Filtration efficiency of face masks used during COVID-19 pandemic

Editor's Note

This study from the University of North Carolina at Chapel Hill evaluated 29 face masks for use by clinicians interacting with patients during the COVID-19 pandemic and  found that expired N95 respirators and sterilized, reused N95 respirators were acceptable alternatives when N95 respirators were not available. All other masks provided less effective filtration.

Among the findings:

• A Controlled Air Purifying Respirator system fitted with a face shield had a fitted filtration efficiency of 99%.
• Expired N95 respirators with intact elastic bands and those that had been sterilized with ethylene oxide and hydrogen peroxide had filtration efficiencies of more than 95%.
• The performance of N95 respirators in the wrong size resulted in decreased filtration efficiencies between 90% and 95%.
• Surgical and procedure masks with ties had substantially lower filtration efficiencies than N95 respirators at 71.5%, and their performance was largely dependent on the tightness of the contact between the material and the face skin.
• Face masks secured with elastic ear loops showed the lowest filtration efficiency at 38.1%, and the filtration dropped to 21.2% when the clinician moved his or her head left and right.
• SARS-CoV-2 virons are 50 to 200 nm in diameter. Based on the mechanisms of particle deposition that govern filtration by face masks (ie, diffusion, impaction, interception, and sedimentation) a count median diameter of 50 nm confers protection. 

This study provides quantitative results on which health care administrators, supply chain leaders, and hospital epidemiologists can make decisions to protect clinicians during a pandemic or mask shortage, the researchers say.