Researchers at the University of Wisconsin-Madison conducted a study showing that wearing face masks in the classroom could be highly effective in reducing aerosol transmission of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – the causative agent of coronavirus disease – 2019 (COVID-19).
However, the filtration efficiency of face masks can be poor if not properly installed, warns David Rothamer and colleagues.
The researchers also showed that classroom ventilation alone did not reduce the likelihood of infection enough, while even the use of moderately effective face masks did sufficiently reduce that likelihood.
“The results reinforce the use of properly donned masks to reduce aerosol transmission of SARS-CoV-2 and other infectious diseases that are transmitted through indoor breathable aerosols,” the team writes.
A pre-print version of the paper is available on the medRxiv * server while the article is being peer-reviewed.
(a) Decorating the classroom with puppets (a) looking back from the front of the room and (b) looking forward from the back of the room.
Using the Wells-Riley equation to evaluate aerosol transfer
Since the first cases of SARS-CoV-2 were identified late last year (2020) in Wuhan, China, researchers have increasingly recognized the long distance route of airborne virus transmission and the need for effective measures to block this mode of transmission.
A common approach to assessing long-range airborne transmission (hereinafter referred to as aerosol transmission) uses the Wells-Riley equation to relate aerosol concentrations to the likelihood of infection.
This approach has already been used extensively in studies of SARS-CoV-2, particularly in the context of super-spreader events.
The researchers say applying the Wells-Riley equation is useful for risk assessment and planning interventions to reduce the risk of disease transmission.
“One of the goals of this paper is to provide recommendations that, if implemented, can reduce the likelihood of COVID-19 transmission in traditionally busy spaces such as classrooms,” they write.
What did the researchers do?
The team used the Wells-Riley equation to assess the effect of ventilation, mask filtration, and a combination of the two on the SARS-CoV-2 probability of infection from aerosols in the classroom.
Images of masks installed on a manikin for filtration tests (a) 4-layer knitted cotton mask, (b) EOC mask, (c) procedural mask, and (d) surgical mask
A polydisperse neutralized salt (NaCl) aerosol generated in a size range consistent with SARS-CoV-2 containing bioaerosols was used as a safe substitute for measurements.
The effective filtration efficiency of the mask was tested by seeding a classroom with the NaCl aerosol and extracting concentrations of the aerosol through four different types of masks attached to dolls with and without the use of mask assemblers.
Two of the masks were washable reusable masks. One was made of 4 layers of knitted cotton and the other was 3 layers of nonwoven polypropylene with a spun bond.
The other two masks were disposable. One was a common, single-use, disposable mask (hereafter referred to as the procedural mask) and the other was an ASTM F2100 Level 2 surgical mask.
The importance of masks fitting properly
Measurements of the mask’s effective filtration efficiency have shown the importance of the masks being properly attached. Ill-fitting masks showed filtration efficiencies that decreased by a factor of six. The estimated leak rate for most poorly fitted masks was typically greater than 50%.
The analysis also found that the knitted cotton mask could only achieve an effective filtration efficiency of 26% even when using a mask fitter.
In all other cases, using a mask fitter significantly improved effective filtration efficiency, with the exception of the polypropylene mask, which performed well without a mask fitter. In the case of the procedural mask, the use of a mask fitter achieved an effective filtration efficiency of almost 95%.
The classroom ventilation alone was inadequate
The study also suggested that classroom ventilation alone is insufficient to keep the chances of infection less than 1%.
On the other hand, the use of masks with medium to high effective filtration efficiency by all persons (dolls) present significantly reduced the likelihood of infection.
Depending on the type of mask and whether a mask assembler was used, the likelihood of infection was reduced by 5, 10 or even more than 100 times.
The team says infection chances of less than 0.1% and even less than 0.0001% could be achieved with masks and mask fitters alone.
What does the team advise?
Rothamer and colleagues say the results increase the use of properly put on masks to achieve decreased aerosol transmission of SARS-CoV-2.
“In particular, the results highlight the great potential for effective mask wear and improved masks or the use of mask fitters to significantly reduce the chances of infection,” the team concludes.
* Important NOTE
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or be treated as established information.