Fitted Facemask Containment: Importance for Determining Transmission of Virus-Laden Aerosols

WILLIAM BENNETT, Steven Prince, Kirby Zeman, James Samet, University of North Carolina at Chapel Hill

     Abstract Number: 211
     Working Group: Aerosol Science of Infectious Diseases: What We Have Learned and Still Need to Know about Transmission, Prevention, and the One Health Concept

Abstract
Background: Facemasks reduce transmission of viruses by 1) protecting the wearer from virus-laden aerosols in the environment and 2) reducing aerosol emissions to the environment from infected individuals. Methods quantifying fitted filtration efficiency for wearer protection (PE) are well established (e.g., Sickbert-Bennett et al. JAMA Intern Med. 2020;180(12):1607). But current methods for assessing facemask containment efficiency (CE) in human subjects rely on measuring a low concentration of aerosols emitted from an infected or healthy individual.

Methods: Expanding on methods used for measuring PE, we designed a small-volume, low-ventilation chamber to accommodate a seated adult male subject. A ported facemask (N95, KN95, Procedure, or Gaiter) enabled introduction of a stream of 0.05um NaCl particles at a constant concentration behind the mask. Ambient chamber concentration was continuously measured by a condensation particle counter sampling 2 feet in front of the face over a series of three 3-minute periods: 1) resting, 2) reading out loud, and 3) repeated forceful coughing. %CE for each mask/procedure was determined as 100*(1 - average of ambient concentration values 30-180 seconds/the same for a “no mask” condition). PE was also measured in the same chamber by referenced/published methods. %Transmission from infected to uninfected is a function of both %CE and %PE = {1-(CE/100)}*{1-(PE/100)} *100.

Results: The average %CE for each mask over all procedures and repeat measures were 94.6, 60.9, 38.8, and 43.2 respectively, for masks listed above. The %PE for each mask was 99.2, 58.9, 30.8, and 6.5. Based on these calculations, %transmission for a KN95 worn by both infected and uninfected is approximately 16% compared to 40% when there is only one wearer.

Conclusion: The best DOUBLE masking for reducing aerosol transmission of virus is two mask wearers.

UNC/USEPA Cooperative Agreement CR83578501.