​Below is a sampling of review articles and original research regarding the science of wearing masks to reduce spread of COVID-19. This list is not exhaustive and we welcome suggestions for more research to include here.  

Please note that original research is separated into studies that discuss particle ingress (viral particles entering the mouth and nose of a healthy person) and particle egress (viral particles leaving the mouth and nose of an infectious patient), as masks seem to be more effective at preventing egress than ingress.

Please also note that the role of aerosols in COVID-19 transmission is not yet clear.

Recent Reviews

  • Researchers conducted a systematic review of 172 observational studies that evaluated at least one of three measures to prevent transmission of the viruses that cause COVID-19 and related diseases (e.g., SARS and MERS): physical distancing, face masks, and eye protection. Findings support physical distance of 1 meter or more (with longer physical distancing related to decreased transmission), face mask use, and eye protection as three effective measures to reduce transmission of viruses.  Read more here >

  • The authors review recommendations from the Infectious Diseases Society of America (IDSA) to contain and mitigate the spread of Covid-19 while gradually reopening society. They review evidence that supports the use of medical masks, cloth masks, and plastic face shields, recommending that these be used, particularly focusing on the practical advantages of adopting plastic face shields. Read more here >

  • ​The author, a primary care physician and researcher at Oxford, addresses some arguments against wearing masks in public to prevent the spread of Covid-19. She applies the “precautionary principle” and sets out “some key findings from basic science, epidemiology, mathematical modelling, case studies and natural experiments” to rebut arguments against wearing masks. Read more here >

particle egress - do masks protect those who are near the wearer?

  • A case report published in MMWR details the transmission patterns of hairstylists who wore masks. Among 139 clients exposed to two symptomatic hair stylists with confirmed COVID-19, no secondary cases were reported. Both the stylists and the clients wore face masks, indicating that face coverings could mitigate the spread of infection. Read more here > 

  • The authors examine how transmission rates changed in a large healthcare system due to universal masking implementation. They found that the rate of growth in new infections changed from an exponential increase to a decrease of 0.49% daily during the intervention period (at a time when new infections were increasing in the general Massachusetts population), indicating that universal masking could help reduce infection spread. Read more here > 

  • ​The authors use an animal model (golden Syrian hamsters infected and uninfected with SARS-CoV-2) to examine the effect of a surgical mask partition in reducing transmission. They found that with no surgical mask partition, 66.7% of non-infected hamsters became infected. In contrast, surgical mask partition for challenged index hamsters significantly reduced transmission to only 16.7% of exposed naive hamsters. Read more here >

  • ​This article presents a 2009 study that compares the efficacy of surgical versus N95 masks in preventing the spread of acute influenza. Infected volunteers coughed onto petri dishes with surgical masks, with N95 masks, and without any masks. Wearing surgical and N95 masks fully prevented growth of influenza on petri dishes, while influenza was detectable on most petri dishes when coughed on without masks, demonstrating that both surgical and N95 masks were equally and fully effective in filtering viral particles and limiting viral dissemination. Read more here >

  • ​The authors describe a recent controlled experiment to determine the efficacy of surgical masks in preventing transmission of respiratory virus. They studied both children and adults suffering from a range of acute respiratory illnesses (including human coronaviruses, influenza virus, and rhinovirus). Viral particles were found in both respiratory droplets and aerosols. The study results provided evidence that surgical face masks reduced transmission of coronavirus particles in both droplets and aerosols, and reduced transmission of viral particles in droplets (but not via aerosols) for influenza virus.  Read more here >

  • This 2013 article tested the efficacy of homemade masks as an alternative to commercial face masks (in the event of a pandemic) in blocking bacterial and viral aerosols. Twenty-one healthy volunteers made their own face masks from cotton t-shirts. Number of microorganisms expelled by volunteers was compared when wearing the homemade masks, surgical masks, and no masks. Both mask types significantly reduced transmission of microorganisms, but surgical masks were three times more effective in blocking transmission. Read more here >

  • A recent study analyzed the transmission patterns in Hong Kong as related to mask-wearing practices. The study found that the prevalence of COVID-19 in Hong Kong was an order of magnitude lower than the prevalence of COVID-19 in Spain, Italy, France, Germany, and the US, and was somewhat lower than the prevalence of COVID-19 in the UK, Singapore, and South Korea. Face mask usage by Hong Kong general public was ~96.6%. Additionally, the study found that there were 11 COVID-19 clusters of 113 people in recreational mask-off settings (restaurant, bars, etc.) as compared to 3 COVID-19 clusters of 11 people in workplace mask-on settings in Hong Kong. The authors concluded that community-wide mask-wearing may contribute to control of COVID-19. (In press) Read more here >

Particle ingress - do masks protect the wearer?​

  • A novel paper recently published in the Journal of Internal Medicine reiterated that universal public masking during the COVID-19 pandemic was one of the most important pillars of disease control. However, most Interestingly, they proposed a new theory why masks are vital. Based on their preliminary modeling, they showed masking reduced the COVID-19 viral inoculum to which a mask-wearer was exposed, leading to milder (or asymptomatic) infection instead of more contagious symptomatic ones. Read more here >

  • This 2013 study examined the efficacy of surgical masks versus N95 masks in protecting the wearer from particle ingress. Surgical and N95 masks were fastened on mannequins and challenged with NaCL aerosol under different breathing conditions (breathing frequency and mean inspiratory flow). Results show that N95 was effective at preventing aerosol ingress, but surgical masks provided much less protection against breathing in particles. Therefore, surgical masks are more effective as a means of preventing the spread of germs via egress; vulnerable populations are encouraged to wear N95 masks to prevent the ingress of germs. Read more here >

  • This 2013 study examined the efficacy of surgical masks versus N95 masks in protecting the wearer from particle ingress. Surgical and N95 masks were fastened on mannequins and challenged with NaCL aerosol under different breathing conditions (breathing frequency and mean inspiratory flow). Results show that N95 was effective at preventing aerosol ingress, but surgical masks provided much less protection against breathing in particles. Therefore, surgical masks are more effective as a means of preventing the spread of germs via egress; vulnerable populations are encouraged to wear N95 masks to prevent the ingress of germs. Read more here >

  • This paper describes the development of a method for assessing the protection of masks against a bioaerosol challenge (i.e., how effective are masks at preventing the ingress of air particles). Microbiologists in the UK attached a dummy test head to a breathing stimulator, placed surgical masks on the mannequins, challenged the dummies/ masks with infectious aerosols, and measured the levels of inert particles and live aersolised influenza virus in the air from in front of and behind each mask. Results demonstrated that the surgical masks reduce exposure to aerosolized infectious virus between a range of 1.5 to 55 fold (average 6-fold), depending on the design of the mask, indicating some limited protective factor of the surgical masks.​ Read more here >

  • This article presents a 2010 study that examines whether wearing surgical masks or wearing surgical masks along with proper hand-washing reduces the risk of contracting influenza-like illnesses in a university residence hall. Wearing surgical masks and proper handwashing significantly reduced the risk of contracting an illness. Wearing a surgical mask alone may have reduced the risk of contracting an illness, although the results were inconclusive. Read more here >

  • This 2010 article tested the ability of fabric materials to filter out aerosol particles as compared to an N95 mask. The authors tested five fabric materials, including cloth masks, sweatshirts, T-shirts, towels, and scarves, and found that the fabric penetration levels ranged from 40-90%, which was much higher than the penetration level of the N95 masks tested. These filtration levels were comparable to those of some surgical masks that had been studied previously. Read more here >

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