A few of the scientific studies & publications that provide a basis for use of masks by date of publication:
September 2021: A study from the Los Angeles County Dept. of Health examined Covid transmission rate during the peak, September 2020 through March 2021. Found that case rates in children & adolescents were 3.4 times lower during the winter peak compared with rates in the community. Their findings suggest that implementing recommended prevention measures – a multipronged prevention strategy, including masking, physical distancing, testing, vaccination of children & adolescents aged ≥12 years – remains critical to reducing transmission in schools. This is promising for children aged <12 years because no COVID-19 vaccine is currently authorized for this age group. These data were published in MMWR Sept 3, 2021. http://dx.doi.org/10.15585/mmwr.mm7035e3
August 2021: A study published last week in Science argues that “unequivocal evidence indicates that airborne transmission is a major pathway for the spread of SARS-CoV-2.” https://www.science.org/doi/10.1126/science.abd9149
July 2021: A study from the Univ. of Waterloo published in the Physics of Fluid found that N95 masks filter exhaled air much better than cloth & surgical masks. Cloth & surgical masks caught 10 to 12 percent of aerosols breathed out during the experiment. Various N95, KN95 & R95 masks filtered over 50 percent.
This study found that increased ventilation/air-cleaning capacity significantly reduced the transmission risk in an indoor environment. Ventilation surpassed the apparent mask filtration efficacy. The researchers concluded that 1) that fresh air exchange or air purification can be very effective in controlling aerosol buildup indoors, & 2) the most beneficial strategy would be to employ both masks & ventilation to minimize the risk of virus transmission. https://uwaterloo.ca/news/media/study-supports-widespread-use-better-masks-curb-covid-19
February 2021: Study published in JAMA examined 11 reports that provided persuasive evidence that universal mandatory mask wearing policies have been associated with reductions in the number or rate of infections & deaths My personal favorite is the one from Kansas in which some counties had mask mandates, & others did not. Those with mask mandates had decreased case rate per 100,000 (by 0.08). Those without mask mandates had increased case rates by 0.11. https://jamanetwork.com/journals/jama/fullarticle/2776536
January 2021 Proceedings of the National Academy of Sciences published “An evidence review of face masks against COVID-19” PNAS January 26, 2021 118 (4) e2014564118; https://doi.org/10.1073/pnas.2014564118
A huge group of researchers conducted an extensive review of the literature (147 medical papers) & found evidence in favor of widespread mask use as source control to reduce community transmission: Nonmedical masks use materials that obstruct particles of the necessary size; people are most infectious in the initial period post-infection, where it is common to have few or no symptoms; nonmedical masks have been effective in reducing transmission of respiratory viruses; & places & time periods where mask usage is required or widespread have shown substantially lower community transmission.
They recommend the adoption of public cloth mask wearing, as an effective form of source control, in conjunction with existing hygiene, distancing, & contact tracing strategies. The available evidence suggests that near-universal adoption of nonmedical masks when out in public, in combination with complementary public health measures, could successfully reduce Re to below 1. This would reduce community spread if such measures are sustained. Economic analysis suggests that mask wearing mandates could add 1 trillion dollars to the US GDP.
Models suggest that public mask wearing is most effective at reducing spread of the virus when compliance is high. The authors recommend that mask use requirements be implemented by governments. When governments do not then by organizations that provide public-facing services. Such mandates must be accompanied by measures to ensure access to masks, possibly including distribution & rationing mechanisms so that they do not become discriminatory. Given the value of the source control principle, it is not sufficient for only employees to wear masks. Customers must wear masks as well. https://www.pnas.org/content/118/4/e2014564118