Besides it being obviously common sense, there's tons of evidence that mask wearing reduces infections and deaths by C-19. For example:
"Face Masks Against COVID-19: An Evidence Review...
This manuscript was compiled on April 10, 2020...
The science around the use of masks by the general public to impede COVID-19 transmission is advancing rapidly...
The preponderance of evidence indicates that mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets in both
laboratory and clinical contexts...
3. Filtering Capability of Masks
...Multiple studies show the filtration effects of cloth masks
relative to surgical masks. Particle sizes for speech are on the
order of 1 µm (20) while typical definitions of droplet size are
5 µm-10 µm (5). Generally available household materials had
between a 49% and 86% filtration rate for 0.02 µm exhaled particles whereas surgical masks filtered 89% of those particles
(21). In a laboratory setting, household materials had 3% to
60% filtration rate for particles in the relevant size range, finding them comparable to some surgical masks (22). In another
laboratory setup, a tea cloth mask was found to filter 60% of
particles between 0.02 µm to 1 µm, where surgical masks filtered 75% (23). Dato et al (2006) (24), note that "quality commercial masks are not always accessible." They designed and
tested a mask made from heavyweight T-shirts, finding that it
"offered substantial protection from the challenge aerosol and
showed good fit with minimal leakage".Although cloth and
surgical masks are primarily targeted towards droplet particles, some evidence suggests they may have a partial effect in
reducing viral aerosol shedding (25).
When considering the relevance of these studies of ingress,
it’s important to note that they are likely to substantially underestimate effectiveness of masks for source control. When
someone is breathing, speaking, or coughing, only a tiny
amount of what is coming out of their mouths is already in
aerosol form. Nearly all of what is being emitted is droplets.
Many of these droplets will then evaporate and turn into
aerosolized particles that are 3 to 5-fold smaller. The point
of wearing a mask as source control is largely to stop this process from occurring, since big droplets dehydrate to smaller
aerosol particles that can float for longer in air (26).
Anfinrud et al (6) used laser light-scattering to sensitively
detect droplet emission while speaking. Their analysis showed
that virtually no droplets were "expelled" with a homemade
mask consisting of a washcloth attached with two rubber
bands around the head, while significant levels were expelled
without a mask. The authors stated that "wearing any kind
of cloth mouth cover in public by every person, as well as
strict adherence to distancing and handwashing, could significantly decrease the transmission rate and thereby contain the
pandemic until a vaccine becomes available."
An important focus of analysis for public mask wearing
is droplet source control. This refers to the effectiveness of
blocking droplets from an infectious person, particularly during speech, when droplets are expelled at a lower pressure and
are not small enough to squeeze through the weave of a cotton
mask. Many recommended cloth mask designs also include a
layer of paper towel or coffee filter, which could increase filter
effectiveness for PPE, but does not appear to be necessary for
blocking droplet emission (6, 27, 28).
In summary, there is laboratory-based evidence that household masks have some filtration capacity in the relevant
droplet size range, as well some efficacy in blocking droplets
and particles from the wearer (26). That is, these masks help
people keep their droplets to themselves.
4. Mask Efficacy Studies
Although no randomized controlled trials (RCT) on the use
of masks as source control for SARS-CoV-2 has been published, a number of studies have attempted to indirectly estimate the efficacy of masks. Overall, an evidence review (29)
finds "moderate certainty evidence shows that the use of handwashing plus masks probably reduces the spread of respiratory
viruses."
The most relevant paper (30), with important implications
for public mask wearing during the COVID-19 outbreak, is
one that compares the efficacy of surgical masks for source control for seasonal coronavirus, influenza, and rhinovirus. With
ten participants, the masks were effective at blocking coronavirus droplets of all sizes for every subject. However, masks
were far less effective at blocking rhinovirus droplets of any
size, or of blocking small influenza droplets. The results suggest that masks may have a significant role in source control
for the current coronavirus outbreak. The study did not use
COVID-19 patients, and it is not yet known whether seasonal
coronavirus behaves the same as SARS-CoV-2; however, they
are of the same genus, so similar behavior is likely.
Another relevant (but under-powered, with n=4) study
(31) found that a cotton mask blocked 96% (reported as 1.5
log units or about a 36-fold decrease) of viral load on average,
at eight inches away from a cough from a patient infected with
COVID-19. If this is replicated in larger studies it would be
an important result, because it has been shown (32) that "every 10-fold increase in viral load results in 26% more patient
deaths" from "acute infections caused by highly pathogenic
viruses".
A comparison of homemade and surgical masks for bacterial and viral aerosols (21) observed that "the median-fit
factor of the homemade masks was one-half that of the surgical masks. Both masks significantly reduced the number of
microorganisms expelled by volunteers, although the surgical
mask was 3 times more effective in blocking transmission than
the homemade mask." Research focused on aerosol exposure
has found all types of masks are at least somewhat effective
at protecting the wearer. Van der Sande et al (33) found that
"all types of masks reduced aerosol exposure, relatively stable
over time, unaffected by duration of wear or type of activity",
and concluded that "any type of general mask use is likely
to decrease viral exposure and infection risk on a population
level, despite imperfect fit and imperfect adherence". Overall
however, analysis of particle filtration is likely to underestimate the effectiveness of masks, since the fraction of particles
that are emitted as aerosol (vs. droplet) is quite small (26).
Analysis of seasonal coronavirus compared to rhinovirus (30)
suggests that filtration of COVID-19 may be much more effective, especially for source control.
The importance of using masks for health care workers
has been observed (34) in three Chinese hospitals where, in
each hospital, medical staff wearing masks (mainly in quarantine areas) had no COVID-19 infections, despite being around
COVID-19 patients far more often, whilst other medical staff
had 10 or more infections in each of the three hospitals.
Masks seem to be effective for source control in the controlled setting of an airplane. One case report (35) describes
a man who flew from China to Toronto and then tested positive for COVID-19. He was wearing a mask during the flight.
The 25 people closest to him on plane/flight attendants were
tested and all were negative. Nobody has been reported from
that flight as getting COVID-19. Another case study involving a masked influenza patient on an airplane (36) found that
"wearing a face mask was associated with a decreased risk for
influenza acquisition during this long-duration flight".
Guideline development for health worker personal protective equipment have focused on whether surgical masks or
N95 respirators should be recommended. Most of the research in this area focuses on influenza. At this point, it
is not known to what extent findings from influenza studies
apply to COVID-19 filtration. Wilkes et al (37) found that
"filtration performance of pleated hydrophobic membrane filters was demonstrated to be markedly greater than that of
electrostatic filters." However, even substantial differences in
materials and construction do not seem to impact the transmission of droplet-borne viruses in practice, such as a metaanalysis of N95 respirators compared to surgical masks (38)
that found "the use of N95 respirators compared with surgical masks is not associated with a lower risk of laboratoryconfirmed influenza." Johnson et al (39) showed that "surgical
and N95 masks were equally effective in preventing the spread
of PCR-detectable influenza". Radonovich et al (40) found in
an outpatient setting that "use of N95 respirators, compared
with medical masks... resulted in no significant difference in
the rates of laboratory-confirmed influenza."
One of the most frequently mentioned papers evaluating
the benefits and harms of cloth masks have been by MacIntyre
et al (41). Findings have been misinterpreted, and therefore
justify detailed discussion here. The authors "caution against
the use of cloth masks" for healthcare professionals compared
to the use of surgical masks and regular procedures, based on
an analysis of transmission in hospitals in Hanoi. We emphasize the setting of the study - health workers using masks to
protect themselves against infection. The study compared a
"surgical mask" group which received 2 new masks per day, to
a "cloth mask" group that received 5 masks for the entire 4
week period and were required to wear the masks all day, to
a "control group" which used masks in compliance with existing hospital protocols, which the authors describe as a "very
high level of mask use". It is important to note that the authors did not have a "no mask" control group because it was
deemed "unethical to ask participants to not wear a mask."
The study does not inform policy pertaining to public mask
wearing as compared to the absence of masks in a community
setting, since there is not a "no mask" group. The results of
the study show that the group with a regular supply of new
surgical masks each day had significantly lower infection of
rhinovirus than the group that wore a limited supply of cloth
masks. This paper lends support to the use of clean, surgical
masks by medical staff in hospital settings to avoid rhinovirus
infection by the wearer, and is consistent with other studies
that show cloth masks provide poor filtration for rhinovirus
(30). Its implementation does not inform the effect of using
cloth masks versus not using masks in a community setting for
source control of SARS-CoV-2, which is of the same genus as
seasonal coronavirus, which has been found to be effectively
filtered by cloth masks in a source control setting (30).
A. Studies of Impact on Community Transmission.
When
evaluating the available evidence for the impact of masks on
community transmission, it is critical to clarify the setting of
the research study (health care facility or community), the res
piratory illness being evaluated and what reference standard
was used (no mask or surgical mask). There are no RCTs
that have been done to evaluate the impact of masks on community transmission during a coronavirus pandemic. While
there is some evidence from influenza outbreaks, the current
global pandemic poses a unique challenge. A review (42) of
67 studies including randomized controlled trials and observational studies found that simple and lowcost interventions
would be useful for reducing transmission of epidemic respiratory viruses. The review recommended that "the following
effective interventions should be implemented, preferably in a
combined fashion, to reduce transmission of viral respiratory
disease: 1. frequent handwashing with or without adjunct
antiseptics; 2. barrier measures such as gloves, gowns, and
masks with filtration apparatus; and 3. suspicion diagnosis
with the isolation of likely cases". However, it cautioned that
routine longterm implementation of some measures assessed
might be difficult without the threat of an epidemic.
http://files.fast.ai/papers/masks_lit_review.pdf
