A comparison of performance metrics for cloth masks as source control devices for simulated cough and exhalation aerosols

Abstract

Universal mask wearing is recommended to help control the spread of COVID-19. Masks reduce the expulsion of aerosols of respiratory fluids into the environment (called source control) and offer some protection to the wearer. Masks are often characterized using filtration efficiency, airflow resistance, and manikin or human fit factors, which are standard metrics used for personal protective devices. However, none of these metrics are direct measurements of how effectively a mask blocks coughed and exhaled aerosols. We studied the source control performance of 15 cloth masks (face masks, neck gaiters, and bandanas), two medical masks, and two N95 filtering facepiece respirators by measuring their ability to block aerosols ≤ 7 µm expelled during simulated coughing and exhalation (called source control collection efficiency). These measurements were compared with filtration efficiencies, airflow resistances, and fit factors measured on manikin headforms and humans. Collection efficiencies for the cloth masks ranged from 17% to 71% for coughing and 35% to 66% for exhalation. Filtration efficiencies for the cloth masks ranged from 1.4% to 98%, while the fit factors were 1.3 to 7.4 on headforms and 1.0 to 4.0 on human subjects. The Spearman's rank correlation coefficients between the source control collection efficiencies and the standard metrics ranged from 0.03 to 0.68 and were significant in all but two cases. However, none of the standard metrics were strongly correlated with source control performance. A better understanding of the relationships between source control collection efficiency, filtration efficiency, airflow resistance, and fit factor is needed.

A Simple Method to Quantify Outward Leakage of Medical Face Masks and Barrier Face Coverings: Implication for the Overall Filtration Efficiency

“…Recently Lindsay and co-workers evidenced that none of the standard metrics such as filtration efficiency, breathability or airflow resistance, and manikin or human fit factors, are direct measurements of how effectively a mask blocks coughed and exhaled aerosols, and that none of them strongly correlated with source control performance [ 41 ]. The proposed TFE should address the need for a comprehensive description of mask efficiency, integrating both filter properties, breathability and fit into a single performance parameter.…”

“…An Andersen impactor composed of six stages is used to collect aerosol with aerodynamic sizes ranging from 7.00 to 0.65 μm. The results of TFE for smaller aerosol generated during breathing (~1 µm) could be better estimated using the results of particle filtration efficiency (PFE) tests, typically working with 0.3 μm sized particles [ 41 ].…”

“…These studies reported only qualitative data, showing leak flow preferential direction, droplet projection distance, or aerosol density distribution, without providing numbers on the total filtration efficiency. Only recently, Lindsley and co-workers reported data about the mass fraction of respiratory aerosol that is blocked by cloth masks by using a complex apparatus composed of realistic head form, a large aerosol chamber, and a multistage aerosol impactor [ 41 ].…”

A highly efficient cloth facemask

“…However, when the mask was worn as designed, with the neck seal tight, the inside-mask concentration upon inhalation rose only to about 6% of the room air concentration, indicative of an effective efficiency of 94% for inhalation, similar to that found for expiratory particle emission. Mask performance is sometimes characterized by the fit factor, which in this case is just the ratio between the room air concentration and the concentration measured in the mask, or ~16.7 [Lindsley et al, 2021]. Distinct from expiratory aerosols, the room air aerosols were primarily smaller than 0.5 microns, with only ~10% of the particles larger.…”

Do They Really Work? Quantifying Fabric Mask Effectiveness to Improve Public Health Messaging

“…To date, the research remains limited on standardized testing method outlined by ASTM International to evaluate (a) FE, (b) differential pressure (dP), and (c) leakage to compare non-medical masks with N95 respirators and surgical masks. However, recent publications have utilized more rigorous methods for evaluation of non-medical-grade masks [ 18 , 19 ]. In the aftermath of shifting public health recommendations on mask wearing, the lack of established research protocols for results dissemination significantly contributes to adoption variance [ 14 , 15 , 20 ].…”