Short answer: yes.
Long answer: While particles of COVID19 are around 0.125 microns (often abbreviated to μm) in size, they are never just floating around in the air naked. Viruses like the coronavirus are often transmitted through respiratory droplets produced by coughing, sneezing, breathing, talking, etc... These droplets COVID19 attaches itself to range in size from 5 microns to 0.5 microns, meaning that that the COVID19 particles attached to these larger particles make the size of the particle even bigger. So again, COVID19 is never just floating around freely by itself, it is always attached to mucus protein, water droplets, or other aerosols and bodily fluids generated by breathing, talking, coughing, etc... These droplets can travel close to 6 meters in the event of a sneeze, 2 meters in the event of a cough, and 1 meter in normal breathing (a big reason why social distancing is a thing and why it is important to wear a mask even if you are asymptomatic).
Facemasks reduce aerosol exposure by a combination of the filtering action of the fabric and the seal between the mask and the face. A study published in Nature Medicine determined that "aerosol transmission is a potential mode of transmission for coronaviruses as well as influenza viruses and rhinoviruses." In the abstract, the authors state that "Surgical face masks significantly reduced detection of influenza virus RNA in respiratory droplets and coronavirus RNA in aerosols, with a trend toward reduced detection of coronavirus RNA in respiratory droplets. Our results indicate that surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals".
For some real world data, a study in Health Affairs compared the COVID-19 growth rate before and after mask mandates in 15 states and the District of Columbia. The study's abstract states that their research "suggest that requiring face mask use in public could help in mitigating the spread of COVID-19," and some of the models in the study "suggested that because of these mandates, 230,000–450,000 cases may have been averted." Essentially, the Health Affairs study found that mask mandates led to a slowdown in daily COVID-19 growth rate, which became more apparent over time. Specifically, after controlling for things like social distancing, the researchers found that masks reduced the cases of COVID19 by 2% every day. Ultimately, "The study provides direct evidence on the effectiveness of widespread community use of face masks from a natural experiment that evaluated the effects of state government mandates in the US for face mask use in public on COVID-19 spread. Fifteen states plus Washington, D.C., mandated face mask use between April 8 and May 15. Using an event study that examined daily changes in county-level COVID-19 growth rates, the study found that mandating public use of face masks was associated with a reduction in the COVID-19 daily growth rate." Another real world examples comes from a Virginia Commonwealth University Department of Ophthalmology study found on Research Gate which looked at coronavirus deaths across almost 200 countries and found that those with cultural norms or government policies favoring mask-wearing had lower death rate. The authors state that, "These results support the universal wearing of masks by the public to suppress the spread of the coronavirus. Given the low levels of coronavirus mortality seen in the Asian countries which adopted widespread public mask usage early in the outbreak, it seems highly unlikely that masks are harmful." The study also identifies that, "societal norms and government policies supporting maskwearing by the public were independently associated with lower per-capita mortality from COVID-19. The use of masks in public is an important and readily modifiable public health measure." Research published in the Infectious Disease Modelling journal developed two baseline mathematical models that utilized mortality data compiled by John Hopkins University, with two U.S. states - New York and Washington - and found "that broad adoption of even relatively ineffective face masks may meaningfully reduce community transmission of COVID-19 and decrease peak hospitalizations and deaths." The effect is small, but nontrivial in the number of lives saved. The research states that "Notably, masks are found to be useful with respect to both preventing illness in healthy persons and preventing asymptomatic transmission" and in their models they find that in New York near universal adoption (80% of the population wearing a mask) of even moderately (50%) effective masks could prevent anywhere from 17-45% of the projected deaths, decrease the peak daily death rate by 34–58%, and decrease the mortality rate by 2-9% and in Washington near universal adoption of moderately effective masks could reduce mortality by 24–65%. Considering these numbers, the research states "Thus, a range of outcomes are possible, but both the absolute and relative benefit to weak masks can be quite large; when the relative benefit is small, the absolute benefit in terms of lives is still highly nontrivial." Figure 4 below shows the results of the models in the study, as does Figure 5.
Finally, we can look at some studies which show that wearing a mask can result in a milder infection, in addition to providing some protection against infection in the first place. One such study published in the Clinical Infectious Diseases journal infected hamsters with COVID19 (yeah kinda animal abuse maybe) and placed them in two different closed system cages with healthy hamsters. One of these cages had a surgical mask partition between the COVID19 hamsters and the healthy hamsters. The surgical mask partition signifgantly reduced infection rate and the hamsters exhibited milder symptoms. The authors conclude "SARS-CoV-2 could be transmitted by respiratory droplets or airborne droplet nuclei which could be reduced by surgical mask partition in the hamster model. This is the first in vivo experimental evidence to support the possible benefit of surgical mask in prevention of COVID-19 transmission, especially when masks were worn by infected individuals," and identify that the results of their experiment "supported the use of community-wide masking to reduce the amount of virus shedding from the asymptomatically infected patients and to protect susceptible individuals." I also like this quote from the study which sums up my ideas in the section rather well: "Masking is a continuous form of protection to stop the spreading of saliva and respiratory droplets to or from others, and to or from the environment to the susceptible individuals by hands through subconscious touching of their nose, mouth, and eyes."
Types of masks
Masks such as the N95 respirator masks are designed to remove more than 95% of all particles that are at least 0.3 microns (µm) in diameter and measurements of the particle filtration efficiency of N95 masks show that they are capable of filtering ≈99.8% of particles with a diameter of ≈0.1 μm. N95 is the United States standard whereas a FFP2 mask is the Eurpoean standard, which works basically just as well. Surgical masks are also significantly effective, as the results of studies "indicate that a surgical mask will reduce the exposure to aerosolized infectious influenza virus." One study has results which prove surgical masks provide roughly twice as much protection as homemade masks. The study states "Surgical masks provided about twice as much protection as home made masks." Essentially, N95/FFP2 masks work the best, then surgical masks, then homemade masks. Another study shows that a surgical mask (that filtered 89% of viral particles) was about three times better in controlling the viral transmission than that of a homemade mask. While homemade masks may not be as effective as the N95 or surgical masks, they still offer some protection, prevent spread, and are better than nothing.
A study from the Extreme Mechanics Letters journal (the full text of which can be viewed here) which looked at how efficient masks made out of different fabrics were at preventing spread of COVID19 found that "most fabrics have substantial blocking efficiency (median values >70%)," and "two layers of highly permeable fabric, such as T-shirt cloth, blocks droplets with an efficiency (>94%) similar to that of medical masks, while being approximately twice as breathable." The median value for blocking efficiency was 70%, meaning that your average fabric blocks 70% of particles from spraying all over the place. Fabrics with two layers have it at 94.1%, and those with 3 layers have it the highest at > 98.1%. Masks with two layers were also very breathable, so lack of oxygen should not be an issue. Overall, the study concludes that in the event of a mask shortage during a pandemic home made masks made of common fabrics with 2 layers "may help prevent the dissemination of droplets by infected individuals, and protect healthy individuals from inhaling droplets, with efficiencies similar to that of commercial medical masks." The results of the study can be viewed below in Figure 6.
Note that the use of masks are 2-fold: control the penetration of droplets from an infectious person into the respiratory tract of a susceptible host, or to control the droplets going out from an infected patient. Masks such as the N95, FFP2, and some surgical masks prevent the former and "homemade" masks, most fabric masks, and some surgical masks prevent the latter. Homemade masks, fabric masks, and most surgical masks are more used to prevent asymptomatic carriers from spreading the virus, meaning the mask will prevent potentially infected particles from traveling as far as they would if the infected person had no mask. A video study from the Chest Clinic which borrows data from a UNSW study provides a helpful visual aid (Figure 1 below). The video can be found here. So N95 masks prevent infection by filtering out droplets and particles that COVID19 could have attached itself to while homemade masks prevent spread and widespread dispersion of droplets and particles (Many anti-mask idiots will argue that cloth/homemade masks don't prevent a wearer from infection and see this as a sign that all masks don't work but this is stupid. Obviously a mask which isn't designed to prevent the wearer from infection, rather to prevent the wearer from spreading their own droplets and particles, would not prevent the wearer from infection). A quote from a large review of different studies which look at the effectiveness of masks against COVID19, aptly titled "Face Masks Against COVID-19: An Evidence Review" describes this quite well: "Our review of the literature offers evidence in favor of widespread mask use to reduce community transmission: nonmedical masks use materials that obstruct droplets of the necessary size; people are most infectious in the initial period post-infection, where it is common to have few or no symptoms; non-medical masks have been effective in reducing transmission of influenza; non-medical masks have been shown to be effective in small trials at blocking transmission of coronavirus; and places and time periods where mask usage is required or widespread have shown substantially lower community transmission."
Masks also serve as a visual reminder for social distancing measures and all should wear them because should a mask be reserved solely for use in symptomatic patients, they become a symbol of illness and could lead to public stigmatization that discourages use, as has been described for patients with tuberculosis.
Addressing common stupid talking points and other thoughts on anti-mask folks
Most arguments against masks or other measures to minimize the impact of COVID-19 essentially boil down to "how come the health measures I didn't follow didn't work?"
Some idiots will bring up some anecdotal stuff like "well I never wore a mask and I never got COVID-19" as evidence that masks don't work. There are several things wrong with this statement. Firstly, the person could have gotten COVID19 and just displayed no symptoms, making them an asymptomatic carrier. Secondly masks generally don't prevent infection, rather they prevent spread. The reason this person did not get infected with COVID19 was likely due to all the other people they encountered who wore masks, thus "protecting" this no mask idiot so this person not getting COVID19 is actually proof masks work. You can think of this by replacing masks with hairnets and COVID19 with hair. Hairnets reduce the likelihood that a cook's hair from getting in food like masks reduce the risk droplets and particles spread. A cook does not wear a hairnet to prevent other people's hair from getting in their food and a person does not typically wear a mask to prevent other people's droplets and particles from getting in them. Imagine a big kitchen with hundreds of cooks, all of whom have long hair. If none of them wear hairnets there is a strong chance of hair ending up in food. If some of them wear hairnets it's less likely hair ends up in food. And if they are all wearing hairnets then there is a very slim chance of hair ending up in food. You'd rather eat food from a kitchen full of long haired chefs all wearing hairnets rather than a kitchen with a few long haired chefs without hairnets right? Same with masks. If everyone was maskless there would be a high chance of a large number of people being infected with COVID, if some wore masks that chance shrinks, and if all wear masks then that chance shrinks even more. So hairnets, like masks, are more effective the more people wear them. Both are non selfish (meaning you do it not to prevent something from happening to yourself but from happening to others) risk mitigation measures.
Saying "well I never wore a mask and I never got COVID19" as evidence that masks don't work would be like saying "I never cover my mouth when I sneeze or cough and I've never gotten the flu, therefore covering your mouth when sneezing or coughing does not prevent you from getting the flu." Duh. It's not supposed to protect you and only you in the first place. It's a measure meant to protect others.
Much of the anti mask movement boils down to people being selfish assholes who resent the fact the government is treating other peoples lives as equally valuable to theirs. in protesting against masks and COVID measures they’re basically saying “noooooo government give me special treatment because I think I’m more healthy than others and they deserve to die because I assume they’re less healthy than me.”
Making fun of people for wearing masks to stay healthy and calling them sheep is like making fun of people for wearing seatbelts to avoid getting in a car accident and calling them sheep - it’s a straw man which misrepresents the intent behind the action and refers to an imaginary person rather than anything based in reality.
Doctors and masks
The reason behind masking during a pandemic is pretty much the same reason why doctors and surgeons wear a mask during surgery: to "prevent large droplets of bodily fluids that may contain viruses or other germs from escaping via the nose and mouth," as per Healthline. Other websites like The Apprentice Doctor reiterate this idea and research from the Journal of the Royal Society of Medicine states "Facemasks have long been thought to confer protection to the patient from wound infection and contamination from the operating surgeon and other members of the surgical staff." For regular folks in a pandemic masking serves to protect others one comes in contact with from potentially infected particles (masks mean less particles expelled and those particles don't travel very far) and for doctors this is to prevent germs and other particles from the surgeon getting inside the open wound of a patient that is being operated on, potentially causing infection. Basically, doctors and surgeons wear masks to prevent pathogens from coughing, sneezing, or talking from getting into the sterile area of surgery so there is no transfer of any pathogens from doctor/surgeon to patient. One study from the Journal of Orthopaedic Translation even found that surgical masks collect more bacteria from the doctor/surgeon wearing them over time which goes to show how masks are effective in reducing particle output. Sounds familiar, right? Even the dumbass Jim Meehan, a physician according to his website and an ophthalmologist according to Health US News and The Frontier (very much not an epidemiologist), recognizes this and states "surgeons do wear masks to prevent their respiratory droplets from contaminating the surgical field and the exposed internal tissues of our surgical patients" on his website.
Various systematic reviews
Various systematic reviews of research and studies similar to the ones above all reach the same conclusions: mask wearing is largely an effort to to reduce virus transmission and spread (rather than prevent or protect the wearer from infection themselves) and has a small, yet nontrivial effect in reducing transmission rates. Coupled with hand washing, it is effective in saving lives and maintaining health.
The use of masks and respirators to prevent transmission of influenza: a systematic review of the scientific evidence
Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis
"Face mask use could result in a large reduction in risk of infection (n=2647; aOR 0·15, 95% CI 0·07 to 0·34, RD -14·3%, -15·9 to -10·7; low certainty), with stronger associations with N95 or similar respirators compared with disposable surgical masks or similar (eg, reusable 12-16-layer cotton masks; pinteraction=0·090; posterior probability >95%, low certainty)."
A rapid review of the use of face mask in preventing the spread of COVID-19
"Face masks use have shown a great potential for preventing respiratory virus transmission including COVID-19."
"Regardless of the type, setting, or who wears the face mask, it serves primarily a dual preventive purpose; protecting oneself from getting viral infection and protecting others. Therefore, if everyone wears a face mask in public, it offers a double barrier against COVID-19 transmission."
A rapid systematic review of the efficacy of face masks and respirators against coronaviruses and other respiratory transmissible viruses for the community, healthcare workers and sick patients
"In the community, masks appeared to be effective with and without hand hygiene, and both together are more protective."
"The study suggests that community mask use by well people could be beneficial, particularly for COVID-19, where transmission may be pre-symptomatic."
"In summary, there is a growing body of evidence supporting all three indications for respiratory protection – community, healthcare workers and sick patients (source control)."
" Universal face mask use is likely to have the most impact on epidemic growth in the community, given the high risk of asymptomatic and pre-symptomatic transmission."
A technical review of face mask wearing in preventing respiratory COVID-19 transmission
"Face mask wearing can block or filter airborne virus-carrying particles through the working of colloid and interface science. This paper assesses current knowledge behind the design and functioning of face masks by reviewing the selection of materials, mask specifications, relevant laboratory tests, and respiratory virus transmission trials, with an overview of future development of reusable masks for the general public. This review highlights the effectiveness of face mask wearing in the prevention of COVID-19 infection."
An evidence review of face masks against COVID-19
"A primary route of transmission of COVID-19 is via respiratory particles, and it is known to be transmissible from presymptomatic, paucisymptomatic, and asymptomatic individuals. Reducing disease spread requires two things: limiting contacts of infected individuals via physical distancing and other measures and reducing the transmission probability per contact. The preponderance of evidence indicates that mask wearing reduces transmissibility per contact by reducing transmission of infected respiratory particles in both laboratory and clinical contexts. Public mask wearing is most effective at reducing spread of the virus when compliance is high."
"Because many respiratory particles become smaller due to evaporation, we recommend increasing focus on a previously overlooked aspect of mask usage: mask wearing by infectious people ("source control") with benefits at the population level, rather than only mask wearing by susceptible people, such as health care workers, with focus on individual outcomes. We recommend that public officials and governments strongly encourage the use of widespread face masks in public, including the use of appropriate regulation."
Community use of face masks and similar barriers to prevent respiratory illness such as COVID-19: a rapid scoping review
"Our best estimate is that the effect of wearing a face mask is between the effects seen in RCTs and the effects seen in cohort studies, or around 6 to 15% reduction in disease transmission."
"Population level studies that consider COVID-19 spread before and after mask-wearing policies (and combinations of other control measures) were introduced in various localities have more often than not concluded that mask-wearing mandates or recommendations seemed to accelerate epidemic decline in early 2020."
