COMMENTARY: What can masks do? Part 1: The science behind COVID-19 protection

Editor’s Note: Part 1 of a two-part commentary explains the variations in material face coverings and surgical masks, the science of respiratory protection, and the hierarchy of illness controls. Part 2, to be revealed tomorrow, will define what makes a great masks examine and why so many fail.

Dr Brosseau is a nationwide skilled on respiratory protection and infectious ailments and a analysis marketing consultant with the Center for Infectious Disease Research and Policy (CIDRAP) on the University of Minnesota (UMN). Dr Ulrich is a UMN assistant professor within the Division of Environmental Health Sciences and a CIDRAP researcher. Dr Escandón is a doctor and infectious ailments researcher within the Division of Infectious Diseases and International Medicine at UMN and likewise collaborates on analysis with CIDRAP. Cory Anderson is a graduate analysis assistant with CIDRAP. Dr Osterholm is CIDRAP director and Regents Professor at UMN.


Confusion continues to abound over the effectiveness of masks to guard folks from COVID-19, and up to date information tales touting imperfect research are solely compounding the state of affairs.

First, there was confusion after which intransigence in regards to the modes of transmission, with the US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) insisting for a lot of months after SARS-CoV-2 emerged that the virus was transmitted solely by giant droplets or hand contact. Only pretty just lately did these companies lastly acknowledge that inhalation of small infectious particles within the air (“aerosols”) is a extra necessary mode of transmission.

That was adopted by substantial misinformation and misunderstanding in regards to the function of material face coverings, surgical masks, and respirators in stopping SARS-CoV-2 unfold. Then we had misunderstandings and poor messaging in regards to the efficacy of various interventions, the effectiveness of 1 intervention versus one other, and why interventions needs to be thought of in a specific order or hierarchy.

In addition, quite a few high-profile research have concluded that face coverings or masks shield towards COVID-19, however they typically lack scientific rigor, making such conclusions an overreach.

So it’s fully comprehensible that the general public and even the scientific neighborhood expresses confusion over how nicely masks shield towards a respiratory illness like COVID-19. In half 1 of this commentary we are going to reduce via the murk by:

  • Explaining the variations between material face coverings (or material masks), surgical masks, and respirators within the context of infectious aerosol inhalation.
  • Delving into the science of respiratory protection.
  • Discussing the function of masks and respirators within the hierarchy of controls for illness prevention.

(For simplicity, we are going to broadly use “masks” to check with each material face coverings and surgical masks, except this distinction is specified. Respirators like N95s should not thought of to be masks [see sidebar at right]).

Part 2, to be revealed tomorrow, will dig deeper by:

  • Detailing how the standard of proof to tell masks insurance policies could possibly be improved by way of well-conducted research.
  • Laying out—utilizing a number of latest publications as examples—methods to assess research about masks, underscoring the significance of ready for the peer evaluate course of carried out by material specialists earlier than drawing conclusions.

Let’s comply with the rising physique of proof

It is important to steadiness the necessity for fast data sharing with rigorous and methodical scientific investigation throughout a pandemic.1 Early within the pandemic, messaging about masks compromised the latter for the previous with the understanding that, with time, extra knowledge would develop into obtainable to clarify the function that masks, together with different interventions, would possibly play in limiting person-to-person transmission of SARS-CoV-2.

Now, over a yr and a half into the pandemic, we’re past the preliminary want to speak data quickly as a part of the emergency response. Rather than a deal with singular research, scientists and the media ought to deal with the rising physique of scientific proof, whereas on the similar time calling for sound examine methodologies that advance the science and fill present data gaps.

At the outset, we need to clarify that we’re not “anti-mask.” Rather, we’re in favor of carrying probably the most protecting facepiece for the setting—reminiscent of a non-fit examined respirator when spending various minutes in a crowded indoor house—and together with different interventions.

The knowledge are clear that the majority material face coverings and surgical masks supply very restricted supply management (protection of others from pathogens by limiting emissions from an contaminated particular person) and private protection towards small inhalable infectious particles and shouldn’t be thought of a alternative for different, simpler strategies of lowering one’s publicity to SARS-CoV-2, reminiscent of vaccination and good air flow.

We strongly help folks carrying simpler facepieces, together with respirators. Even if not fit-tested, an N95 filtering facepiece respirator (FFR)—or some other respirator—will supply higher protection from COVID-19, if sized and worn accurately.2 As there may be now ample provide, it’s now not necessary to order respirators for healthcare employees solely.

The science behind respiratory protection

Aerosol inhalation requires completely different interventions

Despite early overwhelming proof3 that SARS-CoV-2 is transmitted primarily by inhalation of infectious particles within the air (aerosols), it took the WHO and CDC many months to acknowledge this mode of transmission of their scientific briefs.4,5 Neither company, nonetheless, has adequately directed its steering away from droplet and make contact with transmission towards interventions that focus totally on stopping aerosol inhalation.

Droplet transmission, or propulsion of enormous particles from sneezing or coughing into the nostril, mouth, and eyes of these close by, could possibly be prevented by erecting obstacles between folks, bodily distancing, and carrying masks with filters that seize giant particles. None of these interventions, nonetheless, is efficient for smaller inhalable particles, that are the predominant measurement created by folks in excessive concentrations when respiratory, speaking, singing, and so on. Smaller particles keep within the air for lengthy durations (hours), are simply distributed by diffusion and air currents all through an area (thus negating the results of bodily distancing), and can carry many virions. Masks have filters that don’t effectively seize smaller airborne particles and depart gaps across the masks that permit small particles to leak each out and in.3,6

The interventions that forestall aerosol inhalation are those who scale back the focus of small particles in a shared house and the time somebody spends in that house inhaling these small particles. Particle focus can be diminished by having fewer folks within the house, sharing house for shorter durations, utilizing air flow that removes particles shortly close to the supply, and utilizing supply controls (masks and respirators) with good filters and match.

Masks and respirators

Early within the pandemic, CIDRAP revealed an article describing the variations between material face coverings, surgical masks, and respirators.6 The most necessary options of something worn on the face to forestall the emission of respiratory particles (supply management) or shield the wearer from inhaling infectious particles (private protection) are filter effectivity, respiratory resistance, and match, in that order.

The CIDRAP article describes applicable strategies for testing these three necessary variables and the revealed knowledge for every kind of masks or respirator. At that point there have been no requirements for material face coverings and solely a small variety of surgical masks have been examined for filter efficiency (however not respiratory resistance or match).

Masks typically have very low filter effectivity for small particles, growing respiratory resistance with the addition of extra layers of material, and really poor match. Respirator filters, alternatively, are rigorously examined by the National Institute for Occupational Safety and Health (NIOSH) for filter effectivity and respiratory resistance, and the Occupational Safety and Health Administration (OSHA) requires particular person fit-testing of a NIOSH-approved respirator earlier than it’s worn within the office.

There stays no commonplace technique for testing outward leakage (supply management) for any masks or respirator. Despite quite a few research all through the pandemic, only a few have used applicable testing strategies.

A 2021 examine by Lindsley et al,7 which evaluated outward leakage of a spread of face coverings, masks, and respirators on a model, filter effectivity and respiratory resistance utilizing applicable strategies, and match (inward leakage) on human topics, is maybe the very best latest examine to display the variations between material face coverings, surgical masks, and respirators.

In early April 2020, the CDC really useful face coverings in public settings the place bodily distancing was tough, suggesting that two layers of material worn snugly towards the face and secured with ties or ear loops would supply sufficient protection.8 The company, nonetheless, didn’t describe or require any efficiency assessments for filter effectivity, respiratory resistance, or match.

In May 2020, the CDC messaging expanded to “Your cloth face covering may protect them. Their cloth face covering may protect you,” implying that everybody could be protected if everybody wore a face masking.9 This message was predicated on the misunderstanding that SARS-CoV-2 was transmitted solely by giant droplets emitted throughout coughing or sneezing. In July 2020, primarily based on simply two very restricted research in a hospital and hair salon, CDC Director Robert Redfield, MD, touted “Cloth face coverings are one of the most powerful weapons we have to slow and stop the spread of the virus—particularly when used universally within a community setting.”10

It was not till November 2020, nonetheless, that the CDC lastly revealed a scientific transient on face coverings, which once more expanded the message to point that they may additionally shield the wearer from droplets, whereas lastly recognizing that face coverings range broadly in filtration effectiveness.11 The transient fails to acknowledge or focus on the very restricted efficacy of face coverings for stopping the emission or inhalation of small infectious particles and gives no steering for the evaluation of filter effectivity, respiratory resistance, or match.

ASTM, previously referred to as the American Society for Testing and Materials, is a global requirements group that develops and publishes voluntary consensus technical requirements for a variety of supplies, merchandise, and companies. There is now an ASTM commonplace for barrier face coverings (BFCs).12 A BFC is outlined as “a product worn on the face, specifically covering at least the wearer’s nose and mouth, with the primary purpose of providing source control and to provide a degree of particulate filtration to reduce the amount of inhaled particulate matter.” The commonplace contains efficiency assessments for filter effectivity and respiratory resistance and both qualitative or quantitative assessments of leakage. Lacking a standardized technique for evaluating outward leakage (supply management), the usual depends on well-accepted fit-testing (inward leakage) strategies.

NIOSH, the company answerable for approving respirators, designated two efficiency ranges for ASTM BFCs utilized in workplaces13—office efficiency and office efficiency plus—that require, along with a excessive diploma of filter effectivity and low respiratory resistance, a quantitative evaluation of leakage on a panel of human topics. As of Oct 1, there have been 12 ASTM BFCs and a pair of Workplace Performance Plus ASTM BFCs listed on the NIOSH Personal Protective Equipment Information web site.14

Protection afforded by masks, respirators

ACGIH (previously the American Conference of Governmental Industrial Hygienists), an expert scientific group devoted to advancing occupational and environmental well being, created a COVID-19 Task Force in November 2020, which developed a number of truth sheets for office well being and security, together with a desk that illustrates the diploma of protection provided by various kinds of masks compared to a respirator.15 We have modified this desk to display that an N95 FFR, even when not fit-tested, gives extra advantages as each supply management and private protection than any masks (Table 1).

mask table 1 final

The CDC’s 15-minute contact tracing time serves as a baseline for the “time to an infectious dose” when neither the supply (contaminated particular person) nor the receiver (uninfected particular person) is carrying a masks or respirator. The time for somebody to obtain an infectious dose when one or each are carrying a masks or respirator can be calculated by dividing the 15-minute baseline by the share of outward leakage (supply management) or share of inward leakage (private protection), or each.

(These numbers are simply estimates primarily based on the CDC’s 15-minute contact tracing time, which has no scientific foundation. And the extra transmissible Delta variant (lineage B.1.617.2) could require an adjustment to a shorter contact time.)

The receiver’s time to an infectious dose will increase by a really small quantity, to 26 minutes, if each the supply and receiver are carrying a typical material masks. If each are carrying a typical surgical masks, with 50% inward and outward leakage, the receiver’s time to an infectious dose expands to an hour. If each are carrying a non–fit-tested N95 FFR, which can exhibit 20% leakage if sized and worn accurately,2 the time to an infectious dose for the receiver will increase to greater than 6 hours. If each are carrying a fit-tested N95 FFR, the time to infectious dose expands to 25 hours.

Thus, the general public mustn’t depend on typical material or surgical masks to supply them a lot time past quarter-hour in a shared house with potential sources, even when others are additionally carrying masks. Wearing a non–fit-tested respirator permits about an hour of time inside a shared house even when nobody else is carrying a masks or respirator.

Unrealistic expectations about masks

Dr. John Snow, who is taken into account the daddy of contemporary epidemiology, was derided and disbelieved for many of his profession after he clearly demonstrated that sewage-contaminated water was the supply of a big cholera outbreak in London. Ironically, folks believed that “bad air” (or miasma) was answerable for the illness, regardless of knowledge demonstrating in any other case. Today we perceive the significance of protected water provides. Snow’s story illustrates each how straightforward it’s for unproven beliefs to take maintain and the way tough it can be to vary long-standing dogmas even when offered with knowledge exhibiting in any other case.

Scientists, journalists, and the general public all hope, with very restricted proof, that transmission might be stopped by carrying a few layers of material with poor filter effectivity in public settings. They ignore proof of great transmission in households and workplaces,17–20 the place folks spend lengthy durations in shut contact with others.

Infection is dependent upon the organism’s infectious dose (variety of virions that result in an an infection), host components (eg, age, intercourse, comorbidities), and environmental components that have an effect on virus viability (eg, humidity, temperature).21 People’s dose is a perform of non-public publicity, which incorporates each the time they spend in a shared house with a number of infectious folks and the focus of infectious particles within the air of that shared house.

It is comprehensible that masks have come to play an outsized function in folks’s minds, as a result of they seem to supply a comparatively straightforward resolution that, not like different interventions, doesn’t considerably intrude with on a regular basis actions. It is simple to see whether or not somebody is carrying a masks, and it’s straightforward guilty the individuals who don’t for spreading an infection.22 Mask carrying (or the dearth thereof) supplies a simple goal for blame, when there are various much less observable or measurable variables that could possibly be answerable for an infection.

Also, it’s simpler to require folks to put on masks compared to different, simpler interventions, reminiscent of vaccination, limiting the variety of folks or the time they spend in a shared house, or enhancing the air flow in that house. Wearing masks supplies an outsized phantasm of management over a difficult-to-contain and nonetheless largely unknown illness.

The hierarchy of illness management

Public well being tips have emphasised all through the pandemic that stopping an infection and transmission requires a mix of interventions.23,24 It is necessary to think about interventions within the context of a hierarchy, from most to least efficient (Table 2).mask table 2 lgr final

source pathway receiverThe simplest interventions are those who eradicate or reduce sources of an infection (i.e., contaminated folks), reminiscent of vaccination (which lowers the possibility of somebody being contaminated),25 limiting the variety of folks in a shared house (which lowers the probability of multiple particular person being a supply), and limiting the time spent in a shared house (which lowers the focus of infectious particles generated by an contaminated particular person).

Ventilation can even be an efficient intervention, as a result of it dilutes and removes inhalable particles from a shared house.26 It will not be efficient, nonetheless, if many sources share an area for a chronic interval.

Cloth and surgical masks supply a really restricted diploma of supply management, as a result of, whereas they restrict the variety of bigger respiratory particles in an area, they don’t forestall the emission of most small particles (aerosols) exhaled throughout respiratory, speaking, singing, coughing, or different respiratory actions. Because masks supply restricted supply management and far more restricted private protection, their effectiveness can be improved solely by combining them with different interventions.

At the underside of the hierarchy of controls is private protection, reminiscent of respirators like N95 FFRs, which can be efficient at defending the wearer, however depend on applicable match and constant put on.

None of those interventions, absent eliminating contact with different folks, is efficient by itself. But the better the variety of interventions applied, the decrease the chance of person-to-person transmission.22 Some interventions, reminiscent of vaccines, are simpler than others, reminiscent of masks. As mentioned above, anybody carrying a masks needs to be conscious that the longer they spend in a shared house with different folks, the better the chance of being contaminated.


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