius and new or worsening cough or shortness of breath. ”

    Quickly on febrile – each variant has had a variety of symptoms. And all of them have had people who had no fever, no cough, and no shortness of breath.

    Chills, Fatigue, Muscle or body aches, Headache, New loss of taste or smell, Sore throat, Congestion or runny nose, Nausea or vomiting, Diarrhea – and more.

    Nurses follow instructions – no donning/doffing for any of those, in the latter.

    But, it gets worse – more than half of all transmission happens with NO symptoms (multiple studies, but here’s one):


    Moving onto the 3 feet distance.

    Aerosols are thickest nearest the source. Smoke is an aerosol, so think of the thick cloud of smoke near a cigarette smoker (minus the smell).

    85% of respiratory aerosols were found to be sub 5 microns. They float for hours. Coleman et Al (2022).

    We have seen many, many infections beyond 3 feet. Here is one caught on CCTV, 45 feet away – and facing the opposite direction:


    SARS-COV-2 aerosols have been found far from the patients in multiple studies. Such as out at the nurse’s station:

    Yet, in this study which is to measure the efficacy of N95s, the study is set up so that it negates the advantages of the N95s.

    The fit is negated by the 3 foot distance at which the N95 is donned and doffed. The aerosols would have already been breathed in. I am surprised the N95s did as well as they did, because NIOSH does not even consider surgical masks as respiratory protection:


    Finally, in Loeb et Al (2009), he did the same thing in “Surgical Mask vs N95 Respirator for Preventing Influenza Among Health Care Workers
    A Randomized Trial”. 3 foot distancing.

    “….when providing care to or when within 1 m of a patient with febrile respiratory illness,….”


    But he had it listed in the Methods section, where it clearly needs to be, instead of tucked away in an attachment.

    Both studies should be retracted as fatally flawed. If you disagree – would you walk into a building where asbestos abatement is going on in a surgical mask? Or would you put on an N95, before you entered the building?

    Because those are aerosols, too.

  9. Sean Doyle says:

    Loeb et al. already determined in 2009 that medical masks are noninferior to N95 respirators.

    The 2009 N95 vs Surgical Masks RCT conclusion infleunced Toronto public health policy and pandemic planning.

    This Loeb 2022 RCT is about defending the conclusions of his 2009 study that resulted in Nurses donning surgical masks in their workplace during a pandemic when they should have been donning N95.

    Why the 2009 study is not getting equal or more attention is beyond belief.


  10. Anonymous Hedgehog says:

    Laywoman here… Your explanation of a noninferiority study was excellent and I agree with your conclusions. The bee picture gave me the heebie-jeebies, however. LOL

  11. C Weber MD says:

    • N95s are uncomfortable, expensive … and no better at preventing viral respiratory infections than ordinary surgical masks according to a large randomized clinical trial (JAMA. 2019;322(9):824-833. doi:10.1001/jama.2019.11645). The study used matched clusters of clinicians working in adult and pediatric clinics at 7 large U.S. medical centers were randomized to use either N95s or paper masks when caring for sick, coughing patients during influenza season. Most of the 2862 participants were women (85%) who were nurses (41%) or clinical/administrative support staff (21%) in primary care clinics. About 80% were vaccinated against the flu. Self-reported adherence to the assigned mask was about 90%. During four consecutive flu seasons, from 2011 to 2015, cumulative incidence of laboratory-confirmed influenza was similar in both groups, as was incidence of confirmed infection with other respiratory pathogens and incidences of both influenza-like illness and unspecified acute respiratory illness. “There was no significant difference between the effectiveness of N95 respirators and medical masks in preventing laboratory-confirmed influenza among participants routinely exposed to respiratory illnesses in the workplace. In addition, there were no significant differences between N95 respirators and medical masks in the rates of acute respiratory illness, laboratory-detected respiratory infections, laboratory-confirmed respiratory illness, and influenzalike illness among participants. A sensitivity analysis suggested that the primary analysis reported was fairly robust to the missing outcome data with quantitative outcomes varying by less than 5%. This supports the finding that neither N95 respirators nor medical masks were more effective in preventing laboratory-confirmed influenza or other viral respiratory infection or illness among participants when worn in a fashion consistent with current US clinical practice. “

  12. Mark Loeb says:

    Lazarus Long should read the paper and supplements more carefully. It is made clear that by the time enrollment began, universal masking was the policy in place and this is what was followed over the course of the trial. Ninety-one percent of participants reported always using their medical mask and 80.7% their N95 respirator during their entire shift. If Sean Dolye were to read the 2009 JAMA paper he would learn that we stopped the 2009 trial at the start of the pandemic because that is when Ontario policy was changed to the use of N95 respirators. The study obviously did not result in nurses “donning surgical masks in their workplace during a pandemic” as he claims. Sean Doyle should also be reminded that the Radonovich study (JAMA 2019) was consistent with the 2009 study findings. Both Long and Doyle are entitled to their opinions, but hopefully they and other readers can appreciate the benefits of pragmatic randomized controlled trials as well as their limitations and efforts made to mitigate these. Hopefully, readers can also appreciate the major limitation of making firm policy conclusions on the basis of mechanistic aerosol or mannequin studies.

HIV Information: Author Paul Sax, M.D.

Paul E. Sax, MD

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Infectious Diseases

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