Masks Work, Our Comprehensive Review Has Found

Posted on by

Yves here. Lambert was very excited to see that a team led by the suffer-no-fools Trish Greenhalgh of Oxford has decisively shellacked the bizarre and destructive campaign against mask use, with the debunked (but only after it had been widely touted) Cochrane Report a prime example. Greenhalgh and her fellow scientists took the commonsensical step of excluding studies that did not study masking efficacy, like ones that had participants taking off masks while still indoors, or worse, treated various public initiatives to wear masks as if produced proper mask use. A key sentence from the post below: “Most RCTs of mask-wearing by the public were actually trials of advice to wear masks.”

I would have embedded the study below, which both KLG and Ignacio deemed to be magisterial, at the end of the post, but the file size is too big. You can find it here.

And please circulate this post widely!

By Trish Greenhalgh, Professor of Primary Care Health Sciences, University of Oxford; C Raina MacIntyre, Professor of Global Biosecurity, NHMRC L3 Research Fellow, Head, Biosecurity Program, Kirby Institute, UNSW Sydney; and David Fisman, Professor in the Division of Epidemiology, University of Toronto. Originally published at The Conversation

When a Texan farm worker caught bird flu from cattle recently, social media was abuzz with rumours. While bird flu is not a human pandemic, scientists and policymakers the world over are keen to prepare as best they can for when such a pandemic emerges – a tricky task, given that science is messy, policy must be pragmatic and people’s values don’t always align.

It’s time for masks to enter the chat. At the beginning of a pandemic caused by a novel or newly mutated virus, there may be no vaccine, no firm knowledge about how bad things will get and no specific treatment. Slowing transmission until more is known will be critical.

Getting most people to wear a mask could nip the outbreak in the bud, preventing a pandemic or lessening its impact. Wearing a mask is inconvenient, but not as inconvenient as lockdowns.

But do masks work? A review of masks and respirators, that looked only at clinical trials, concluded that there was not enough evidence to assess whether mask wearing reduces the risk of spreading or contracting respiratory diseases. However, we disagree with that.

The review, by the not-for-profit Cochrane Collaboration, failed to influence recent guidance issued by the US Centers for Disease Control and Prevention (CDC) in response to the troubling news of bird flu transmission to humans. The CDC recommended well-fitting respirators – along with overalls and safety goggles – for anyone working with potentially infected cattle until the bird flu threat subsides.

Is this latest guidance based on sound evidence? According to our new review of the evidence, yes. Like the Cochrane team, we pooled data from randomised controlled trials (RCTs) and analysed the combined data – a so-called meta-analysis.

Unlike them, we also examined non-RCT evidence, including dozens of laboratory studies which showed that respiratory infections, including the common cold, COVID, flu, measles and TB, spread mainly through the air.

Laboratory evidence showed that different mask materials are better or worse at filtering tiny particles, and more or less breathable – especially when damp. This explains why a cloth or paper mask that’s become soggy from the moisture in exhaled air gets harder to breathe through and may be less protective.

Whereas medical masks are typically tied loosely around the face (hence air can bypass the filter), respirators fit snugly and if worn at work must be fit-tested to make sure that all air inhaled or exhaled passes through a high-grade filter.

All this non-RCT evidence is crucially important for the design of RCTs. Because respiratory viruses float in the air, to be optimally effective a mask must be made of high-filtration material and must be fit closely. It should not be removed while indoors or the person will immediately be exposed to infectious particles in the air.

It follows that we should not expect RCTs of badly designed masks, masks that don’t fit or masks that are worn only some of the time, to show an effect. Neither should we expect mere advice to wear masks to have any effect unless it is followed.

Finally, when comparing respirators with masks in places where there is a high risk of infection, such as a hospital, the respirator needs to be worn continuously until the person leaves the building, not just popped on occasionally when doing so-called “aerosol-generating procedures” – such as intubating a patient.

If we take these crucial details of RCT design into account, rather than just comparing any masks-on, masks-off experiments, we find that masks are effective, and respirators even more effective, in reducing the spread of respiratory disease. We can also explain why some previous reviews appeared to show that this wasn’t the case.

Most RCTs of mask-wearing by the public were actually trials of advice to wear masks. In both RCTs and observational studies (such as real-world experiments), there was a dose-response effect: the more people wore their masks, the more effective the masks turned out to be. And when there’s a looming pandemic, people do tend to wear their masks.

The Bottom Line

When we looked at RCTs, we found that masks do protect in the community, and N95 respirators (masks made using higher-grade filtration material and designed to fit closely around the face to protect against airborne contaminants) are superior to masks in healthcare workers, especially when respirators were worn continuously at work. Non-RCT evidence also shows that masks work and respirators work better.

Let’s hope we’re not heading for another pandemic. But as we contemplate that possibility, the bottom line from our recent review is masks work. Along with improving indoor air quality and avoiding crowded, underventilated places, they provide the best way to avoid catching a respiratory infection. And our findings support previous advice to not just wear any mask but wear the best mask available.

Print Friendly, PDF & Email


    1. bailey's in SC

      How long will we wait for Media to hold Trump & Biden accountable for not MAKING N95 masks available to EVERYONE? Vote RFK, Jr., we have NO other choice!

      1. aj

        But isn’t RFK Jr a big covid denier? My guess is he would be as big an anti-masker as the others.

        1. bailey's in SC

          My guess is, you’re a troll. NC readers are too well-informed to make such a statement.

          1. Yves Smith Post author

            You are the one who is accumulating troll points by making a personal attack, which is a violation of our written site Policies, rather than marshaling any evidence and forcing me to waste my time doing so.

            aj’s guess is correct, RFK, Jr. is derangedly anti-mask:

            Newly emerged video shows Democratic presidential candidate Robert F. Kennedy Jr. comparing mask mandates during the COVID-19 pandemic to Nazi medical experimentation on Jews in concentration camps.


            1. Baiey's in SC

              I’m NOT arguing against N95-type masks, nor do I believe RFK, Jr. is. From the earliest days (thanks Lambert), N95-type masks made sense. I’m questioning why both Trump and Biden have gotten a media pass after they chose to protect private sector’s patent rights & profitability over OUR health and safety.

              BOTH Trump and Biden deliberately excluded the WORTHINESS of the masks they REQUIRED us to wear. And, they left us to compete with each other in the open market to purchase the few masks available that were best for us. Of course I’ll side with RFK, Jr. on this. If Gov’t. health directives aren’t science-based, they’re misdirection and political charade.

              As to the article, “MASKS” do not work! “N95-TYPE” masks work. But, very few of us were wearing them, even fewer correctly. So, while conflating the two certainly benefits our private sector and BOTH Political Parties, I believe it does a disservice to us. The blame isn’t ours. N95-type masks, with instructions, could easily have been made available and accessible to all.

              The irrefutable lessons I took from Covid are: First, BOTH Trump and Biden failed us miserably when we needed Gov’t. leadership and policy most. Second, WE urgently need to break OUR private sector’s strangle-hold on OUR Gov’t.

            2. aj

              Thanks Yves. I am not a troll. Biden and Trump both responded horribly to Covid. However, based on my understanding of RFK Jr’s attitudes toward covid and masking, I don’t think he would have handled the situation any better than the other 2. I did some googling and found the below video straight from the horses mouth. There is no discussion of N-95 vs baggy blues. There are no other qualifications besides lumping masks into one group. I won’t be voting for either of the two main parties this time around, but RFK is not the solution.


    2. Angie Neer

      But it does need to be supported by sound information about what makes a mask good. I think many people would assume baggy blue masks are good because they associate them with medical professionals.

  1. Terry Flynn

    My “reverse taxi driver experience” I mention in another thread today provides ample experience that there are people out there that understand this. I routinely wear an FFP2 mask. Not perfect but better than baggy blue.

    My minicab driver was positively angry at the fact society made his elderly parents very ill and his mum barely survive ICU during 1st phase COVID. I had been reticent (as usual – I’ve been physically threatened in the street for masking when going from one shop to the one next door). Then I opened up and he lapped it up – just stuff I knew as a medical statistician plus stuff I knew from this site. He totally “got” why the western “it’s all about me” approach was shite.

    However, the horse has bolted and I fear closing the stable door will do nothing to stop the consequences now. Meanwhile my local General Practice has almost completed installation of new weird UV lights, whole lotta extra filtration via lowering of the ceiling etc.

  2. John Beech

    N95 mask on whilst in LARGE waiting room yesterday. Been there maybe 10-minutes when an itch made me raise the mask to scratch. Inadvertently breathed and IMMEDIATELY caught the scent of perfume. Yes, N95 masks work!

    Anecdotal? Yes, but try this for yourself because I’ve noticed it more than once.

    That, and no matter what . . . hork with saline/PVI once home. I run 250ml through my nasal cavities without fail. Four years on and the entire family unit is NOVID.

    1. Lunker Walleye

      Had a final physical therapy session the other day and the therapist asked me why I was wearing a mask (respirator). I explained I always wear a mask in a public setting. She made a “Covid is over” type statement and mentioned that she wears a mask if she starts feeling unwell and is in a patient’s presence or if she thinks a patient might be ill. Since I will likely see her again in the future I simply stated that I wear a mask because I have read a lot about Covid.

  3. EarthMagic

    Personal experience, I’m a barber/stylist trimming beards and bangs every shift, and get very close to people’s faces. I use an N95 or a KN95 at work.
    I have never gotten covid, or the flu, or even a cold, at work in these 4+ years. Even when our whole crew caught covid in January 2021.
    The only time I got covid was during time off after contact with a positive family member. We kept air purifiers going, opened windows and wore masks outside of our rooms. Four people in the home and covid did not spread in the house. If people want to tell me masks don’t work, they might as well tell me socks don’t keep you warm. From experience, THEY WORK.

    Has anyone posted the old Outbursts of Everett True comics from the early 1900s about the mask controversy during the Spanish Flu?

  4. John Smith

    I think any comprehensive review here would find that masks work, COVID is still a pandemic, and virus mutation is dangerous, no matter what the evidence to the contrary, lol. Try reviewing the book Expired by Claire Craig, to broaden your perspective.

  5. Revelo

    I made this comment previously and the usual “little knowledge is a dangerous thing” crowd here blasted me with the typical “links please? ha ha!” comment but the forum had moved on before I posted links, so no one read my reply. Anyway, i guess people here incapable of formuling their own search. I used “effect of high co2 from masks” with bing (because i like beibg contrary), but Google search should give similar results. First link that popped up was
    I’ll quote title and part of abstract, since people here maybe unwilling to click a link that challenges their beliefs:

    Possible toxicity of chronic carbon dioxide exposure associated with face mask use, particularly in pregnant women, children and adolescents – A scoping review
    Fresh air has around 0.04% CO2, while wearing masks more than 5 min bears a possible chronic exposure to carbon dioxide of 1.41% to 3.2% of the inhaled air. Although the buildup is usually within the short-term exposure limits, long-term exceedances and consequences must be considered due to experimental data. US Navy toxicity experts set the exposure limits for submarines carrying a female crew to 0.8% CO2 based on animal studies which indicated an increased risk for stillbirths. Additionally, mammals who were chronically exposed to 0.3% CO2 the experimental data demonstrate a teratogenicity with irreversible neuron damage in the offspring, reduced spatial learning caused by brainstem neuron apoptosis and reduced circulating levels of the insulin-like growth factor-1. With significant impact on three readout parameters (morphological, functional, marker) this chronic 0.3% CO2 exposure has to be defined as being toxic. Additional data exists on the exposure of chronic 0.3% CO2 in adolescent mammals causing neuron destruction, which includes less activity, increased anxiety and impaired learning and memory. There is also data indicating testicular toxicity in adolescents at CO2 inhalation concentrations above 0.5%
    It’s pretty obvious that right fitting respirators protect against airborne diseases. Question is tradeoffs. If the respirator is filtration in input only and there is ekectro-mechanical assistance to push air through the filter, then probably no negative other than discomfort. But if filtration on both input and output with no electro-mechanical assistance, then CO2 buildup might easily offset benefits of filtration for mild (for most people) illnesses like ordinary flu or covid19.

    1. Revenant

      The excerpts are interesting but:

      1) the risk, if any, from these data is in gestating females and juveniles. Non-gestating adults (this is getting very LGBTQIA…) will lower their risk by wearing an N95 mask;

      2) it would quickly confirm this risk or not if you also sourced data on the measured CO2 concentration in respirators. 1.4-3.2% seems very high. This study found respirators only increased CO2 beyond 0.5% (5000 ppm) in limited subjects / conditions.

      3) I have not read the studies you cite but from the citations some of them are animal studies and the rest uncertain so the toxicity / pathology being observed may simply not be relevant to humans. Canaries asphyxiate quicker than humans, famously; house birds are also exquisitely sensitive to PTFE fumes from burning nonstick saucepans and so only human studies are acceptable here – especially given they are largely easy and ethical to perform..

      For me, the case against masks for capnotoxia is unproven. We would have observational data of widespread harm to medical staff, for one thing….

        1. Adam1

          That’s not surprising. Your auto-desire to physically breath is regulated by the body’s estimation of the % of O2 to CO2 in the lungs.

          When I learned to scuba dive years ago, they talk you through how hyperventilation works. Anyone can hyperventilate, but when you are a scuba diver you’ve 20-30 lbs of weight on your back and you often/usually jump into water. When you are jumping into the water you need to be aware of the force of the water pushing back on you. There’s nothing at the moment you can do, but that pushing can cause to exhale more than normal and inhale more than normal and now your lungs MIGHT have more O2 than expected – this is the physiological definition of hyper ventilation.

          There is NOTHING wrong with that state and your body is fine with it, but your conscious brain, if not prepared, will likely go into panic and you will artificially gasp for MORE air only increasing the O2 concentration in the lungs and a panicked person in the water is ripe for drowning. So in scuba diving training, at least I was, you’re trained to recognize hyperventilation symptoms because it is extremely likely you will be your only savior or as a diver you should not be risking other people’s lives to save you over this type of situation.

          There is a reason in the movies they give someone who is hyperventilating a paper bag… it’s so they can re-breath the CO2 they are exhaling and which will eventually bring about the bodies desired natural O2-CO2 balance. As a diver you can’t rely on someone handing you a paper back, you need to overcome the conscious brain’s panic that you’ve stopped physically breathing and realize that once your body’s normal O2-CO2 levels have returned you will start breathing again as normally expected.

          Long story short… your body knows what your O2-CO2 mix should be. If it’s not right, it will adjust and/or at a minimum you should be conscious of it.

          Yes, a slow onset loss of O2 can be easily ignored by people and lead to issues, but even a masked person removes it at some point in the day. If it is a real issue their body will likely signal at that moment (or a few breaths later) that OMG we got the O2 we’ve been so in desire for the last 8 hours.

    2. steppenwolf fetchit

      How mild is covid19 for most infectees? What is the threshhold between mild and not-mild-anymore post-infection coviditis( long term organ/tissue damage)? What is the threshhold between mild and not-mild-anymore long covid? Knowing that would help know what risk is being traded off for what risk.

    3. IEL

      Having worn N95s (3M Aura) all day at work for years, the one downside I noticed was feeling less mentally sharp with the mask on. That is a known side effect of elevated CO2 levels, and it had some effect on my efficacy at work. On the other hand, I have not yet gotten COVID, much less long COVID.

    4. Yves Smith Post author

      A mere careful reading shows this study to be crap, merely via the use of language designed to imply they have the goods when they don’t.

      “Possible chronic”. That = PURE SPECULATION. And the comparison was “chronically exposed” with no definition of what that mean.

      The simple and more useful check is blood ox levels, which can easily be done via inexpensive blood ox monitors. My mother had COPD and so we would watch her blood ox as a proxy for Covid. It’s an n of only 1, but I weight train hard with an N95 on and have never detected any impact on performance or perceived exertion levels. Ditto I have checked my blood ox with an N95 and see no reduction.

      BTW I was told but have been unable to confirm that airlines circulate air not as much as they could/should not just to save money but to create higher CO2 levels to get passengers to fall asleep. So I doubt any mask CO2 effect, to the extent some might experience it, is as pronounced as what happens on planes….and flight crew experience for hours.

    5. skippy

      Dog pile …

      What mask is the baseline here – ???? – duh …

      I have used masks since the military, forced marches in full mop and kit doing forced marches with full kit, decades at work, huge sample with work mates, know all the VOC science, et al, till this day with my work in removing coatings on old 130 yr old homes and …. no one has been injured by wearing a mask …. full stop …

      Now if you want to argue a fat obese population in some nations cant breathe with a mask on after consuming a life of eating corp rubbish and not being fit due to programing we might have a conversation.

  6. Societal Illusions

    There seems not to be any analysis done on the impacts of masks on the wearer due to reduced oxygen intake and increased re-breathing of what is exhaled (a bodily waste product). Our lungs are of significant import to our waste management.

    We also should be breathing through our noses which have inbuilt this natural mechanism to reduce injurious particulates.

    I choose no masks for this reason. I trust my body to work as designed and support its natural function.

    1. Yves Smith Post author

      Help me. We are not living in anything approximating a state of nature. Your lungs were not designed to cope with air pollution, FFS. Particles get deep in your lungs and do lasting damage. I have no idea how people get indoctrinated to believe this nonsense.

      Here in Thailand, where Bangkok has moderately to pretty bad air due to pollution from cars + burning season (the fields in the north are burned off in the spring, leading to nearly two months of really bad air) most people wear KN95s outdoors and even indoors then and you see >75% mask wearing outside generally. Less common where I am because on the coast so generally better air.

      Many many workers have to use P-100 masks when working with nasty chemicals for entire work days. If there were bad health effects from that we would have had all sort of class action litigation long ago. Given that living or working near high traffic roads has been found to have detrimental health effects, wearing masks also filters the PM2.5 particles that pose an active health hazard. From the California Air Resources Board:

      Why is CARB Concerned about PM10 and PM2.5?

      CARB is concerned about air-borne particles because of their effects on the health of Californians and the environment. Both PM2.5 and PM10 can be inhaled, with some depositing throughout the airways, though the locations of particle deposition in the lung depend on particle size. PM2.5 is more likely to travel into and deposit on the surface of the deeper parts of the lung, while PM10 is more likely to deposit on the surfaces of the larger airways of the upper region of the lung. Particles deposited on the lung surface can induce tissue damage, and lung inflammation.

      What Kinds of Harmful Effects Can Particulate Matter Cause?

      A number of adverse health impacts have been associated with exposure to both PM2.5 and PM10. For PM2.5, short-term exposures (up to 24-hours duration) have been associated with premature mortality, increased hospital admissions for heart or lung causes, acute and chronic bronchitis, asthma attacks, emergency room visits, respiratory symptoms, and restricted activity days. These adverse health effects have been reported primarily in infants, children, and older adults with preexisting heart or lung diseases. In addition, of all of the common air pollutants, PM2.5 is associated with the greatest proportion of adverse health effects related to air pollution, both in the United States and world-wide based on the World Health Organization’s Global Burden of Disease Project.

      Short-term exposures to PM10 have been associated primarily with worsening of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), leading to hospitalization and emergency department visits.

      Long-term (months to years) exposure to PM2.5 has been linked to premature death, particularly in people who have chronic heart or lung diseases, and reduced lung function growth in children. The effects of long-term exposure to PM10 are less clear, although several studies suggest a link between long-term PM10 exposure and respiratory mortality. The International Agency for Research on Cancer (IARC) published a review in 2015 that concluded that particulate matter in outdoor air pollution causes lung cancer.

      1. Arnold Ehret

        >Particles get deep in your lungs and do lasting damage.

        You can get rid of them through chelation and detoxification. The medical doctors you rely on for your health do not understand this.

        1. Yves Smith Post author

          Quackery is absolutely not on here. Chelation as a remedy has risks of its own. It’s net beneficial for heavy metal toxicity. It’s recommended by alternative medicine con artists for ailments where it has NO benefit like Alzheimers.

          Chelation has already been studied for inhalation of damaging small particles (the affected population is law enforcement officers at shooting ranges, where they inhale what is called rifle smoke). It’s been found to be ineffective:

          See more generally on the overhyping of chelation:

  7. steppenwolf fetchit

    One more reason for reality-based covid-cautious people to wear masks is to protect themselves from the ” but . . muh Freedumm!” mask-boycotters who are bucking for a Darwin Award or a Herman Cain Award or maybe both.

    I notice that after semi-broad mask adoption in my workplace environment ( a large Academic Midwestern Hospital), that whereas before covid I was getting several colds per year, some of them pretty bad, I have only gotten 2 or so colds that I can remember over the past several years. So I would anecdotally surmise that masking, including by me, has largely prevented me from getting my pre-covid-normal number of colds.

    ( By the way, I wonder whether another word might be coined for the pandemic, given the deliberate nature of the deliberate spread-facilitation of covid to as many people as possible by most of the relevant authorities . . . and that word would be ” plandemic”. As in ” planned pandemic”. One could quibble that the pandemic was not “planned” in the sense of being deliberately engineered, spread, etc. But the authorities very certainly glommed onto the accidental emergence of covid as an opportunity to make it spread on purpose and on purpose obstruct stopping it until it could no longer be stopped . . . as in Gebreyesus’s careful refusal to allow WHO to call it a “pandemic” until he could be assured that it really WAS a “pandemic” and could no longer be stopped. WHO Director Gebreyesus’s approach to covid
    reminds me of Science Officer Ashe’s approach to the Alien Xenomorph in that movie Alien.)

  8. GC54

    I always shave my beard down to a small goatee +mustache and smooth facial skin before N95 Aura masking for air travel including terminal phases (must raise for instantaneous TSA view :). No food or drink airborne. So far no COVID.

  9. GM

    If “masks don’t work”, i.e. serious masks worn properly don’t work, then that means that all the PPE regulations in various contexts are useless.

    It logically follows — if it doesn’t work for SARS-CoV-2 out in the wild, then it won’t work for e.g. working with viruses in the lab. Right?

    So then we can just abolish all BSL3/4 regulations and allow everyone to work with whatever they want with zero safety precautions and regulations. Right? It logically follows.

    So who is up for working with Ebola at BSL1?

    Yeah, I thought so too.

    That is the reduction-ad-absurdum here.

  10. etudiant

    The study indicates that well adjusted respirators and snugly fitted N95 masks are helpful.
    The vast majority of the people never used a respirator.
    At best, they loosely fitted a ‘Made in China’ masks of unknown quality. It is remarkable that even these very modest measures appear to have had some beneficial effect.

  11. Matthew

    I’m disappointed to see this here. For one, what’s unfortunate about academia is you can find pretty much any result you want – including RCTs, meta studies, etc. The biggest correlation is who funds the study or the professor’s grant. There are so many ways to do this, including subconscious ways, such as through inclusion criteria, choice of statistical methods, and so on.

    One interesting paper I saw recently was a survey of all the statistical valid methods to interpret certain data, and getting a result from that – it was a distribution with many positive and negative results.

    The bottom line is that saying one study proves anything is useless. That’s exactly what the “trust the science” was saying: shut up and believe the papers I show to you, but don’t believe the papers I say are bad. And dissing the Cochrane Review does show bias, because while they’re not perfect, they’re as close to neutral in science as you can get.

    I remember Ian Miller creating a ton of graphs at the population level in areas with mask mandates vs no mandates, including measuring percent of people using it regularly as they should. No noticeable difference between the two.

    But again, why is this posted here?

Comments are closed.