Bridging the Gaps Between WHO and Aerosol Scientists (Part I)
Face masks in the middle of a controversy
A key question in the coronavirus disease 2019 pandemic has been whether SARS-CoV-2 could be transmitted via the airborne route. The answer is clear: yes, it can. Now the controversy among aerosol scientists and the World Health Organization (WHO), a leading mediator between science and policy, is: Which is the predominant route of SARS-CoV-2 transmission?
WHO agrees that airborne transmission of COVID-19 is plausible in some specific situations. However, according to the WHO, SARS-CoV-2 spreads predominantly through virus-containing droplets emitted by the infected person and contact routes. In this case, the critical control measures are “reducing direct contact, cleaning surfaces, physical barriers, physical distancing, use of masks within droplet distance, respiratory hygiene, and wearing high-grade protection only for so-called aerosol-generating healthcare procedures”.
On the other hand, many scientists, experts in aerosols, claim that SARS-CoV-2 is transmitted primarily by infectious aerosols. Since these tiny particles can remain suspended in the air for hours and move long distances, control measures should include “ventilation, air filtration, reducing crowding and time spent indoors, use of masks whenever indoors, attention to mask quality and fit, and higher-grade protection for healthcare staff and front-line workers”.
This disagreement is not a minor controversy, since WHO plays a leading role in the global policy response to COVID-19 as a mediator between science and policy. WHO advice influences both allocation of resources and management of the pandemic.
Aerosol scientists and many other scientist say:
SARS-CoV-2 is mainly transmitted through aerosols. If we don’t acknowledge airborne transmission, we will not be able to stop the spread of the pandemic.
WHO argues that one important reason that indicates that the virus is mainly transmitted through droplets because, surgical masks and medical respirators (required to prevent aerosol transmission) provide equivalent protection in health care settings.
High quality random control trials have shown that respirators do not offer, in practice, a protection superior to what can be achieved with surgical masks. This evidence supports droplet and contact transmission.
No health workers were infected in March and April 2020 despite not wearing respirators. This data support that the virus is not airborne.
For WHO, these findings are an indication that aerosols are not the predominant route of COVID-19 transmission. In this article, we are going to see if the objections related to masks that WHO uses to support that this virus is not airborne are founded or not.
Are respirators essential to protect ourselves against SARS-CoV-2?
Some people say, «This can be easily checked in a lab». However, in the labs there are ideal conditions and the results obtained may not reflect reality.
To see what is happening in real conditions, randomized controlled studies provide valuable information. A few have been done. Such studies aimed to compare the degree of protection of respirators versus surgical masks. Some of them have not found any difference between respirators and surgical masks.
«The results obtained — says WHO — do not support this virus is airborne».
However, if we delve into the subject, we will see that the results do not necessarily lead to that conclusion.
We will dig a little bit deeper.
Whether transmitted through droplets or aerosols, distancing and masks are essential. Nevertheless, there are substantial differences as regards the role of face masks in each case. If spread through droplets, any material that acts as a physical barrier would suffice to protect against COVID-19.
However, if SARS-CoV-2 spreads through aerosols (as an airborne disease), both high filtration efficiency and a good fit between the mask and the face’s surface are essential to ensure protection. Respirators certified for filtration and seal leakage would protect against aerosols.
Let’s do a quick review. There are three main groups of masks, from highest to lowest filtration efficiency:
- Respirators (such as N95, KN95, FFP2 and FFP3).
- Surgical masks and procedure masks.
- Cloth masks.
The effectiveness of any mask to block a virus depends on its:
- Filtration capacity (which, in turn, depends on the role that each particle size plays in virus transmission).
- Fit. The correct fit of respirators prevents tiny airborne particles can find their way around any gaps between the mask and face.
- Breathability (the ease with which air passes through the material). Breathability is not just crucial for comfort. If breathability is low, air tends to leak around the edges, and users occasionally remove uncomfortable masks. When choosing a material for manufacturing face masks, the challenge is to find a material that is effective enough to capture particles, and that allows you to breathe normally.
Protection depends also on comfort.
A suitable mask should not:
1. Irritate the skin.
2. Cause headaches.
3. Lead to an excessively high concentration of CO2 in the gap between the mask and the face.
All these factors are essential features for an excellent mask.
In principle, the use of surgical masks would be enough to minimize exposure to respiratory droplets. “Loose-fitting” masks are designed to protect against droplets, but not aerosols.
Do surgical mask protect us from the coronavirus?
Since the COVID pandemic began, we have been told: “Surgical masks do NOT protect the wearer” or: «Surgical masks are ineffective if most infectious viruses are carried in fine droplets».
You may have seen infographics like this:
This kind of infographics spread even faster than the virus. And most of the information they contained was suspiciously specific. How could researchers have known so soon what the probability of contagion with each type of mask was? In some cases, they did not even specify what kind of mask it was.
By pulling the thread, we can get to the original infographic, which comes from Turkey.
Too much opinion and speculation and too little data.
To what extent surgical masks protect us from the coronavirus?
So far, most of the evidence for the effectiveness of masks comes from in-vitro experiments with non-biological particles. Those experiments may not reflect what happens not with artificial particles but with viruses. And not in laboratories, but as they are used in real life.
What happens in practice?
Scientists have done some experiments on this (although there are still very few). And what does that data say? Let us see.
This study has important implications since researchers found that none of the healthcare workers (HCWs) got infected with COVID-19 when they used surgical masks and used other precautions.
However, in other studies the outcome was different. These authors reported for the first time SARS-CoV-2 nosocomial infections despite using surgical masks and physical distancing.
This report may provide possible evidence for airborne transmission of SARS-CoV-2.
In this study, HCWs wearing surgical masks became infected with SARS-CoV-2, despite not being involved in aerosol-generating procedures (interestingly, patients and employees wore an ASTM level 1 mask with ear loops and no mask fitters):
We describe 3 instances of SARS-CoV-2 transmission despite medical masks (and eye protection).
In a recent study of health-care workers (HCWs) in Finland, occupational COVID-19 infections occurred in those workers using surgical masks but not in those wearing FFP2/3 respirators and following aerosol-prevention guidelines. The authors concluded that surgical masks are not adequate protection against SARS-CoV-2 and that the use of FFP2/3 respirators in all patient contacts is recommended.
Most of the data has not been obtained with this new coronavirus, SARS-CoV-2, but with other viruses, especially the influenza virus. One way to measure the effectiveness of surgical masks is to compare them to the N95. That is what they did. The researchers randomly divided more than 2,000 nurses into two groups. One of them wore surgical masks, and the other one, N95 masks.
Researchers tracked how many nurses were infected with influenza and other respiratory viruses. No significant difference in the incidence of laboratory-confirmed influenza was found. The infection rates were the same!
We might think that the results of that study are due to chance. (In science, a single study is rarely enough to conclude anything.)
However, it is not the only one. Another research team did a similar study in Canada with 446 nurses during cold and flu season (September through December).
They were tracked all those months to see how many got the flu or colds caused by various types of viruses. Again, there was no significant difference between those who wore the surgical masks and those who wore the N95s. Approximately 10% of the nurses were infected with each type of mask. There was no difference.
However, two other studies conducted in China found lower infection rates with N95 masks when they randomly assigned two groups of nurses to use both masks.
These N95 masks are as effective as their fit, as their weakest point is leakage from the edges of the mask (we will explain details soon). In the last two studies, researchers found surprisingly low fit test failure rates (1.1 to 2.6 %). However, a previous study had found a failure in the fitting in 60 % of cases with the same N95 masks.
Other authors have carried out a systematic review and meta-analysis of randomized controlled trials. Their conclusion was:
Medical masks and N95 respirators offer similar protection against infection respiratory viruses, including coronavirus, in health care workers during non-aerosol medical care. The (low-certainty) evidence suggests that medical masks and N95 respirators offer similar protection.
There are more studies, reviews, meta-analyses…
Although N95 respirators appeared to have a protective advantage over surgical masks in laboratory settings, our meta-analysis showed that there were insufficient data to determine definitively whether N95 respirators are superior to surgical masks in protecting health care workers against transmissible acute respiratory infections in clinical settings.
Surgical and N95 masks were equally effective in preventing the spread of PCR-detectable influenza.
The use of N95 respirators compared with surgical masks is not associated with a lower risk of laboratory-confirmed influenza.
Compared to masks, N95 respirators conferred superior protection against clinical respiratory illness and laboratory-confirmed bacterial, but not viral infections or influenza-like illness.
These other authors have done a systematic review and meta-analysis of randomized controlled trials. The aim was to compare medical masks to N95 respirators in preventing laboratory-confirmed viral infection and respiratory illness, including coronavirus, specifically in healthcare workers. Their conclusion was:
Low certainty evidence suggests that medical masks and N95 respirators offer similar protection against viral respiratory infection including coronavirus in healthcare workers during non-aerosol-generating care.
The authors of this Cochrane Systematic Review concluded:
There is uncertainty over the effects of N95/P2 respirators when compared with medical/surgical masks on the outcomes of clinical respiratory illness and influenza‐like illness.
There is a need for large, well‐designed RCTs addressing the effectiveness of many of these interventions in multiple settings and populations, especially in those most at risk.
To summarise, a few studies that have been done in real situations have found that both types of masks (respirators and surgical masks) offered similar protection against respiratory infections caused by viruses, including SARS-CoV-2.
Many people will be sceptical:
But how are surgical masks going to work as well as N95 masks to capture viruses? It is not possible that surgical masks can capture such small particles!
Besides, what about articles like this, in which “FFP2 masks provided approximately 50 times more protection than homemade masks and 25 times more protection than surgical masks”?
This study uses the so-called Protection Factor (PF). This factor indicates how many times the air inside the mask is cleaner than the air outside. If we considered the results of the experiments of the studies in real situations that we have seen in the previous thread, we would expect to see much more similar protection between surgical masks and FFP2.
So why do they say there is such a huge difference? If we want to report on the protection offered by a mask, the most understandable way is to present the percentage of captured particles. It is simple and straightforward. However, the mask industry does not do it like that but indicates it with the protection factor, or PF.
The percentage captured particles is calculated as follows:
% Captured [1 — (1 / PF)] x 100
This method can be confusing since it exaggerates the differences. Suppose that a mask captures 50 % of the particles in an experiment. The protection factor would be 2. So far, it is easy. But PF begins to rise very rapidly as the percentage captured by the mask increases.
To illustrate this, we can plot the actual percentage of captured particles (X-axis) versus the protection factor (Y-axis). We can see how the protection factor changes abruptly on the right side of the graph.
If a surgical mask blocks 75 % of the 0.2 to 0.5 µm particles, and an FFP2 blocks 95%, when applying the formula it turns out that the FFP2 provides 25 times more protection, as stated in the article.
In summary, the use of the protection factor exaggerates the differences between the masks as the percentage of captured particles increases and minimizes the differences between, for example, 0% and 50% of the captured particles.
What is going on? Can a surgical mask block aerosols?
So far, we have been told that while a surgical mask can be effective in blocking large particle splashes and droplets, a surgical mask does NOT filter or block very small particles that are floating in the air that can be transmitted by coughing, sneezing, or certain medical procedures.
From studies in military training and decontamination drills, we know that surgical masks prevent the spread of germs and other contaminants. However, due to the loose fit between the mask and the face’s surface, surgical masks do not provide complete protection.
It has been found that surgical masks were able of retaining the aerosols expelled by coronavirus patients (although we do not know if this is also the case in the case of SARS-CoV-2).
In one study, researchers used covid-infected hamsters* and healthy hamsters. Each group was in a cage. A fan between the cages allowed the transmission of particles between them. The researchers created three different scenarios:
- No barrier masks between cages.
- Masks placed as a barrier in the cages with the infected hamsters.
- Barrier masks in the cage of healthy hamsters.
*Hamsters have cell receptors that are very similar to humans. That is why they were chosen to do the experiments.
They waited seven days. And this is what happened. Surgical masks partially protected healthy hamsters from SARS-CoV-2 infected hamsters.
The hamster experiment suggests that if infected humans wore surgical masks, they would protect other people. And that, if healthy humans wore a surgical mask, they would also protect themselves, although to a lesser extent. This experiment agrees with the scientific consensus: it is more important that the infected person wears a mask than the healthy person wears it. But the problem is that we do not know who is infected, and that is why everyone should carry it.
Two major mistakes regarding face masks
Mistake number 1: Not taking into account the size (and nature) of the particles in which the virus travels (and other parameters) to test the masks.
When some experts in their respective areas talk about filters and masks, they look at the virus size. But viruses do not leave an infected body naked, nor do they spread naked, but in the form of aerosols and droplets of a wide range of sizes; from about 0.2 µm to more than 100 µm.
When we think about whether a mask will be effective or not, we must not consider the size of the virus but those of the particles that contain the viruses.
Particles of around 0.3 µm are the most difficult to retain. Still, most of the aerosols emitted are probably larger than 1 µm. For these larger particles, the difference between respirators and surgical masks would be not so great.
Surgical masks retain 75 % of the most difficult particles to filter, but almost 100 % of particles> 2–3 µm.
The method used to evaluate surgical masks is not adequate to determine whether the masks are good at retaining aerosols containing viruses. The majority of these respiratory particles are in the range of 0.5 to 10 microns (µm), most frequently between 1 and 2 µm. They are composed of proteins, mineral salts, cell debris and water.
It would be advisable to modify the test method so that the filtration of particles smaller than 1 micron is evaluated, and not just the 3-micron particles as it is done now (in Europe).
As for respirators, this is the proposal of the most suitable test, according to John Volckens et al., Colorado State Univ.
Mistake number 2: Considering that every mask that looks like a surgical mask is a medical surgical mask.
An article published in 2008 received much attention. Researchers use particles (latex spheres) of three different diameters to evaluate filter and fit performance of surgical masks (those used in hospitals) and of those masks typically used in dental settings.
Although they all seemed similar (the bacterial filtration efficiency was ≥95 %; they all met the standard), there was much difference between one and the other. But researchers threw them into a single package.
There are huge differences between the two groups of masks and within each grupe. Compare the result obtained, for example, by mask A, which allows more than 80 % of the particles to pass through, with any of the hospital masks (graph B), which retain more than 95 % of the particles of the three sizes.
As we can see in this and other studies, not all surgical masks are equal. The filtration efficiency of surgical masks can vary greatly. Medical-surgical mask (those used in hospitals) are better than non-medical masks (single-use face masks); nevertheless, they are all called “surgical” masks.
Inexplicably, the conclusion of the authors was:
None of these surgical masks exhibited adequate filter performance and face-fitting characteristics to be considered respiratory protective devices. We conclude that surgical masks do not offer comparable protection to respiratory protective devices.
Although they are all called “surgical” masks, some have higher quality requirements than others
Safety standards are higher in the United States than in Europe or in China. In the United States, they are required to capture at least 95 % of 0.1 µm particles.
And when it comes to fit, some masks are also better than others:
Surgical masks with ties (the middle one in the figure below) fit better and block particles much better than surgical or procedure masks with ear loops.
In summary, the efficiency of different types, models, and fastenings of surgical masks vary greatly. This large difference in the filtration capacity between some masks and others is due to the different characteristics of the materials, the design, and the number of layers.
And what about respirators?
As with surgical masks, respirators can vary significantly in their ability to protect the user, both for their filtering material and the airtight seal around the edges.
Besides, many FFP2, FFP3, KN95, etc., are fake. You have to do detective work to find out if you have bought a real mask. But even if the respirator is conforming, the so-called FFP2 earmuff-type masks (with rubber bands to hold them on the ears) are far from the originals (the ones with straps to adjust behind the head). They have eliminated some critical tests, and generally manufacturers and regulators pay more attention to the filter material than to the fit.
People like this type of masks with ear loops, because it is not difficult for them to breathe with them. This is not surprising since a very high proportion of the air is not passing through the filter.
These masks are being bought in hospitals to protect healthcare workers because they have the correct certificate.
Regardless of the type of mask, fit is essential
An excellent fit is essential since small leaks reduce filtration efficiency. Leaks representing only 0.5 % to 2 % of the total mask area reduce total filtration efficiency by half to two-thirds. Therefore, the leak area must be kept at a minimum.
Leaks increase the faster we breathe. For example, when we exercise (more breaths/minute).
In this video, you can see the importance of a good fit.
For a mask to provide the expected protection, it must fit very well. Do you see any hole? Does your finger easily fit under the chin, along the sides, or above the nose? Can you feel the warm air from the sides when you exhale? Are your glasses fogging up? If you answer yes to any of these questions, the mask will not protect you well.
The more particles that are able to pass through the mask (both through the filter and along the edges, unfiltered), the worse the mask will be.
During exhalation, breathing movements increase the pressure inside the mask and force the air through the filter. The opposite occurs during inspiration. If an ineffective seal is formed around the mask, the contaminated air will take the path of least resistance and flow through the spaces around the mask. This leakage will reduce its overall effectiveness substantially.
But the face is not a static surface; for example, the jaw’s movement when swallowing, grimacing, or speaking is likely to disrupt the seal.
Even if all these variables are insignificant, there is the effect of gravity, which pulls the mask down.
On January 22, 2021, an interesting article was published. It compared the efficacy of N95 masks, several KN95s, a surgical mask (low quality), and several cloth masks.
Scientists asked participants to put on the appropriate mask and perform a fit check (according to the UK National Health Service (NHS) guidelines). The researchers then performed a quantitative fit analysis. This test continuously measures particles’ concentration inside and outside the mask while it is being worn.
There was a high proportion of fit test failures and, consequently, a reduction in the masks’ effectiveness.
Four of the seven participants were unable to fit properly any of the N95 respirators tested.
Poorly fitted N95 respirators offered a highly variable range of protection, in some cases comparable to surgical and cloth masks. The best-fitting mask, N95 8511, fitted well only 3 of the 7 participants. Other N95 masks did not fit properly in any of the participants. Or, put another way, 4 of the 7 participants could not achieve a proper fit with any of the N95 respirators tested.
And it is noteworthy that this occurs with the best kind of mask on the market, the N95.
Another key finding was that the surgical mask, KN95, and cloth masks offered similar protection levels. All of them had a bad fit.
Therefore, although many people consider KN95 a better mask than a surgical mask, this study showed that the KN95 tested did not fit in any participant well enough to offer protective benefits superior to those achieved with the surgical or the cloth masks.
Minor anatomical differences made a big difference!
The results of this study indicate that we should not assume that any N95 respirator will fit most of the population.
Anatomical differences that may appear insignificant, such as the amount of subcutaneous fat under the chin, were found to impact fit significantly. And some adjustment problems were only identified when the participant was performing certain activities.
Participant F-28, being younger, had more subcutaneous fat on the skin under her chin. This extra padding allowed the respirators to seal when her jaw was tightly closed or when swallowing. Measurements showed that the subcutaneous fat only differed by approximately 3 mm extra thickness from her mother (participant F-68).
All these factors help to make checking the fit of a mask particularly difficult. For these reasons — the researchers say — fit testing is critical to ensure a respirator fits properly and acts as an extension of the breathing apparatus, rather than simply a shield to block some of the incoming particles.
Performing a self-test of the fit is an unreliable way to determine the fit
Our research findings indicate that even if a mask is well constructed, we cannot predict the protection it will provide.
People in the study could not accurately assess their mask fit through self-assessments, even though some were trained to do so.
Three of the seven participants worked in the healthcare or healthcare-related field and had received mask-fitting training. Another participant worked in hazardous industry and was used to wearing masks. These participants were somewhat more likely to correctly identify whether the mask was a snug fit than those unfamiliar with mask use.
However, all but one of these more experienced participants incorrectly identified fit of 4 of the 5 N95 respirators.
All participants made at least one incorrect determination of fit. They could not reliably predict whether the respirators fitted correctly or not. That is, they were unable to realize whether the fit was good or not.
This study shows that the fit verification method used in many hospitals is unreliable and has high failure rates.
If mask fit issues arise in laboratory studies, where details are taken care of, and participants are familiarized with self-filtering masks (some of the best models on the market), let us consider what will occur in the general population, unfamiliar with respirators (some of them dubious quality) and its facial fit.
The poor fit of these FFP2 / N95 type masks in real (not ideal) situations has been seen before
This article published in 2010 highlighted the magnitude of the problem. The researchers found that in N95 masks the amount of air that passed through the edges (leaks) was up to 20 times greater than that passed through the filter. In the case of surgical masks, this amount was six times higher.
Leaks increased further if the mask wearer leaned forward or moved their head up and down.
The researchers concluded:
The future efforts in designing new RPDs for health care environments should be increasingly focused on the peripheral design rather than on the further improvement of the filter media. The faceseal leakage was found to represent the main pathway for the submicrometer particles penetrating into the respirator/mask.
Thus, we believe that the priority in product development should be given to establishing a better fit that would eliminate or minimize the faceseal leakage. The implementation of this recommendation should, of course, not undermine the effort to develop RPDs with appropriate comfort level, which requires maintaining relatively low pressure drop through the respirator/mask.
Leaks were higher for infectious aerosol-sized particles
The difference was less pronounced for ultrafine particles because they are captured mainly by diffusional and electrical polarization forces; in contrast, the leaks were more pronounced for larger (0.3 to 1 μm) particles, as these are removed by several mechanisms, including inertial deposition. Contrary to what happened with the N95, in the case of surgical masks the proportion of the leak did not depend on the particle size.
What do all these results imply?
Although the mask itself has a high filtration efficiency, unless a tight seal is achieved, the overall efficiency will be much lower.
Even a small fit problem not detected by the user when performing a fit check can significantly diminish these self-filtering masks’ actual protection.
The overall efficiency of any mask or respirator accounts both for air that passes through the mask and for the edges of the mask. It is well established that some fraction of the expiratory airflow leaks around the edges of the mask. Althouh the actual leakage flowrate in each direction is difficult to measure, it is estimated that its magnitude depends on the direction of scape (out of the top, the sides, or the bottom).
Thus, the overall efficiency at blocking emission and inhalation of respiratory aerosol particles will be lower (sometimes much lower) than expected.
If a mask leaks (because of the design or incorrect use), it will not fully protect the user, even if it has a quality certificate that states that it filters at least 99 % of the particles. This filtering efficiency may not reflect the effective capacity of facemasks to protect users.
We probably idealize the effectiveness of respirators
Almost everyone considers respirators to be better than surgical masks. The protective effects of these respirators have generally been studied in laboratories, under ideal conditions. In the laboratories, respirators have an excellent filtration efficiency and an excellent fit, superior to those of any other mask. But it is not the same to assay the filtration efficiency of the material, or the fit, on a mannequin in a laboratory, than to establish the masks or respirators’ effectiveness on people moving around and using the mask as they usually do.
The evidence supporting surgical mask usage for the prevention of COVID-19 in health care settings is currently inconclusive. We need more data from real word
Because of the paucity of high-quality studies in the healthcare setting, the advocacy of respirators is not entirely evidence-based.
Although respirators appear to have a protective advantage over surgical masks in laboratory settings, there is insufficient data to determine definitively whether respirators are superior to well-fitted surgical masks in protecting HCWs against transmissible acute respiratory infections in clinical settings.
Bridging the Gaps
In the absence of direct evidence to support a prevalence of either droplets or aerosols in SARS-CoV-2 transmission, WHO argues against the latter that droplet-contact precautions are enough to prevent transmission effectively in HCWs.
The WHO rationale is as follow: 1) If mask fit is required to prevent aerosol transmission; 2) If the main difference between surgical masks and respirators is fit requirements; 3) If both types of mask provide the same protection in health care settings, then mask fit is not essential. That implies that COVID-19 is not transmitted through aerosols (except during aerosol-generating procedures). (Note that this rationale assumes that the users fit their masks appropriately.)
On the other hand, no COVID-19 infections were observed in March and April 2020 when HCWs wore the personal protective equipment recommended (by WHO): gown, gloves, medical masks and face shields or goggles in routine care and, in addition, N95 respirators for aerosol-generating procedures.
«The fact that HCWs were not infected despite not wearing N95 respirators indicates — conclude the WHO — , that this virus, SARS-CoV-2, is not transmitted through aerosols».
In the majority of studies, important confounders were not addressed. Without a proper description of the mask type and use, a study is almost usefulness because there is so much variability among masks and users. An important confounder is the lack of control of the fit of respirators and masks while being used.
Other confounders that could have reduced the differences in the effectiveness of surgical masks and respirators in some studies may be:
i) the quality and the compliance of use of the surgical masks or respirators. Infections might have occurred in the period of non-use of respirators since they are more uncomfortable to wear for long periods and are more frequently removed;
ii) the impact of contamination from improper doffing of masks or ocular inoculation;
iii) the effectiveness of an excellent ventilation and negative pressure rooms(frequent in hospitals), or the adoption of other infection control measures.
A) How it is possible that, in some studies, respirators do not offer, in practice, a protection superior to surgical mask to prevent respiratory viral infections?
It could be that the respirators did not fit well enough to outperform surgical masks. Respirators are only as effective as their fit, as their weakest point is leakage from the edges of the mask. As some studies have shown, a correct mask fit is difficult to achieve and could be at the origin of this controversy. A good fit takes a lot of time and skill. They must be moulded until a complete seal is achieved. Most people do not do well with commercially available N95 / FFP2 type masks. Even a small fit problem not detected by the user can greatly diminish the actual protection offered by respirators.
On the other hand, respirators are more uncomfortable to wear and (if properly adjusted) it is more difficult to breathe with them than with surgical masks. This discomfort could lead them not to be used for as long as surgical masks. Infections might have occurred in the period of non-use of respirators.
B) And how it is possible that no health workers were infected in March and April 2020 despite not wearing N95 respirators?
First, hospitals have good ventilation and special rooms under negative pressure. Second, it could be that the surgical masks they used were the best-fitting models, with the best overall efficiency.
To sum up, neither of the two findings (neither A nor B) can lead to the conclusion that this virus is not transmitted through aerosols. Thus, WHO should not use these studies to dismiss the prevalence of the airborne route.
