The World Health Organization and the Centers for Disease Control recommend that a certain distance be maintained between people to prevent the spread of COVID-19. These recommendations for social distancing are calculated from various studies, but further research is still needed on the exact mechanism of transmission of the virus from one person to another.
IN Fluid physics, researchers at Stony Brook University, Harvard, ETH Zurich and Hanyang University demonstrate that normal indoor breathing without a mask can transport droplets of saliva capable of carrying viral particles at a distance of 2.2 meters or 7.2 feet, within 90 seconds.
Using a face mask significantly reduces the distance these droplets travel. After almost two minutes, the saliva droplets, limited by a mask, traveled just 0.72 meters, below 2.4 feet and well below the distance of 1.8 meters or 6 feet suggested by the CDC.
The study uses computer simulations with a more realistic model of the situation of interest than those used in previous studies. Previous work looked at aerosol transport after coughing or sneezing, while this study looked at normal human breathing. Normal breathing produces periodic jets containing droplets of saliva, but the speed at which the jet moves is less than one-tenth the rate of coughing or sneezing.
Researchers have found that even normal breathing creates a complex field of vortices that can displace droplets of saliva from a person’s mouth. The role of these vortices has not been understood before.
“Our results show that normal breathing without a face mask generates periodic delayed jets and leading circular vortex rings that propagate forward and interact with the vortex flow structures created in previous breathing cycles,” said author Ali Hosronejad.
This complex vortex field can transport aerosol droplets over long distances. A face mask dissipates the kinetic energy of the jet produced by the exhaled breath, disrupting vortices and restricting the movement of virus-laden droplets.
The researchers looked at the effect of the evaporation of saliva droplets. In the absence of a mask, they found that the droplets of saliva near the front of the exhaled breath had partially evaporated, reaching only one-tenth of a micron. In stagnant indoor air, droplets of this size would not settle on the ground for days.
The use of a mask partially redirects the exhaled breath downwards and significantly restricts the movement of the plume forward, so that the risk of hanging droplets remaining in the air is significantly reduced.
“To simplify the breathing process, we did not take into account the flow of air-saliva mixture through the nose and only considered the flow through the mouth,” Hosronejad said. “In future studies, we will examine the effect of normal breathing through both the nose and mouth.”
Humid air can prolong the life of virus-laden aerosol droplets
Ali Khosronejad et al, Computational study of exhalation particle transport and vortex dynamics during respiration with and without face masks, Fluid physics (2021). DOI: 10.1063 / 5.0054204
Provided by the American Institute of Physics
Quote: Normal breathing sends droplets of saliva 7 feet; masks shorten this (2021, June 9) downloaded on June 9, 2021 from https://phys.org/news/2021-06-saliva-droplets-feet-masks-shorten.html
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