The World Health Organization and the Centers for Disease Control recommend keeping a certain distance between people to prevent the spread of COVID-19. These social distancing recommendations are estimated from a number of studies, but further research is still needed on the exact mechanism of virus transmission from one person to another.
IN Liquid physics, researchers from Stony Brook University, Harvard, ETH Zurich and Hanyang University show normal indoor breathing without a mask capable of transporting saliva droplets capable of carrying virus particles to a distance of 2.2 meters or 7.2 feet during of 90 seconds.
Using a face mask significantly reduces the distance these drops move. After nearly two minutes, the saliva droplets bounded by a mask had traveled only 0.72 meters, below 2.4 feet and well below the distance of 1.8 meters or 6 feet, proposed by the CDC.
The study used computer simulations with a more realistic model of the interesting situation than those used in previous studies. Previous work considered aerosol transport after coughing or sneezing, while this study specifically looked at normal human breathing. Normal breathing produces periodic currents that contain droplets of saliva, but the rate at which the jet moves is less than one-tenth of a cough or sneeze.
Investigators found that even normal breathing produces a complex field of vortices that can move saliva droplets away from the person’s mouth. The role of these vortices has not been previously understood.
“Our results show that normal breathing without a face mask generates periodic subsequent rays and leading circular vortex rings that propagate forward and interact with the vortex flow structures produced in previous breathing cycles,” said author Ali Khosronejad.
This complex vortex field can transport aerosol droplets over long distances. A face mask disperses the jet’s kinetic energy produced by an exhaled breath, disrupts the vertebrae and restricts the movement of virus-loaded droplets.
Investigators considered the effect of evaporation of the saliva droplets. In the case of no mask, they found the saliva droplets near the front of the exhaled breath had partially evaporated and reached a size of only one-tenth of a micron. In stagnant indoor air, droplets of this size would not settle on the ground for several days.
The use of a mask partially redirects the exhaled breath and significantly restricts the forward movement of the liquid, so that the risk of suspended droplets remaining in the air is significantly reduced.
“To simplify the breathing process, we did not consider the flow of air-saliva mixture through the nose and only took into account the flow through the mouth,” Khosronejad said. “In future studies, we will investigate the effect of normal breathing through both nose and mouth.”
Moist air can prolong the life of virus-laden aerosol droplets
Ali Khosronejad et al., A computational study of expiratory particle transport and vortex dynamics during breathing with and without face masks, Liquid physics (2021). DOI: 10.1063 / 5.0054204
Provided by the American Institute of Physics
Citation: Normal breathing sends saliva drops 7 feet; masks abbreviate this (2021, June 9) retrieved June 9, 2021 from https://phys.org/news/2021-06-saliva-droplets-feet-masks-shorten.html
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