The authors reviewed existing evidence for filtration efficiency of cloth masks vs medical masks, and they found that cloth masks fared surprisingly well. Layers matter, as does the material used.
Cloth masks may prevent spread of coronavirus disease 2019 (COVID-19) because they reduce contamination of air and prevent droplets from reaching surfaces; thus, health officials are just in promoting their use alongside social distancing and hand washing, according to authors from a leading Canadian medical school writing today in Annals of Internal Medicine.
Whether non-medical masks had any value beyond making people feel safer has been a topic of debate for months; the CDC initially shied from such a recommendation but changed course April 3, 2020, as it became clear that asymptomatic persons were unwittingly spreading the virus. The CDC advisory distinguishes between medical-grade N95 masks, which it says must be reserved for first-responders and health care workers, and more simple cloth masks that can be sewn or fashioned out of household materials. This second type is the topic of today’s Annals essay.
“Cloth does not stop isolated virions,” say the team of authors from McMaster University and St. Joseph's Hospital. “However, most virus transmission occurs via larger particles in secretions,” whether through aerosol or droplets as people speak, eat, cough, or sneeze. The point, they say, is not that some particles may pass through cloth but that many more are stopped, and each one trapped by the mask, “is not available to hang in the air as an aerosol or fall to a surface to be later picked up by touch.”
The authors reviewed existing evidence for filtration efficiency of cloth masks vs medical masks, and they found that cloth masks fared surprisingly well. Layers matter, as does the material used. The single randomized controlled trial of cloth masks compared an “unusually inefficient mask” with a medical mask, unfortunately, they noted, there was no comparator group of no mask, which would be a more useful data point. Health care workers wearing cloth masks contracted the flu at a rate of 2.3% compared with 0.7% for workers wearing the medical masks as indicated, and 0.2% for those wearing them continuously.
“When we apply the principles of evidence-based medicine to public policy, there is high-quality, consistent evidence that many (but not all) cloth masks reduce droplet and aerosol transmission and may be effective in reducing contamination of the environment by any virus, including SARS-CoV-2,” the authors wrote.
“No direct evidence indicates that public mask wearing protects either the wearer or others. Given the severity of this pandemic and the difficulty of control, we suggest that the possible benefit of a modest reduction in transmission likely outweighs the possibility of harm.”
The authors say that in low-income areas, public distribution of masks may be needed, so that responsibility for wearing a mask does not fall on an individual.
Clase CM, Fu EL, Joseph M, et al. Cloth masks may prevent transmission of COVID-19: an evidence-based, risk-based approach [published online May 22, 2020]. Ann Intern Med.