One of the most politicised scientific issues of 2020 was the question about whether face-masks stop the spread of COVID-19.

Here are some specific examples:

In particular:

When worn by members of the public who are infected, are surgical masks effective at slowing the spread of COVID-19 ("source control")?

Note that this question is about effectiveness (not efficacy:

Intervention studies can be placed on a continuum, with a progression from efficacy trials to effectiveness trials. Efficacy can be defined as the performance of an intervention under ideal and controlled circumstances, whereas effectiveness refers to its performance under ‘real-world' conditions"),

so answerers and voters are invited to weight evidence about the spread of COVID-19 in real-life conditions above evidence from lab experiments, and in turn, evidence from lab-experiments above theoretical models.

This question is one of a number of similar questions on Skeptics.SE.

Here is a master list (note that some of these questions have been automatically deleted due to age and low votes):

Cohort Mask Type Wearer's Status Question
Public Surgical Mask Healthy deleted
Public Surgical Mask Infected this question
Public Cloth Mask Healthy link
Public Cloth Mask Infected deleted
Health Care Worker N95 Infected deleted

Older, related questions that these questions are intended to supersede:

  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Oddthinking
    Commented Jan 2, 2021 at 12:30
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    Adding a +500 bounty. Consider looking into this study as it has some of the best data so far: aip.scitation.org/doi/10.1063/5.0057100, although still not a full answer. Commented Aug 25, 2021 at 19:13
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    this question is way overlooked. 1 year in, indeed a "master question", and still no satisfying answer.
    – cregox
    Commented Jan 23, 2022 at 21:56
  • @oddthinking i mean i honestly still see zero evidence to believe in whatever mask analysis, be it in favour, against, or even somewhere in the middle (mostly because nobody seems to accept we are still somewhere in the middle). i don't think anyone ever argued that surgical masks are less effective than cloth, for instance. or that we can't find ways for populations to mask up. in any case, i could use one good analysis that answer this very specific question, and i think the fact we don't have any in here, in itself, is a great evidence that we may simply have no answer yet.
    – cregox
    Commented Jan 25, 2022 at 11:39

4 Answers 4


We finally have a study that examines surgical masks and their effectiveness under different viral-load situations and compares their effectiveness to other types of masks. The research provides the mechanisms for surgical mask protection against COVID-19 infection and incorporates opposing findings

The findings demonstrate that surgical masks are effective in most situations but not in high-load situations (such as treatment rooms) when respirator-style masks are likely better suited.

Cheng et al. (2021) note that

We find that most environments and contacts are under conditions of low virus abundance (virus-limited) where surgical masks are effective at preventing virus spread.

  • 4
    It helps a lot when infection probability is already low (a big slice from a small pie), and helps less and less as infection probability increases (a small slice from a big pie). A catch-22 almost. When you really need them is when they kind of stop helping.
    – user11643
    Commented May 29, 2021 at 0:22
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    Major problem of this report, it is not a real world study, but: "Here, we develop a quantitative model… ". As garbage in garbage out goes, it is a theoretical 'study' entirely dependent on assumptions and chosen parameters etc. Do they account for hand hygiene, procedure, leakage, prolonged & repeated wear? It disregards so much real-world effects, influences, confounders, that at the very gratuitous best, it maybe approximates an ideal world best case scenario, ie nothing to evaluate mandated masks as worn by the public in the most ridiculous situations. Commented May 29, 2021 at 10:43
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    @LangLаngС I do not want to get into a long debate with you but the empirical study you seem to require would be immoral and never get approved or be published. A model is as good as we can really get. Modeling I think is very suitable for something such as masks considering the ethical constraints of an observational study (which could lead to deaths). You jump from a model to it must be garbage. I think I'll trust the authors, the peer-reviewers, and the editors of Science over someone on the web. The authors do provide balance as well, as does my answer. It is not biased or one-sided. Commented May 29, 2021 at 14:48
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    @LangLаngС Empirical is not always better than modeled. Sometimes it is, sometimes it is not. General Relativity! It was purely theoretical and counterred centuries of observation. Also, early climate change models far outperforming earlier observational data. Again, I am not sure what you mean when saying statements such as, "I conclude from the garbage that's the model's input to its output", maybe it is an English as a second language thing. You can contest all you want (publish then) but you are a person on the internet and I call tell from your comments you have never run a clinical RCT. Commented May 30, 2021 at 0:18
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    Notably this study was not performed by some random assortment of interested researchers, but by globally-recognized experts in aerosols and epidemiology. It has been cited in other papers such as "Association of social distancing and face mask use with risk of COVID-19" nature.com/articles/s41467-021-24115-7 in Nature Communications which incorporated reported efficacy of mask-wearing in clinical settings. Commented Aug 25, 2021 at 19:32

First, surgical masks (especially doubled up) work fine for keeping you safe from aerosols. Cloth masks also work to some extent, but not quite as much. This has been shown in a number of studies, but I'll link only a single recent one here that I think is well-performed.


Filtration Efficiency of Masks

This is important to note, because it's a mechanism - observational or RCT studies which show an effect of something are great, but in the absence of a plausible mechanism are often showing some confounding effect. If you have mechanism + effect, you can be a lot more confident that you have a causal relationship then either individually.

There's now a working paper with a RCT for mask use in Bangladesh. This was cluster randomized - some villages were given free masks, instruction on how to wear them and why they're important, with the support of village elders. Others were in the non-intervention arm of the trial (obviously a study about masks could not be blinded). The authors estimate that their interventions increased mask-wearing by 29%. This is just about the best you could do for a RCT on something like mask-wearing (it would be most accurate to say that it's a trial of public policy associated with mask-wearing, but that's starting to split hairs a bit).

enter image description here

Table A17 is showing that the promotion of surgical masks resulted in a statistically significant reduction in symptomatic disease prevalence in all age groups, after adjusting for baseline controls. Cloth masks promotion resulted in a much weaker difference in outcomes between the two groups, so we cannot be sure if they're effective or not. I would at this point, based on the evidence, be confident in saying that there is a causal link between surgical mask wearing and a decrease in symptomatic COVID cases.

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    Comments are not for extended discussion; this conversation has been moved to chat.
    – Jamiec
    Commented Sep 6, 2021 at 18:01
  • @Jamiec You moved a 'discussion' to chat before it reached the level needed for the chat to not roomba-auto-delete—thereby removing very important on-topic criticism concerning this post. There are now merely 8 chat messages. If you had just pruned the fruitless insults made by OP and the self-comment at the start from this 'discussion', there even would have been two insulting comments and a chatty one less. That was unnecessary? Commented Sep 6, 2021 at 19:25
  • @JonathanReez You may be interested in this: today.umd.edu/…
    – CJR
    Commented Sep 13, 2021 at 19:10
  • some of the healthy participants will be asked to wear face shields and wash their hands every 15 minutes, while others will not be given those precautions - huh, they're not testing masks? This seems like a very weird choice. Commented Sep 13, 2021 at 19:21

It would appear to be difficult to test this with a single direct experiment, but we can approximations based on aerosol transmission models.

A recent paper, SARS-CoV-2 aerosol transmission in schools: the effectiveness of different interventions is the best attempt to do this I've seen so far. Here's an excerpt of their summary:

Methods: We adapted a previously developed aerosol transmission model to study the effect of interventions (natural ventilation, face masks, HEPA filtration,and their combinations) on the concentration of virus particles in a classroom of 160 m3 containing one infectious individual. The cumulative dose of viruses absorbed by exposed occupants was calculated.

They provide the following summary table for their simulations (I've narrowed it down to the 1 and 6 window simulations, full table is in the PDF):

Type of intervention Natural ventilation Season HEPA filters Surgical masks Mean Cumulative Dose
Baseline no NA no no 167
Natural ventilation 1 window partly open at all times Winter no no 102
1 window fully open at all times Winter no no 54
6 windows partly open at all times Winter no no 31
6 windows fully open at all times Winter no no 12
HEPA filters no NA 2.5 ACH no 68
no NA 5 ACH no 43
Face masks no NA no yes 21
Combined interventions no NA 2.5 ACH yes 9
no NA 5 ACH yes 6
2 windows partly open at all times Winter no yes 9
2 windows partly open at all times Winter 2.5 ACH yes 6
2 windows partly open at all times Winter 5 ACH yes 4

From this summary table its apparent that everyone wearing masks is as good as keeping every window open at all times. Wearing masks and keeping windows open is even better. Compared to the baseline scenario (no masks, no open windows), adding masks reduces the number of suspended viral particles by a factor of 8, showing that they can have a significant effect on the potential number of infections. That being said, obvious caveats apply:

  1. Surgical masks are not mandated in most countries, you can often wear a loose fitting cotton mask instead which reduces efficacy.
  2. People don't wear masks perfectly - taking them off to drink water, unlock their phone, scratch their nose, etc. Though this concern would apply even in the ideal experiment described in the first part of the answer.
  3. We don't have good data on how many COVID particles have to be inhaled in order to get infected. The authors of the paper agree:

The probability of on-site transmission, i.e.,the probability that one susceptible exposed person gets infected, based on the absorbed dose. At the time of writing, the dose-response relationship for persons exposed to aerosolized SARS-CoV-2 viruses is not known to the authors. A few studies with other coronaviruses suggest an exponential response,26meaning that a slight reduction in the inhaled dose would relate to an exponential reduction in the probability of contracting the disease, independently of the infective dose for SARS-COV-2. Preliminary experimental studies on SARS-CoV-2 suggest an infection dose between 10-1000 infectious virions. However, due to the high variability in infectious dose between SARS-CoV-2 variants, we only compared the relative effectiveness of different indoor preventive measures on the cumulative dose absorbed.

So the answer to the question based on the paper is as following:

  1. Under simulated conditions, surgical masks reduce viral loads emitted by an infected person by a factor of 8 compared to not wearing masks.
  2. We don't know by how much this reduces the likelihood of others in the room getting infected. We do know that surgical masks cannot guarantee 100% protection from the virus.
  3. We don't know how viral loads are reduced in real time conditions where masks are worn imperfectly.
  • 1
    It's not technically difficult to test this source control issue in re Covid, just unethical politifact.com/article/2021/mar/12/… ; medpagetoday.com/infectiousdisease/covid19/87870 Imagine what the control group has to have: someone confirmed with Covid but not wearing a mask being put in contact with other (unprotected) people. That's why there are no RCTs on this source-control angle basically. Commented Aug 27, 2021 at 13:50
  • OTOH some such trials have been approved in the UK now, so maybe they'll do one for masks, Commented Aug 27, 2021 at 14:00
  • I still see a potential problem because insofar they still give antivirals to the volunteers after infection is confirmed, in order to limit symptoms, which may be an issue if you want to test someone's ability to spread the virus... Commented Aug 27, 2021 at 14:15
  • @Fizz I think it should be perfectly safe to test now that we have effective vaccines. Get a bunch of healthy vaccinated people under the age of 30 and there’s basically a non existent chance of them suffering any significant consequences from the experiment. The vaccines would still affect the outcome but with enough participants you could get a statistically significant result. Commented Aug 27, 2021 at 14:36

Note/preamble: I'm not sure why I got more DVs for this answer, but do note that I tried to answer the Q as asked namely

When worn by members of the public who are infected, are surgical masks effective at slowing the spread of COVID-19 ("source control")?

Answering with population studies on mask mandates in general, would have been really easy and given a nice what-you-probably-want-to-hear-message that mask work, but this question is more specific than that, i.e. wants to isolate the effect of source control as opposed to a group wearing masks.

And the Q also wants

answerers and voters to weight evidence about the spread of COVID-19 in real-life conditions above evidence from lab experiments

Ask yourselves if the other answers have even tried to follow these two requirements before you cast more stones at mine.

As of now I'm not aware of any RCTs that have been conducted on the issue, with Covid-19 as the infectious agent. And there has been plenty of debate, even among experts, whether it's mostly larger droplets or mostly aerosols that matter for this particular agent, so if you want a high degree of certainty, it's going to be difficult to rely on inferences from other/older studies on different agents...

Some such RCTs were conducted with influenza (and "influenza-like illnesses"), before this pandemic; at least one in China and a smaller one in France. The results were underwhelming, showing a trend but not reaching statistical significance (the Chinese study) or basically no effect whatsoever (France); their design may have been overly ambitious though as they measured transmission from confirmed index cases [randomized to with and without mask "treatment" groups] to other household members, with whom there were substantial opportunities for prolonged and varied forms of (unmonitored) interaction. The household "targets" were not required to wear any form protection.

A good number of sources argued that conducting this kind of trail (with subjects) during the pandemic is unethical. On the other hand, some Covid-19 challenge trials have been now (i.e. 2021) approved and conducted in the UK, but insofar nothing that involved testing masks as source-control, as far as I now. (The staff did wear PPE, as you might expect, and one could bet they were vaccinated too.)

So, we'll still have to rely on "natural experiments" outside of the technical laboratory ones that have tested mask efficacy in some artificial/proxy/model setting. (The other answers mostly cover these kinds of experiments.)

Of course, you could say I'm cherry picking, which is inevitable for this kind of natural experiments, but I'm going to point out two CDC case studies, one in which the infected sources wore masks (and seemingly the "targets" too), and one in which the source mostly did not, but the targets were reported to have mostly adhered to masks.

Among 139 clients exposed to two symptomatic hair stylists with confirmed COVID-19 while both the stylists and the clients wore face masks, no symptomatic secondary cases were reported; among 67 clients tested for SARS-CoV-2, all test results were negative. [...]

On May 12, 2020 (day 0), a hair stylist at salon A in Springfield, Missouri (stylist A), developed respiratory symptoms and continued working with clients until day 8, when the stylist received a positive test result for SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19). A second hair stylist (stylist B), who had been exposed to stylist A, developed respiratory symptoms on May 15, 2020 (day 3), and worked with clients at salon A until day 8 before seeking testing for SARS-CoV-2, which returned a positive result on day 10.

A total of 139 clients were directly serviced by stylists A and B from the time they developed symptoms until they took leave from work. Stylists A and B and the 139 clients followed the City of Springfield ordinance and salon A policy recommending the use of face coverings (i.e., surgical masks, N95 respirators, or cloth face coverings) for both stylists and clients during their interactions. Other stylists at salon A who worked closely with stylists A and B were identified, quarantined, and monitored daily for 14 days after their last exposure to stylists A or B. None of these stylists reported COVID-19 symptoms. [...]

Stylist A worked with clients for 8 days while symptomatic, as did stylist B for 5 days. During all interactions with clients at salon A, stylist A wore a double-layered cotton face covering, and stylist B wore a double-layered cotton face covering or a surgical mask. [...]

The Greene County Health Department (Missouri) conducted contact tracing for all 139 exposed clients back to the dates that stylists A and B first developed symptoms. The 139 clients were monitored after their last exposure at salon A. Clients were asked to self-quarantine for 14 days and were called or sent daily text messages to inquire about any symptoms; none reported signs or symptoms of COVID-19. Testing was offered to all clients 5 days after exposure, or as soon as possible for those exposed >5 days before contact tracing began. Overall, 67 (48.2%) clients volunteered to be tested, and 72 (51.8%) refused; all 67 nasopharyngeal swab specimens tested negative for SARS-CoV-2 by PCR. Telephone interviews were attempted 1 month after initial contact tracings to collect supplementary information. Among the 139 exposed clients, the Greene County Health Department interviewed 104 (74.8%) persons. [...]

Among the 104 interviewed clients, 102 (98.1%) reported wearing face coverings for their entire appointment, and two (1.9%) reported wearing face coverings part of the time (Table 2). Types of face covering used by clients varied; 49 (47.1%) wore cloth face coverings, 48 (46.1%) wore surgical masks, five (4.8%) wore N95 respirators, and two (1.9%) did not know what kind of face covering they wore. Overall, 101 (97.1%) interviewed clients reported that their stylist wore a face covering for the entire appointment; three did not know. When asked about the type of face coverings worn by the stylists, 64 (61.5%) reported that their stylist wore a cloth face covering (39; 37.5%) or surgical mask (25; 24.0%); 40 (38.5%) clients did not know or remember the type of face covering worn by stylists. When asked whether they had experienced respiratory symptoms in the 90 days preceding their appointment, 87 (83.7%) clients reported that they had not. Of those who did report previous symptoms, none reported testing for or diagnosis of COVID-19.

Six close contacts of stylists A and B outside of salon A were identified: four of stylist A and two of stylist B. All four of stylist A’s contacts later developed symptoms and had positive PCR test results for SARS-CoV-2. These contacts were stylist A’s cohabitating husband and her daughter, son-in-law, and their roommate, all of whom lived together in another household. None of stylist B’s contacts became symptomatic.

The teacher reported becoming symptomatic on May 19, but continued to work for 2 days before receiving a test on May 21. On occasion during this time, the teacher read aloud unmasked to the class despite school requirements to mask while indoors. [...]

The index patient [=teacher] became symptomatic on May 19 with nasal congestion and fatigue. This teacher reported attending social events during May 13–16 but did not report any known COVID-19 exposures and attributed symptoms to allergies. The teacher continued working during May 17–21, subsequently experiencing cough, subjective fever, and headache. The school required teachers and students to mask while indoors; interviews with parents of infected students suggested that students’ adherence to masking and distancing guidelines in line with CDC recommendations was high in class. [...]

During May 23–26, among the teacher’s 24 students, 22 students, all ineligible for vaccination because of age, received testing for SARS-CoV-2; 12 received positive test results. The attack rate in the two rows seated closest to the teacher’s desk was 80% (eight of 10) and was 28% (four of 14) in the three back rows (Fisher’s exact test; p = 0.036). During May 24–June 1, six of 18 students in a separate grade at the school, all also too young for vaccination, received positive SARS-CoV-2 test results. Eight additional cases were also identified, all in parents and siblings of students in these two grades.

enter image description here

All classrooms had portable high-efficiency particulate air filters and doors and windows were left open. [...]

Specimens for WGS were collected during May 26–June 12; all 18 positive specimens with detectable virus (cycle threshold value <32) were sequenced using ClearDx instruments (Clear Laboratories), Oxford Nanopore MinION sequencing technology, and SARS-CoV-2 ARTIC V3 protocol for amplicon sequencing. Consensus genome assembly was performed in Terra using Titan Clear Laboratories workflow. All sequences generated were classified as the Delta variant. A phylogenetic tree was constructed using the UShER pipeline and visualized using Auspice.us [...]. Eleven sequences were genetically indistinguishable from one another; seven sequences contained additional single nucleotide variations. Among the indistinguishable specimens, six were from students of the index patient, four were from students in the separate grade, and one was from a sibling of a student in the index patient’s class, suggesting that infections occurring in the two grades likely were part of the same outbreak. [...]

[Unfortunately,] the teacher’s specimen was unavailable for WGS, which prevented phylogenetic identification of the outbreak’s index patient. [...]

There is the obvious confounding factor that in the first natural experiment the virus variant involved was not delta, but in the latter one it was. Also, the total interaction time of the stylists with any of their clients was probably smaller than that of the teacher with the students... and the clients didn't interact with each other, but the students almost certainly did to some degree. So, yeah, this is hardly a prefect comparison.

Even in the first natural experiment, if you want to take the close/family contacts as controls... for one of the stylists they all developed infections, while for the other one, none of such "controls" developed infection. (It's worth noting though that only five (4.8%) of the clients wore professional grade PPE, i.e. N95 respirators, so the natural experiment in this regard was fairly different from hospital staff interaction with known infected persons.)

The 2nd natural experiment, i.e. the classroom one, was partially "ruined" by the fact that the teachers' sample was not subjected to deep/whole genome sequencing, so there's only confirmation that the children had the exact same or one-mutation-away [sub-]strain. (The tests were not conducted by the same lab.) Also, there isn't much in the CDC report on how long the teacher was unmasked; no structured interviewing of the students was reported (unlike for the clients of the stylists).

So, yeah, this is basically the kind of field evidence that the CDC has to base their recommendations on, besides the lab sneeze simulator experiments etc. from the other answers. The CDC itself says (or at least said in May 2021):

Data regarding the “real-world” effectiveness of community masking are limited to observational and epidemiological studies.

The first study they cite there is the one the two hair stylists, by the way. There's generally little or no effort made in that text to separate studies between source-control and wearer-protection when it comes to “real-world” effectiveness, and this is because it's hard to do that with observational data. Practically all evidence cited regarding source control (one para) is from technical studies on masks:

Source Control to Block Exhaled Virus

Multi-layer cloth masks block release of exhaled respiratory particles into the environment,[citations] along with the microorganisms these particles carry.[citations] Cloth masks not only effectively block most large droplets (i.e., 20-30 microns and larger)[citations] but they can also block the exhalation of fine droplets and particles (also often referred to as aerosols) smaller than 10 microns;[citations] which increase in number with the volume of speech[citations] and specific types of phonation.[citations] Multi-layer cloth masks can both block up to 50-70% of these fine droplets and particles[citations] and limit the forward spread of those that are not captured.[citations] Upwards of 80% blockage has been achieved in human experiments that have measured blocking of all respiratory droplets,[citations] with cloth masks in some studies performing on par with surgical masks as barriers for source control.[citations]

They do cite something like 12 different papers on such technical/lab studies on masks; omitting the list here since the page is open-access. A typical paper cited e.g. for the last sentence is Kawaoka et al..

Here, we developed an airborne transmission simulator of infectious SARS-CoV-2-containing droplets/aerosols produced by human respiration and coughs and assessed the transmissibility of the infectious droplets/aerosols and the ability of various types of face masks to block the transmission. We found that cotton masks, surgical masks, and N95 masks all have a protective effect with respect to the transmission of infective droplets/aerosols of SARS-CoV-2 and that the protective efficiency was higher when masks were worn by a virus spreader.

The keyword there is "simulator" as far as “real-world” effectiveness is concerned. (The simulator was basically two mannequin heads [facing each other] enclosed in an airtight box, with some pumping gear to simulate breathing through the heads.) Also note the study's conclusion that masks had more effect when worn by the source; it's probably from studies like these that such conclusion made its way into the guidelines.

The Cheng et al. paper that has been laconically quoted in another answer actually has something to say (based on its theoretical model/simulation) about source-control too. This is most detailed in its supplementary material.

Figure 3 shows that source control is more effective than wearer protection in reducing Pinf,pop [probability of infection, at population level] via airborne transmission of SARS-CoV-2. Besides airborne transmission, other pathways have also been suggested for the transmission of respiratory viruses like SARS-CoV-2. For example, through direct or indirect contact between people and contaminated surfaces; or through respiratory droplets that are larger than 100 μm and would typically fall to the ground in seconds within 2 m of the source and not affect distant people (30, 49). Source control is very efficient in removing large droplets >100 μm (~100% efficiency) as illustrated in Figs. 4 and S5. Thus, source control can also reduce eye infections by droplets, which would not be prevented by wearer protection. Moreover, source control reduces the overall release of respiratory viruses, and thus their availability for contact transmission.

So the theory/simulation in this paper (which is largely a model/simulation of a large hall: 500m2with 200 people -- based on a Fangcang hospital) seems to agree with the physical simulation data from previous "micro" works that used only two "people". I'm mentioning it as being towards some "expert/model consensus" on source control being the more significant mechanism by which masks work.

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    I’ll note that a lot of sources were pushing back against “ethicists” and correctly claiming that risking the health of a few thousand volunteers is perfectly justified in a pandemic expected to kill 50 million people. Commented Aug 28, 2021 at 20:40
  • I'm curious what deserved a DV in my answer (I know it was probably not Johnathan who DV--just saying because he's probably going to be notified even if I don't @ him--something to do with posting under someone's comment..) Commented Aug 28, 2021 at 22:52
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    I did not downvote, but I think this answer could do with some editing, improved organization, shorter quotes, and clear conclusions. I'd also consider removing the long preamble with the frame challenge to the question.
    – LShaver
    Commented Sep 2, 2021 at 2:11

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