Masks: When to Wear Them, When Not to Wear Them, Should We be Wearing Them? Answers to Critical Questions

Masks When to Wear Them, When Not to Wear Them, Should We be Wearing Them Answers to Critical Questions

Recently, two studies were published which led many to question whether masks are protective against COVID-19 and if they are even necessary. Let’s answer these questions by beginning with a review of the study findings; in one study from Denmark, they found that masks did not offer additional protection to citizens who socially distance while outdoors. The 2nd study was of a group of military recruits where they found that consistent mask wearing over a two-week period both indoors and outside did not appear to protect against transmission. Yet, as everyone is aware, mask wearing, even outside, is still strongly recommended by the vast majority of health care agencies.

The goal of this review is to provide the physiologic insights that can both reconcile and modify these three seemingly conflicting conclusions between the recent mask studies and the prevailing mask recommendations which, although they are largely correct, often go a bit too far to the extreme. The following will hopefully allow all to understand exactly when and where and what kind of masks are needed to protect yourself from getting COVID-19;

The three prevailing observations to reconcile:

1) wearing standard masks doesn’t offer additional protection while social distancing outside (Danish Outdoor study) 

2) wearing standard masks doesn’t offer much additional protection while in close quarters in quarantine for prolonged periods (Military Recruit study)

3) wearing masks are critical to reduce transmission (CDC, WHO recommendations)

 To understand how all three of the above can be simultaneously true, the predominant mode of spread of this virus must be agreed upon. The 3 possible modes of transmission are:

  • direct contact/hands/surfaces (prevented with hand hygiene ) 
  • large droplet spread from person to person in close proximity (prevented by social distancing) 
  • airborne spread by inhaling tiny floating droplets directly into the nose/lungs (prevented by either ubiquitous standard masking indoors or by any wearer of an N95)

The CDC and WHO have long claimed that there is little transmission via direct contact or surfaces and that COVID-19 is instead predominantly spread via large droplets jumping from person to person. However, many scientists, after astutely observing the near collapse of social and economic life across the world as well as the manner of the rapid, massive global spread of COVID-19, instead concluded that the predominant mode of person-to-person transmission must be via “airborne” transmission. Although they were entirely correct, and despite the rapidly accumulating reports of “super-spreader” events from around the world, the WHO’s infection control committee was reluctant to adopt this position without “sufficient evidence”. The discord in opinion on this issue erupted when a group of 239 scientists from 32 countries wrote an open letter to the WHO with the evidence “proving” airborne spread.

Well before that letter was published, I had written an Op-Ed with my mentor and friend Professor Paul Mayo back in May of 2020, to warn the world that SARS-CoV-2 was being transmitted via airborne spread and that N95 mask production was critical. Although our essay was submitted to and accepted by the New York Times Opinion page, the editors that accepted it were abruptly fired days later for publishing a disturbing Op-Ed by Senator Tom Cotton which called for the US military to be sent into the streets to quell the George Floyd protests. The newly appointed editors unfortunately dropped all the previously accepted Op-Eds. First time in my life I had a NYT Op-ed accepted… and the Op-Ed board gets themselves fired just days before publication? Well, at least it wasn’t as if I had a historically important scientific manuscript pass expert peer-review and then some compromised #%&$! desk jockey editor named Fred Fenter decided to retract it days before publication.. I mean that would be just beyond crushing (I still have not recovered from that by the way).  

Fortunately, we then got the Op-Ed accepted and published in USA today (#1 ranked newspaper circulation in the country at 2.6 million daily readers!). In that Op-Ed, we provided overwhelming evidence that the predominant form of spread was via the airborne route, citing the work of many of the scientists that wrote the protest letter to the WHO two months later. Despite this Op-Ed and the open letter by 239 top scientists in the world… the WHO… still has not accepted this conclusion and thus has caused some scientists to devote precious research equipment and time to obtain the “sufficient evidence” needed to prove to the WHO committee the glaring reality of airborne spread. One such researcher, Kim Prather, a Professor at Scripps Institution of Oceanography, recently quoted, one year later, on March 31, 2021; “We were all baffled that [public health agencies] didn’t take aerosol transmission as being more serious to begin with,” and “It’s so frustrating because we’re all scrambling.…We cannot keep doing this” (hmm… where have I heard that phrase before?). Well, at least it was not like some WHO committee, despite mountains of evidence and millions dying, recommended against use of one of the safest, broadest, and most effective anti-viral in history (cue crushing chest pain…again)

Now, if the above is not disturbing enough, here is the most terrifying part of the delays in and lack of proper guidance around mask use: the reality and risks of airborne spread was almost certainly known as early as the first 30 cases of this pandemic, at the end of December 2019, when a public health announcement fleetingly appeared on a Wuhan health ministry website. This fact was detailed in a Wall Street Journal article describing the WHO pandemic surveillance system that first detected COVID-19 by continually scouring the internet for words suggesting illness outbreaks. That Wuhan notice, although it was quickly taken down, had read, “Avoid closed public places and crowded places with poor air circulation.” Thus, it was known by at least one health official in Wuhan that the new virus was likely spread by airborne means… in December of 2019. Yet the WHO still only considers airborne transmission “a possibility” at this time. Palm to forehead… once again… at the innumerable, perplexing actions and positions adopted by multiple national and international health care agencies throughout the pandemic. 

So, after reviewing the open letter and Op-Ed, if one can accept that the predominant mode of spread is via the airborne route, what follows is that;

Masks are critical in protecting yourself from COVID-19… but only indoors. It is very difficult, if not nearly impossible to give the virus to others outdoors via the airborne spread of tiny floating droplets because those droplets are, in most circumstances quickly dispersed as a result of wind, air, or person movement. Further, mounting evidence suggests that exposure to outdoor UV light alone could neutralize the virus. Thus, the exhaled particle clouds get quickly diluted or killed such that there is insufficient inoculum to infect others outside. In fact, at the time I wrote the Op-Ed above, there was only one true contact-traced, confirmed, documented outdoor transmission – and that was between two Chinese friends who spoke at close range for over an hour. In fact, not only do studies show that spending time outside reduces risk of COVID-19 infection, but in a study from the Journal of Infectious Diseases, less than 10% of all COVID-19 cases were transmitted in outdoor settings. Further, the investigators admitted that the risk was likely far lower than 10% given many of the locations, like construction sites or summer camps, had both indoor and outdoor exposure risks.

Most instructive is the data from the Health Protection Surveillance Centre in Ireland which found that just 262 of over 200,000 infections occurred outside — just 0.1% of all cases.

So, those who argue against masks should simply amend their argument to say that masks don’t work or are likely and almost definitely unnecessary… OUTDOORS… in fresh air, sunshine, rain, while walking, in a field, on the sidewalk etc. With the rare exception of being in an extremely congested crowd and/or stagnant air, it is almost certainly unnecessary. 

How long it will take for most of the world to narrow down mask requirements to just these two conditions is frustrating to contemplate. In fact, I found the Dutch study silly and their conclusions unsurprising given all the participants were socially distancing as well! I have long resented having to wear a mask while outdoors on the sidewalk, or anywhere outdoors not crowded by others or just quickly passing them (do you hear that San Francisco Bay area?). But, people are scared and are being over-cautious and I get that. However, the Danish study supports this point – if outside and social distancing, masks aren’t necessary and that I have long agreed with (I already argued this in my Op-Ed above from May).

Now, how do we interpret the military recruit study? How come masks did not offer much protection there? Easy – because, in that study, “standard” masks (non-N95’s) will not protect you if you violate any of what I maintain are the 4 main risk factors that predict transmission within indoor spaces: Density, Duration, Dimensions, and Draft.

  1. Density – # of people in the room
  2. Duration – # of hours spent in the room
  3. Dimensions – # of square feet and ceiling height of the room
  4. Draft – # of air exchanges per hour or the rate of fresh air entry/speed of air flow 

I have long maintained that if you violate any one or multiple of the “4 D’s” above in a significant way, you will get sick, even with a “standard” mask, That is what the military recruit study showed, given that, in the group wearing masks, nearly all transmission occurred between roommates or within platoons, and it should be noted that in these situations, likely all the 4 D’s above were violated given they spent time indoors, among a high density of recruits, for prolonged durations, in small dimensioned rooms, with likely little draft. Standard cloth or surgical masks just won’t offer sufficient protection in these settings if an infected person is in their midst.

A recent newspaper article from Spain beautifully illustrated exactly what occurred in the recruit study using sophisticated, animated graphics. So, all must understand that masks are critical to decrease the likelihood of getting COVID-19 and/or prolong the duration that you can avoid getting COVID-19… when indoors for prolonged, but finite periods in not-too-close quarters with non-household members and wearing either no mask or just standard cloth masks. Hence the large amounts of data showing crowded restaurants and bars as the most common sources of spread of COVID-19– people are eating/drinking and thus not wearing masks for prolonged periods in indoor, crowded environments. This can be deadly to high-risk people! 

Note that, as per the Spanish article, even if you wear a standard mask in such situations, although it will protect you for a prolonged period it will not do so indefinitely. Spend 6 hours in such an environment, even with a standard mask and you can run a high risk of getting COVID-19 if another (typically pre-symptomatic) person is there. Note this super-spreader event that occurred on a long plane flight from Ireland where everyone was wearing standard masks – 59 people still got sick in this situation!  And this is another reason why I wear N95’s when I fly – however, I would argue that N95’s are likely most needed for long flights rather than short ones but who knows the time cutoff?

The main point is that transmission eventually occurs if a sick person is present, likely due to violating the “D” for “duration,” the “D” for density, the “D” for dimensions (aircrafts are small). In other spaces like a small room with a lot of people, you can add the “D” for draft. In these situations, transmission can happen even if everyone is wearing “standard masks” which, although highly protective when all cohabitants of a space are wearing them, the protection wanes over prolonged periods in close, confined settings. To understand how standard masks protect in the short term, see the explanations in my Op-Ed as well as those found on the #Masks4All organization website, a historically important group that served as one of the main forces in influencing much of the world as to the critical need for ubiquitous mask wearing.

Now, we should note that, in contrast to standard cloth masks, N95 masks will protect against transmission/inhalation of droplets, even over prolonged periods indoors and even surrounded by many infected COVID patients such as occurs in emergency rooms, hospital rooms/wards and ICU’s.

Thus, I argue that it would and could be safe to do any activity, in any crowd, in any confined indoor space… but only if everyone present wore an N95. The problem with N95’s is that they are uncomfortable to wear for long periods and they are also in short supply in the US due primarily to the total lack of an organized federal government N95 production initiative (ah hem – we argued for this in our Op-Ed above and apparently Jared Kushner never read it– ouch) but also due to the persistently high national and global health care worker needs in order provide safe care in the high risk outbreaks when many patients rapidly fill hospitals. 

The discomfort of N95’s is real though – imagine a birthday party or wedding dance floor with everyone wearing N95’s – it would be safe to do so… but so not fun. We just cannot have it both ways anymore it seems, i.e., participate in activities that are both safe and fun. 

The original title of my Op-Ed above was “N-95’s for All” given that it argued for the production of more N95’s for the citizen population to safely allow activities in both high-risk indoor situations and/or places where many fellow-citizens refuse to wear masks

The safety of N95’s can be illustrated by the fact that I have been caring for critically ill patients with Covid-19 for over a year in ICU’s… and I haven’t gotten sick… because I wear N95’s around infected patients and standard masks away from them while also, with others, taking care not to overcrowd communal workspaces. It works – many of my colleagues have also been working in ICU’s and hospitals have not gotten COVID… since we all started wearing N95’s. However, in the time before widespread use of N95’s and mask wearing in hospitals became the standard, many doctors and nurses and aides were getting COVID – I had a number of scary Covid-19 illness episodes in my network of colleagues – with several deaths among the wider New York City community of physicians.

So, my recommendation: wear masks indoors when non-household members are present. Avoid close quartered, crowded conditions amongst non-household members for prolonged periods unless the mask is an N95 – in all other situations indoors, standard masks are sufficiently protective. 

In conclusion, I agree that constant, ubiquitous mask wearing does not make sense in almost all outdoor settings, but they are critical in many indoor spaces, unless the space is some large, cavernous, uncrowded space and/or you are there for a brief period and/or it is a very well-ventilated space (or everyone is on ivermectin ☺). But making rules for each space would be far too complicated and dangerous mistakes would inevitably be made. Thus, it is best to err on the side of safety and wear your masks in public indoor spaces people ☺. I hope this helps clear up some of the questions and confusion triggered by the recent trials that seemingly suggest that “masks don’t work”. They absolutely do, and are critical to protect yourself, you just need to understand which mask and in what situations. 

Pierre Kory, MD, MPA

President & Chief Medical Officer

Front-Line COVID-19 Critical Care Alliance


  1. Dear Dr. Kory,
    I LOVE your work with FLCCC and Ivermectin. You are truly a hero in my book, and you singlehandedly inspired me to research Ivermectin since last December when I’ve watched your Senate testimony. Since then I’ve dedicated all my spare time to advocate for Ivermectin and expose the corruption hindering its approval for COVID-19.

    That’s why it’s quite painful for me to say this: I am disappointed in this article, and your conclusion (i.e. that masks “absolutely” work) is simply wrong. You start out fantastic with all the correct science on aerosol transmission, outdoor transmission, and actual real-world scientific studies showing that standard surgical masks don’t work, and your four D’s are sound general rules as well, but then you somehow make a logical leap that standard (surgical) masks “will protect you for a prolonged period” and “ubiquitous mask wearing … [is] critical in many indoor spaces”. You offer no real scientific evidence at all for these statements neither in this article nor in your linked Op-Ed. And I am sorry, but the Masks4All website is a sad propaganda. Their main quote is from Dr. Fauci? Seriously? Their “science” page is all models, lab experiments, and poor cherry-picked data without real comparisons, time periods, or controls. At most vague correlations for a short period.

    You call the Danish study (not Dutch, btw) “silly” while this is the only real-world RCT mask study for COVID-19 in the world. Are you pooping on RCTs now in general? Are you not for real-world experiences with Ivermectin? Then why you favor a model-touting Masks4All site over real-world data when it comes to masks? Be very careful of models and lab experiments: they almost never translate to actual real-world experiences.

    You offer no real-world scientific evidence that surgical masks protect you indoors, even for a short period, even with universal masking, and even with following your 4 D’s. On the contrary: all actual real-world scientific evidence and comparisons show that even universal mask wearing everywhere doesn’t make any difference in the course of the pandemic.
    If you want to look at just one place for comprehensive scientific data on masks, I highly recommend Swish Policy Research’s page on masks:

    It has all the evidence you need, including many comparion charts and a section with rebuttals on “masks work” evidence. Interestingly, your referenced Masks4All also appears:
    “A review by members of the lobby group ‘Masks for All’, published in the journal PNAS, claimed that masks are effective as a source control against aerosol transmission in the community, but the review provided no real-world evidence supporting this proposition.” (i.e. it’s only a hypothesis without evidence)

    And while N95 masks, worn properly, surely should protect you from catching COVID-19, your hypothesis of universal N95 mask wearing offering almost perfect protection is also not supported by real-world data: see the German case in this subsection on the same SPR page:

    Additionally, your own anecdotal evidence of wearing N95 mask and not catching COVID-19 is not a proof. It’s anecdotal, one isolated example, and correlational – as I am sure you have been prophylaxing with IVM and other medications. Plus you are wearing it properly – don’t expect that from most people.

    I am sorry but the real-world evidence clearly shows that masks, whether cloth, surgical, or even N95, and whether indoors or outdoors, don’t make any long-term difference in the course of COVID-19 infections. Aerosol transmission defeats any masks in the long run, and your 4 D’s themselves are probably way better protections than any masks.

    In conclusion, I would suggest that reaching heard immunity both through vaccination and naturally acquired immunity through COVID-19 infection WITH universal early therapeutic treatment of Ivermectin, Fluvoxamine, HCQ, and others, is the best way to defeat this COVID-19 pandemic. Not masks, not lockdowns, not school closures, nor restrictions of any kind on the body, mind, and soul of humans – but letting these work. That’s what always made human health and progress possible.

    Dear Dr. Kory: I will die on the hill of Ivermectin with you if necessary. Yet, I have to call you out on this one: you are wrong on masks.

    With deep respect,
    Norbert Szolnoky

    1. That is the first time I have seen anyone state or imply that they will die for the cause of ivermectin. Furthermore, I have never seen any expression about violence by those advocating for the use of ivermectin. In all the writings/videos etc that I have come across, advocates think of this conflict as purely a matter of getting the message out.

      1. Dear Peter,
        I love and follow your writings here on TrialSite News, but with all due respect, is this the main thing you got out of my long reply to Dr. Kory? And you think “die on the hill of” is expressing violence? It means I will defend and fight for (figurately) the cause of Ivermectin at any cost to me – just like Dr. Kory is doing. If you think I am expressing intention at real physical violence then you totally misread my words.
        And I am quite disappointed that this is what you got out of my reply, not the point that the real-world evidence shows that surgical masks don’t work in any settings as the aerosol spread will defeat them easily. And even if N95 masks would hypothetically work (again, the evidence linked in my original reply shows otherwise) it’s totally unfeasible to think that people would wear this in everyday settings in a proper way. Plus imho it’s really a wrong direction anyway, and the focus should be reaching herd immunity asap with all the available medicines like Ivermectin, NOT prolonging this pandemic with wrongheaded non-pharmaceutical interventions. I am quite sad actually that seemingly many Ivermectin enthusiasts don’t know the science regarding NPIs and still advocate for lockdowns, school closures and masking, and even vaccinating kids.
        Anyway, I love your reporting on Ivermectin, and I hope you will stand your ground on that hill with me. Perhaps this expression is more to your likings… 😉