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Laboratory evaluations of surgical and N95 masks by researchers at the University of California, Davis, show that they do cut down the amount of aerosolized particles generated during breathing, coughing and talking.
Tests of homemade fabric face coverings, however, show that the fabric itself releases a large amount of fibers to the air, underscoring the importance of washing them.
As the COVID-19 pandemic continues, using masks and other face coverings has emerged as a significant tool alongside contact tracing and isolation, hand-washing and social distancing to reduce the spread of coronavirus.
The goal of wearing face coverings is to prevent individuals who are infected with COVID-19 but asymptomatic from transmitting the virus to others. But while evidence shows that face coverings generally reduce the spread of airborne particles, there is limited information on how well they compare with each other.
Sima Asadi, a graduate student working with Professor William Ristenpart in the UC Davis Department of Chemical Engineering, and colleagues at UC Davis and Icahn School of Medicine at Mount Sinai, New York, set up experiments to measure the flow of particles from volunteers wearing masks while they performed”expiratory activities” including breathing, speaking, coughing and moving their jaw as if chewing gum.
Asadi and Ristenpart have previously studied how people emit small particles, or aerosols, during address. These particles are small enough to float through the air over a considerable distance, but large enough to carry viruses such as influenza or coronavirus. They have found that a fraction of individuals are”superemitters” who give off many more particles than average.
The 10 volunteers sat in front of a funnel in a laminar flow cabinet. They wore no mask, a medical-grade surgical mask, two types of N95 mask (vented or not), a homemade paper mask or homemade one- or two-layer fabric mask made from a cotton T-shirt based on CDC directions.
The tests simply measured outward transmission — if the masks could block an infected individual from giving off particles that might carry viruses.
Without a mask, talking (reading a passage of text) gave away about 10 times more contaminants than simple breathing. Forced coughing produced a variable amount of particles. Among those volunteers from the study was a superemitter who consistently produced nearly 100 times as many particles as the others when coughing.
In all of the test situations, surgical and N95 masks blocked as much as 90 percent of particles, compared to not wearing a mask. Face coverings also reduced airborne particles from the superemitter.
Homemade cotton masks really produced more particles than not wearing a mask. These appeared to be tiny fibers released from the cloth. Because the cotton masks produced particles themselves, it’s hard to tell if they also blocked exhaled particles. They did seem to at least reduce the amount of larger particles.
The results confirm that masks and face coverings are effective in reducing the spread of airborne particles, Ristenpart said, and also the importance of regularly washing cloth masks.
University of California-Davis
Asadi, S., et al. (2020) Efficacy of masks and face coverings in controlling outward aerosol particle emission from expiratory activities. Scientific Reports. doi.org/10.1038/s41598-020-72798-7.
