Masks for COVID: Updating the evidence

Notes taken whilst preparing a paper on mask efficacy from Nov to Jan 2022. My previous paper on this was written in April 2020 and published in the Proceedings of the National Academy of Science

Jeremy Howard


July 4, 2022

These are notes I took whilst preparing a paper on mask efficacy from Nov 2021 to Jan 2022. In the end, I gave up on the paper, because I felt like people had given up on masks, so there wasn’t much point in finishing it. I’ve decided to publish these notes in the hope some people will find them a useful starting point for their own research, and since I’ve noticed some signs in recent weeks that people might be open to avoiding COVID again. My previous paper on this topic, in which I led a team of 19 experts, was written in April 2020, and published here in the Proceedings of the National Academy of Science.

The rise of better masks

In the US, 400 million N95 masks are being distributed for free, coming from the 750 million stored in the US’ Strategic National Stockpile. A similar campaign to distribute 650 millions masks in the US in 2020 was cancelled.

KN95 masks are being given to US congressional staff, and masks are required for federal workers and whilst in federal buildings.

The Los Angeles school district has required students to upgrade from cloth masks to “well-fitting, non-cloth masks with a nose wire”.

Masks work

A review paper discussed both lab evidence and empirical evidence for the importance of face masks, with eight “seminal studies” showing a reduction in transmission when masks are used, and one Danish study of surgical masks with “several design limitations” which “demonstrated only a modest benefit in limiting COVID-19 transmission”. The authors note that “laboratory studies have demonstrated the ability of surgical masks to block SARS-COV-2 and other viruses”, with the masks “60%–70% effective at protecting others and 50% effective at protecting the wearer”.

An evidence review from early in the pandemic concluded that “given the current shortages of medical masks, we recommend the adoption of public cloth mask wearing, as an effective form of source control”. It noted that “by the end of June 2020, nearly 90% of the global population lived in regions that had nearly universal mask use, or had laws requiring mask use in some public locations.” The review said that “There has been one controlled trial of mask use for influenza control in the general community. The study looked at Australian households, was not done during a pandemic, and was done without any enforcement of compliance” – and yet still found “masks as a group had protective efficacy in excess of 80% against clinical influenza-like illness.”

An observational study of Beijing households analyzed the impact of mask use in the community on COVID-19 transmission, finding that face masks were 79% effective in preventing transmission, if used by all household members prior to symptoms occurring.

One study used a multiple regression of policy interventions and country and population characteristics to infer the relationship between mask use and SARS-CoV-2 transmission. It found that transmission was around 7.5 times higher in countries that did not have a mask mandate or universal mask use, a result similar to that found in an analogous study of fewer countries. Similar results were found by numerous other papers.

A mathematical model of mask use estimates that mask wearing reduces the reproduction number R by (1−mp)^2, where m is the efficacy of trapping viral particles inside the mask, and p is the percentage of the population that wears masks.

A report in Nature explained that researchers running a randomized controlled trial (RCT) of community mask use in Bangladesh “began by developing a strategy to promote mask wearing, with measures such as reminders from health workers in public places. This ultimately tripled mask usage, from only 13% in control villages to 42% in villages where it was encouraged”, and “then compared numbers of COVID-19 cases in control villages and the treatment communities”. They found that the number of infections in mask wearing communities decreased, with a reduction of COVID symptoms using surgical masks to 0.87 times the incidence in unmasked communities, and 0.91 times when using cloth masks. The report noted that “the researchers suggest that the true risk reduction is probably much greater, in part because they did no SARS-CoV-2 testing of people without symptoms or whose symptoms did not meet the World Health Organization’s definition of the disease.” The researchers concluded that “promoting community mask-wearing can improve public health”.

The Johns Hopkins School of Public Health reviewed the work and concluded that “This study is the largest and best-designed randomized controlled trial to date of a realistic non-pharmaceutical intervention on SARS-CoV-2 transmission.”

A paper investigating an upper bound on one-to-one exposure to infectious human respiratory particles concludes that “face masks significantly reduce the risk of SARS-CoV-2 infection compared to social distancing. We find a very low risk of infection when everyone wears a face mask, even if it doesn’t fit perfectly on the face.” They calculate that “social distancing alone, even at 3.0 m between two speaking individuals, leads to an upper bound of 90% for risk of infection after a few minutes”, but that when both source and susceptible wear a well-fitting FFP2 mask, there is only 0.4% after one hour of contact. They found that to achieve good fit it is important to mold the nose piece wire to the size of the nose, rather than leaving it in a sharp folded position.

A similar study “quantifies the extent to which transmission risk is reduced in large rooms with high air exchange rates, increased for more vigorous respiratory activities, and dramatically reduced by the use of face masks.” The authors describe the six-foot rule widely used to ensure social distancing as “a guideline that offers little protection from pathogen-bearing aerosol droplets sufficiently small to be continuously mixed through an indoor space.” Instead, they develop a safety guideline based on cumulative exposure time,” the product of the number of occupants and their time in an enclosed space. In particular, they identify that the greatest risk comes in places where people are speaking (other than quietly) or singing, and that “the benefit of face masks is immediately apparent”, due to the multiplicative effect when both source and susceptible wear a mask. They further note that “Air filtration has a less dramatic effect than face mask use in increasing the CET bound. Nevertheless, it does offer a means of mitigating indoor transmission with greater comfort, albeit at greater cost.”

Another study of the combined impacts of ventilation and mask effective filtration efficiency in classroom settings found that “ventilation alone is not able to achieve probabilities <0.01 (1%)” of transmitting COVID in a classroom. However, they found that good masks reduce infection probability by >5× in some cases, and that “reductions provided by ventilation and masks are synergistic and multiplicative”. However they also noted that “most masks fit poorly”, recommending that work be done to ensure that high quality masks are used.

Similar results were found in a study of community public health interventions, which concluded that “control the pandemic, our models suggest that high adherence to social distance is necessary to curb the spread of the disease, and that wearing face masks provides optimal protection even if only a small portion of the population comply with social distance”.

Guidance from the independent scientific advisory group OzSAGE points out “that school children are able to wear masks. As an example, all children over two years of age in San Francisco are required to wear masks at school”.

Omicron changes the game

An analysis of fine aerosol emissions found that, compared to the original wild type (WT) virus:

“Delta and Omicron both also have increased transmissibility: the number of cells infected for a given number of ribonucleic acid (RNA) virus copies was found to be doubled and quadrupled respectively. Furthermore, Omicron also seems to be better at evading the immune system. This implies that the critical dose of virus copies above which a situation is potentially infectious needs to be lowered. For the WT, we had proposed a critical dose of 500 virus copies. If the above-mentioned capacity to infect cells translates into an infection risk, this would imply a critical dose of around 300 virus copies for Delta and around 100 virus copies for Omicron.”

The study finds that “surgical masks are no longer sufficient in most public settings, while correctly fitted FFP2 respirators still provide sufficient protection, except in high aerosol producing situations such as singing or shouting.”

Data from Hong Kong shows that “Omicron SARS-CoV-2 infects and multiplies 70 times faster than the Delta variant and original SARS-CoV-2 in human bronchus”.

A study of transmission in Danish households estimated the secondary attack rate (SAR) of omicron compared to delta, finding it 1.2 times higher for unvaccinated people, 2.6 times higher for double-dosed, and 3.7 times higher for boosted. The authors conclude that “the rapid spread of the Omicron VOC primarily can be ascribed to the immune evasiveness”.

According to UK statistics, the risk of hospitalization from omicron when unvaccinated is about the same as the wildtype virus, which is about half the risk of the delta variant.

The journal Infection Control Today reported that many experts are concerned that “‘Omicron the Pandemic Killer’ Idea Ignores Dangers of Long COVID”:

“Linda Spaulding, RN-BC, CIC, CHEC, CHOP, a member of Infection Control Today®’s Editorial Advisory Board (EAB), says that she’s “seen athletes in their 20s on the wait list for double lung transplants because of long COVID. That’s something that has long-term consequences. Some people talk of COVID fog. They just can’t put their thoughts together.” In addition, even the treatments for those with long COVID can put toll on a patient’s body.”

“As noted by Kevin Kavanagh, MD, another member of ICT®’s EAB, a core difficulty in society’s attempt to guide COVID-19 from pandemic to endemic is that COVID is not just a respiratory virus. Kavanagh wrote in October that SARS-CoV-2 is similar to HIV because it can “silently spread throughout the host’s body and attack almost every organ.””

Better masks work better

The US Centers for Disease Control and Prevention (CDC) explains that:

“Loosely woven cloth products provide the least protection, layered finely woven products offer more protection, well-fitting disposable surgical masks and KN95s offer even more protection, and well-fitting NIOSH-approved respirators (including N95s) offer the highest level of protection.”

Unfortunately “well-fitting disposable surgical masks” do not exist out of the box, since there are large gaps on each side of the mask. Surgical masks require modifications to achive a good fit. That’s because they are made to stop liquid splashes during surgery, rather than made to stop airborne transmission. There are two methods shown by the CDC to improve fit:

  • Knot and Tuck: Tying the sides of the mask together to remove the side gap
  • Double masking: Wearing a tight fitting cloth mask over a surgical mask

Research shows that both of these approaches dramatically reduce exposure to aerosols emitted during a period of breathing:

“…adding a cloth mask over the source headform’s medical procedure mask or knotting and tucking the medical procedure mask reduced the cumulative exposure of the unmasked receiver by 82.2% (SD = 0.16) and 62.9% (SD = 0.08), respectively (Figure 2). When the source was unmasked and the receiver was fitted with the double mask or the knotted and tucked medical procedure mask, the receiver’s cumulative exposure was reduced by 83.0% (SD = 0.15) and 64.5% (SD = 0.03), respectively. When the source and receiver were both fitted with double masks or knotted and tucked masks, the cumulative exposure of the receiver was reduced 96.4% (SD = 0.02) and 95.9% (SD = 0.02), respectively.”

An airborne transmission simulator was used to estimate the ability of various types of face masks to block COVID-19 transmission. In this experiment, “cotton mask led to an approximately 20% to 40% reduction in virus uptake compared to no mask. The N95 mask had the highest protective efficacy (approximately 80% to 90% reduction)”. All of the masks were much more effective at source control than at protecting the wearer, with the N95 stopping all detectable transmission.

The American Conference of Governmental Industrial Hygienists (ACGIH) say that “workers need respirators”, noting that a worker with an “N95 filtering facepiece respirator… has 1-10% inward leakage and outward leakage”, but with a surgical mask “has 50% inward leakage and outward leakage”, and with a cloth face covering “has 75% inward leakage and outward leakage”. They explain that “N95 FFRs have an assigned protection factor of 10 (10% inward leakage) but must receive a fit factor of 100 (1% inward leakage) on an individual worker.” ACGIH created a table showing how, if we start with an assumption that it takes on average 15 minutes to get infected if no-one is wearing a mask (based on CDC contact tracing premises), we can calculate the time it would take on average to get infected if one or both of source and receiver are wearing various types of mask. This is calculated by simply dividing the base time of 15 minutes by the leakage factor for the source’s mask (if any), and then dividing that by the leakage factor for the receiver’s mask (if any).

This approach is, however, an over-simplification. Reseach based on a a single-hit model of infection shows that the probability of infection “shows a highly nonlinear sensitivity” to inhaled virus number. Therefore, “In a virus-rich regime… wearing a mask may not suffice to prevent infection.”

Research undertaken by the National Personal Protective Technology Laboratory (NPPTL) found that respirators with an exhalation valve “reduce particle emissions to levels similar to or better than those provided by surgical masks, procedure masks, or cloth face coverings”. Furthermore, “surgical tape secured over the valve from the inside of the FFR can provide source control similar to that of an FFR with no exhalation valve”.

Pushing back against masks

Professor Alison McMillan, Commonwealth Chief Nursing and Midwifery Officer in Australia claims that “there is no evidence to suggest that we should be moving towards… N95 respirators in the community setting.” She added “I am aware that there are some publications out there suggesting a move to N95 (masks). But that’s not supported in the empirical evidence”.

According to Norman Swan, host of the ABC’s Coronacast, “If you’re wearing an N95 that hasn’t been fit tested – and it’s not an easy process to do yourself at home – there’s no guarantee that it’s an awful lot more effective than wearing a surgical mask. Professor Catherine Bennett, chair in epidemiology at Deakin University, claims that”Technically, the instructions say you shouldn’t reuse” respirators, and that “If you’re not particularly checking its fit, you’re probably wasting your time”. Occupational environment physician Malcolm Sim agrees: “If you put it [an N95 mask] off and put it on, they’re not meant for that purpose… They’re easily damaged in somebody’s handbag,” adding that the integrity of the masks can be compromised. He says that “If you’re handling them a lot, taking them on and off, there’s much more potential for you to get it [the virus] on your hands, your face, different parts of your body.”

University of New South Wales epidemiologist Mary-Louise McLaws claimed that “There’s no evidence yet that a N95 mask will protect you more than a surgical mask for Omicron.”

An opinion piece in Newsweek claims that “the effectiveness of respirators is vastly overestimated, and there is scant evidence that they stop community transmission. Moreover, NIOSH-approved respirators are tight, uncomfortable, and can impede breathing.” The article further claims that “For respirators to work, they must be well fitting, must be tested by OSHA, and must be used for only short time windows as their effectiveness diminishes as they get wet from breathing.”

Recently there has been particular pushback against the use of masks by children, with the Newsweek article alleging that “Respirators are not necessary to protect children from COVID-19 because of the astoundingly low risk COVID-19 presents to them”, and that in fact wearing masks involves “existing well-documented harms”. There hasn’t been any documented harms to children from wearing masks,

Respirators can be reused

According to mask manufacturer 3M, respirators (which they refer to as “Filtering Facepiece Respirators (FFRs)”) “can be used many times.” They say that “There is no time limit to wearing an FFR. Respirators can be worn until they are dirty, damaged or difficult to breathe through.”

In reporting from CNN, Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, explained that an N95 mask’s material and filtration ability aren’t “going to degrade unless you physically rub it or poke holes in it.”You’d have to be in really polluted air … for several days before it lost its ability to filter out particles. So, you can really wear them for a long time. People have been talking about 40 hours – I think that’s fine. Really, it’s going to get gross from your face or the straps will get too loose or maybe break before you’re going to lose filtration ability… One of the first indicators of being able to change it if it looks nice and clean is that it just feels a little harder to breathe through. There appears to be more resistance with every breath.” She also noted that the contamination risk in reusing N95 masks is “lower, much lower, than the risk of you not wearing an N95 and breathing in particles”.

The CDC has prepared guidelines for optimizing the supply of respirators which recommend reusing respirators at most five times. This guidelines were created for people “implementing policies and procedures for preventing pathogen transmission in healthcare settings”. They have been widely shared, incorrectly, by reporters as being recommendations for community use.

The inventor of N95 mask material, Peter Tsai, says that “N95 masks can be rotated, 1 mask every 3–4 days”, and that in doing this “there is no change in the mask’s properties.”

According to the NIOSH Guide to the Selection and Use of Particulate Respirators N95 respirators must maintain at least 95% filtration after a total mass loading of 200mg. This is designed to ensure they continue to work in sites with high particulate matter, such as some construction environment. However in normal use, even outside in a city with high levels of population, it would take over 200 days of 24 hour per day use to get to this level. The guide says that “generally, the use and reuse of N-series Ž lters would also be subject only to considerations of hygiene, damage, and increased breathing resistance”. The NIOSH guidelines are well supported by research.

Fit tests are not required for respirators to be effective

In one study non-experts were asked to read the instructions that come with a respirator, and then to don the respirator without assistance and complete a fit test. The average fit factor achieved was 88, and the lowest fit factor of the subjects was 15, with nearly half achieving a fit factor greater than 100.

Surgical masks have been found to have a much poorer fit in practice. One study showed that for surgical masks “quantitative fit factors ranged from 2.5 to 9.6”, and another found an average fit factor of 3.0.

Guidance from the US Food and Drug Administration (FDA) explains that:

“Fit Factor is a means of expressing the difference in particle concentration inside the mask and outside the mask during use. For example, a fit factor of 2 means that the concentration of particles within the mask is ½ or 50% of the concentration outside the mask; a fit factor of 5 means the concentration of particles within the mask is 1/5 th or 20% of the concentration outside the mask.”

The guidance says that failing to achieve a fit factor of 2 “may suggest that respirator fit will not be sufficient to assure that the device will help reduce wearer exposure to pathogenic biological airborne particulates.”

An analysis of the fitted filtration efficiency (FFE) of surgical masks found that, unmodified, they only achieved an FFE of 38.5%. The “knot and tuck” technique improved that to 60.3%, and a DIY mask fitter consisting of three rubber bands increased it to 78.2%. A 3-layer cotton mask had an FFE of just 26.5%. An N95, on the other hand, achieved an FFE of 98.4%. Furthermore, the N95 FFE had a standard deviation of only 0.5% — that is, it was effective for multiple tests during “a series of repeated movements of the torso, head, and facial muscles”. Interestingly, a 2-layer nylon mask had an FFE of 79.0% (standard devatiation 4.3%), showing that some cloth masks can be quite effective. These findings were replicated in a study of numerous types of cloth mask, which found that hybrids of 600 TPI cotton with silk, chiffon, or polypropelene achieved 72-96% filtration efficiency.

Researchers have calculated that “the particle size most likely to deposit in the respiratory tract when wearing a mask is ∼2μm”. Unfortunately, this particle size is not considered in N95 or similar standards. Instead, 0.3 μm particles are used.

A 2010 study of fit testing respirators for public health medical emergencies found that 98% of non-experts wearing masks without training achieved a fit factor of over 5 (20% leakage) and 75% of them achieved a fit factor of over 10 (10% leakage).

Donning and doffing masks is not complex or risky

Analysis by the CDC concludes that the risk of infection through surfaces (fomites) “is generally considered to be low”, a view that was supported by the evidence as early as July 2020. An analysis of “418 samples from mask fronts, cell phones, paper money, card machines, sewage, air and bedding” during a COVID surge “did not detect any trace of SARS-CoV-2 in all samples analyzed”.

We should not reserve respirators for healthcare workers

According to Anne Miller, executive director of Project N95, there are many U.S. manufacturers of N95 masks and an ample supply.

The Economist reported that in Europe “at the start of the pandemic, FFP2 masks were scarce and costly. Even governments fell victim to price gouging, paying more than €4 ($4.50) per mask. Demand had previously been low, so stockpiles and production capacity could not satisfy the sudden surge. Governments wanted to reserve supplies for those most at risk of contracting the virus, such as health-care workers.” However they reported that by the end of 2021 “FFP2 masks are in healthy supply, and as the highly transmissible Omicron variant spreads across the world, updating guidance to recommend their wider use could be one way to help reduce transmission.”

In the first 6 months of 2020, over 70,000 new face mask companies were registered in China, many run by people with no previous experience and no registration or licensing. The Chinese government stepped in to make licensing more stringent, shutting down many companies, and international demand fell over quality concerns.

Due to “a dramatic reduction in demand for N95s”, US mask factories are closing. In June 2021 the American Mask Manufacturer’s Association said that “we have 28 members who are going to go out of business in the next 60 to 90 days.” By July 2021 they estimated “that 5,000 workers have been laid off across its member companies”. However following school mask mandates and demand during the omicron surge, demand in the US spiked in early 2022.

The CDC has found that 60% of KN95s are counterfeit.

In Australia it has been reported that “general practitioners have been left without highly protective N95 masks as consumers rush to stock up after a sharp rise in COVID-19 cases.”

In May 2021 the CDC stated that “The supply and availability of NIOSH-approved respirators have increased significantly over the last several months. Healthcare facilities should not be using crisis capacity strategies at this time and should promptly resume conventional practices.”

Demand distortions can increase as we proceed up the supply chain, creating inefficiencies for upstream firms. This is known as the Bullwhip Effect.

Respirators need not be uncomfortable

In an analysis of the physiological impact of the N95 filtering facepiece respirator (FFR) “in healthy healthcare workers, FFR did not impose any important physiological burden during 1 hour of use, at realistic clinical work rates”.

A study of KF80, KF94, KF99, N95, and N99 masks found that self-reported comfort levels were nearly perfectly correlated with the ease of inhalation.