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Respiratory infections spread from person to person through virus-carrying droplets via airborne transmission or by contact with a surface contaminated by droplets. Infected persons often expel these droplets by coughing or sneezing—a telltale sign that others should steer clear to avoid infection. But transmission actually depends on a large range of factors, including the number of droplets, their size, and their velocity during expiratory events like coughing, sneezing, and breathing.
Sneezing, for example, can expel thousands of large droplets at a relatively high velocity, whereas coughing generates 10-100 times fewer droplets. Talking expels considerably fewer droplets still, about 50 per second, and they are smaller. These small droplets are more likely to suspend in the air, travel farther distances, and transmit infection once they are inhaled. Large droplets, on the other hand, are more likely to contaminate surfaces and transmit infection by touch.
As the team notes in the paper, many studies to accurately measure how droplets are generated and transported have already been conducted. However, consensus on droplet behavior remains elusive due to the complex nature of the phenomena, as well as the difficulty of making such measurements.
Containment strategies for COVID-19 are based on what policymakers think they know about flow physics. But Mittal and Ni caution that much of that is based on outdated information.
For instance, even months into the pandemic, many questions still surround the use of face masks. Face masks are often designed to protect the person wearing the mask—think a construction worker trying to avoid inhaling dangerous dust, for instance. But face masks to combat COVID-19 transmission should offer both inward and outward protection, protecting others as much as it protects the wearer.
Scientists can better understand how to improve outward protection by simulating the flow leakage caused by gaps around the nose and mouth, says Jung-Hee Seo, associate research professor of mechanical engineering. He's working with Mittal and Koroush Shoele from Florida State University on state-of-the-art simulations to analyze air flow and droplet dispersion in face masks. Their simulations take into account different face shapes and mask structures, allowing them to evaluate the effectiveness of various mask designs.
A computational simulation of a cough shows the airflow velocity of droplets moving through a simple face mask. IMAGE CREDIT: COURTESY OF JUNG-HEE SEO
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