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Humidity helps virus particles remain airborne, travel farther

New research suggests humidity can help exhaled droplets remain airborne for longer. Photo by Piqsels/CC
New research suggests humidity can help exhaled droplets remain airborne for longer. Photo by Piqsels/CC

Aug. 18 (UPI) -- Scientists mostly agree that viral particles are airborne, but to what extent is a point of contention -- how long virus particles remain suspended and how far they can travel from their source remain open questions.

When physicists at the University of Missouri used a new model to measure the influence of airflow and fluid flow on the movement of exhaled droplets, they found high levels of humidity can prolong the lifetime of medium-sized droplets by a factor of 23.

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The research team published their results Tuesday in the journal Physics of Fluids.

Though a solitary COVID-19 particle typically measures less than one-tenth of a micron, the majority of droplets exhaled by coughs, sneezes and breathing are larger -- between 50 to 100 microns in diameter. For reference, a human hair boasts a diameter of roughly 70 microns.

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"Our paper is not particularly designed for coronavirus," lead study author Binbin Wang, assistant professor of civil and environmental engineering at Missouri, told UPI in an email. "Rather, we focus on the fundamental physics of droplets from human respiratory flows."

Exhaled droplets feature water, lipids, proteins and salt, in addition to virus. The model developed by Wang and his colleagues considered how environmental conditions and other substances in the atmosphere might impact these droplets.

When researchers compared the results of their model to the predictions of models designed to simulate the movements of similarly sized particles, like corn pollen, they found significant agreement.

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At 50 percent relative humidity, researchers found medium-sized droplets traveled no more than 11 feet, but at 100 percent humidity, the same droplets traveled as far as 16 feet.

"Our finding did show strong influences of temperature and humidity on spreading of droplets both in time and space," Wang said. "Generally, high humidity will increase evaporation time of a droplet. How long a droplet will stay in air strongly depends on the initial size of the droplet."

While the latest research offers clues as to the effects of air flows, temperature and humidity on exhaled droplets, the relationship between droplets and actual virus particles remains an area of uncertainty.

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"We don't know how a virus is correlated to the droplets," Wang said. "What would happen to virus once droplets end their life is largely unknown. We need to be quite careful in interpreting our findings."

Still, Wang suggests the findings support the consensus recommendations among public health experts.

"Social distancing and face covering could significantly mitigate potential virus transmission," Wang said.

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