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Two recently published studies have led some to question whether masks are actually 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, the researchers found that masks did not offer additional protection to citizens who socially distance when outdoors ( Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask Wearers). In the second study, a group of military recruits who wore masks consistently over a two week period both indoors and outside did not appear to be protected against transmission (  SARS-CoV-2 Transmission among Marine Recruits during Quarantine). Yet, as everyone is aware, mask wearing is still strongly recommended by the vast majority of health care agencies.

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

The three 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 (C.D.C., W.H.O. 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 three 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 C.D.C. and W.H.O. have long claimed that there is little transmission via direct contact or surfaces, and that COVID-19 is instead predominantly spread via large droplets from person to person. However, many scientists, after astutely observing the near collapse of social and economic life across the world due to the massive global spread of COVID-19, instead concluded that the major mode of person-to-person transmission must be the “airborne” route. Although they were entirely correct, the relevant W.H.O. committee was reluctant to adopt this position without “sufficient evidence.” The discord on this issue erupted when a group of 273 scientists wrote to the W.H.O. with the evidence “proving” airborne spread:  It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (Oxford Academic). Well before that letter, I wrote an Op-Ed with my mentor and friend Professor Paul Mayo last May as we tried to warn the world that SARS-CoV-2 was being transmitted via airborne spread. Although the Op-Ed was accepted by The New York Times Opinion page, the editor who accepted it was fired before it could be published, and unfortunately for the world, the newly appointed editor dropped all the previously accepted Op-Eds. It was not until another two critical months passed before it was accepted and published in USA Today. In this Op-Ed, I 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 W.H.O. two months later:  ICU doctors: Many more Americans need to wear N95 masks to slow COVID-19.

So, if we accept that the predominant mode of spread is via the airborne route, what follows is that;

1. 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 a person’s movement. Thus, the exhaled particle clouds get quickly diluted to the extent that there is not enough of a concentrated inoculum to infect others nearby. 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.
2. 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.. Except in congested crowds or perhaps stagnant air, it is unlikely to help. 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 minding my own business away from people or quickly passing them. But, people are scared and are being overly cautious, and I get that. The Danish study, however, supports this point: if outside and social distancing, masks aren’t necessary. I have long agreed with, and already argued, this in my Op-Ed (above) last May.
3. Then how do we explain 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 the four main risk factors that predict transmission within indoor spaces; Density, Duration, Dimensions, and Draft:

• Density – # of people in the room
• Duration – # of hours spent in the room
• Dimensions – # of square feet and ceiling height of the room
• Draft – amount of fresh air entry/speed of air flow

If you violate any of the four D’s above in a significant way, you will get sick, even with a “standard” mask. The military recruit study showed that, in the group wearing masks, nearly all transmission occurred between roommates or within platoons, and it should be noted that these situations violate all the four D’s above. 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 article illustrated exactly what occurred in the recruit study using sophisticated, animated graphics:  A room, a bar and a classroom: how the coronavirus is spread through the air (El País). These examples concur with what I have long maintained, in that, if you are in small, closely confined, poorly ventilated spaces with a large number of people for prolonged periods. you will get sick, even while wearing a mask.

What we all need to take from this is 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-houshold members. Hence the large amounts of data showing crowded restaurants and bars as the main source of spread – people are eating/drinking and thus not wearing masks for prolonged periods in indoor, crowded environments. This is deadly; although, if you wear a standard mask in such situations it will protect you for a prolonged period, but not indefinitely. Spend six hours in such an environment, even with a standard mask, and you will run a high risk of getting COVID-19 if another (typically pre-symptomatic) person is there who has the disease. One case in point is that of a “super-spreader” event on a long plane flight from Ireland where everyone was wearing masks – 59 people still got sick in this situation.  Researchers link 59 Irish COVID cases to inbound long-haul flight. This is why I wear N95’s when I fly – however, I would argue that N95’s are most likely needed for long flights rather than short ones, but who knows the time cutoff?

In the example from the El Pais article above, when a group of friends spends time together in an indoor space (i.e. a living room) for a prolonged period they will still contract the virus even though they socially distanced and wore masks. Transmission eventually occurs, likely due to violating the “D” for “duration” and possibly the “D” for draft if the windows were not open and there was no fresh air circulation. Recognize that this 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 explanation in my Op-Ed  ICU doctors: Many more Americans need to wear N95 masks to slow COVID-19 (USA Today), along with multiple examples of their efficacy on  masks4all.co.

N95 masks, on the other hand, will protect against transmission/inhalation of droplets, even over prolonged periods indoors. Thus I would argue that it would and could be safe to do any activities in any crowd or confined indoor space, but only if everyone present wore an N95. The problem with N95s is that they are uncomfortable to wear for long periods. They are also in short supply due primarily to the total lack of an organized federal government N95 production initiative (ahem – we argued for this in our Op-Ed above), and also due to the persistently high national and global health care worker demand for them, for the purpose of providing safe care to the many patients filling hospitals. The discomfort of N95s is real, though – imagine a birthday party or wedding dance floor with everyone wearing N95s. it would be safe to do so, but not so much fun. We 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-95s for All” given that it argued for the production of more N95s for the citizen population, to allow people the chance to avoid transmission in both high-risk indoor situations and also in indoor situations where others 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 for 11 straight months in ICU’s… and I haven’t gotten COVID That’s because I wear N95’s around infected patients and all the health care workers wear masks and try not to overcrowd communal workspaces. It works – many of my colleagues working in ICU’s and hospitals, have not gotten COVID since we all started wearing N95’s. But, in the time before widespread use of N95’s and mask wearing became standard in hospitals, many doctors and nurses and aides were getting COVID. I had a number of scary COVID illness episodes hit my network of colleagues, with several deaths among the wider New York City community of physicians.

So, my recommendation: wear masks indoors. Always. Avoid close quartered, crowded conditions among non-household members for prolonged periods unless the mask is an N95. In all other situations indoors, standard masks are sufficiently protective. Here is the most disturbing part of this story: the reality of airborne spread was known as early as the first thirty cases of this pandemic, at the end of December 2019, when a public health announcement fleetingly appeared on a Wuhan health ministry website (this notice was detected by a W.H.O. pandemic detection system that continually scours the internet for words suggesting illness outbreaks). That notice, although it was quickly taken down, was known by the W.H.O. to have read, “Avoid closed public places and crowded places with poor air circulation.” This fact was detailed in a Wall Street Journal article:  How Coronavirus Overpowered the World Health Organization. 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 W.H.O. 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. I just hope, once this is over, all can learn from the many frightening mistakes that have been made.

In conclusion, I agree that constant, ubiquitous mask wearing does not make sense in almost all outdoor settings, but they are absolutely critical in nearly all indoor spaces. This is 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. 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 indoors, people ☺.

I hope this helps clear up some of the questions and confusion triggered by these recent trials suggesting 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.

January 2, 2021