How well do masks prevent the spread of aerosol particles?

Why is wearing masks so important to keep viruses from spreading?

As a result of the global SARS-CoV-2 pandemic, wearing masks has become more and more common worldwide. One of the main benefits of wearing masks is that it prevents the virus from spreading. That said, people who are already infected with coronavirus are less likely to infect others if they wear a mask. Masked opponents point to the lack of quality evidence to support this claim. This is technically correct. For ethical and logistical reasons, there are no high-quality experimental studies that prove the influence of masks on preventing the spread of SARS-CoV-2.

In the absence of COVID-specific evidence, the scientific community must instead rely on more general mechanistic studies to demonstrate the benefits of wearing masks. A study published in Nature Scientific Reports is an interesting addition to the growing body of such mechanistic evidence (1).

The study examines the effectiveness of a variety of mask types in reducing aerosol particle emissions during a range of activities. To conduct these tests, the research team recruited 10 participants, four female and six male, in different age groups. An aerodynamic particle sizer (APS) was placed in a HEPA-filtered laminar flow hood. The HEPA filter minimizes particles in the background, while the APS can identify and count particles between 0.3 and 20 µm in size. A funnel was placed at an opening in the flow hood and connected to the APS.

Participants were then asked to place their mouth in front of the funnel and do a variety of expiratory tasks such as speaking, coughing, etc. These activities were repeated with each type of mask and in the absence of a mask. A microphone next to the funnel recorded the duration and intensity of actions such as speaking and coughing. The types of masks used included surgical masks, N95 respirators, KN95 ventilated respirators, and a variety of homemade cloth masks.

Wearing surgical masks or KN95 ventilators resulted in a significant reduction in particle emissions when breathing, coughing, and speaking. However, a slight increase in particulate emissions was observed in participants who wore single- or double-layer cotton T-shirt masks. The authors conclude that this increase is not due to expiratory particles, but rather to particles from the mask. Cotton is a friable cellulose fiber that can be worn away by shear forces. The suggested explanation for the unexpected results here is that expiratory forces cause cotton particles to loosen and be recognized by the APS, rather than cotton-based masks causing expiratory droplets to increase.

Interestingly, one of the study participants was considered a "super emitter" because his particle emissions when coughing were two orders of magnitude above average. Surgical masks and KN95 respiratory protection masks also remained effective with this “super emitter”. The audio recorder confirmed that people wearing masks speak louder but not cough louder.

Overall, the study provides some interesting results. The results of tests with surgical and medical masks confirm the effectiveness of these masks in reducing droplet emission. However, the results of tests with homemade masks, especially cotton masks, raise a few more questions. Research is needed to confirm the hypothesis that the observed increase in particulate emissions when wearing these masks is due to fabric particles. However, it is also important to investigate whether these substance particles can contain viral material.

Written by Michael McCarthy

1. Asadi S., Cappa CD, Barreda S., Wexler AS, Bouvier NM, Ristenpart WD. Effectiveness of masks and face coverings in controlling the emission of aerosol particles to the outside through expiratory activity. Sci Rep. 2020; 10 (1): 15665.

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