Aerosols vs. droplets
Researchers model the spread of the SARS-CoV-2 virus in various
temperatures and relative humidities in typical indoor situations

Date:
October 14, 2020
Source:
University of California - Santa Barbara
Summary:
A new study investigates the secret of the SARS-CoV-2 virus's
unusual success: its transmissibility, or how it manages to get from
host to host. The dominant mode, it turns out, changes according
to environmental conditions.



FULL STORY ========================================================================== Winter is on its way. And in this year of coronavirus, with it comes
the potential for a second wave of COVID-19. Add in flu season and our
tendency to head inside and close our windows to the cold, wet weather,
and it appears the next several months are going to present us with new
health challenges.


==========================================================================
UC Santa Barbara researchers Yanying Zhu and Lei Zhao hope to arm
people with better knowledge of how SARS-CoV-2 spreads as the seasons
change. Their new study investigates the secret of this virus's unusual success: its transmissibility, or how it manages to get from host to
host. The dominant mode, it turns out, changes according to environmental conditions.

"Back at the beginning of April a lot of people were wondering if COVID
would go away in the summer, in the warmer weather," said Zhu, a professor
of mechanical engineering and one of the authors of a paper that appears
in the journal Nano Letters. "And so we started to think about it from
a heat transfer point of view, because that's what our expertise is."
The virus, of course, did not disappear during the summer as hoped, and in
fact COVID cases across the country continued to climb. To understand how
the novel coronavirus manages to persist in circumstances in which the flu virus fails, Zhu, Zhao and colleagues modeled different temperatures and relative humidities along a continuum from hot and dry to cold and humid
in typical indoor spaces, where the virus is distributed by normal speech
and breathing -- and, according to the paper, where people "only sneeze
or cough into a tissue or their elbows." To these scenarios they added
emerging knowledge about the highly contagious microbe; in particular,
how long it remains infectious outside a host.

The results are sobering. For one thing, respiratory droplets -- the
most common mode of transmission -- don't obey our social distancing guidelines.

"We found that in most situations, respiratory droplets travel longer
distances than the 6-foot social distance recommended by the CDC," Zhu
said. This effect is increased in the cooler and more humid environments
to distances of up to 6 meters (19.7 feet) before falling to the ground
in places such as walk-in refrigerators and coolers, where temperatures
are low and humidity is high to keep fresh meat and produce from losing
water in storage. In addition to its ability to travel farther, the virus
is particularly persistent in cooler temperatures, remaining "infectious
from several minutes to longer than a day in various environments,"
according to several published studies.



========================================================================== "This is maybe an explanation for those super-spreading events that have
been reported at multiple meat processing plants," she said.

At the opposite extreme, where it is hot and dry, respiratory droplets
more easily evaporate. But what they leave behind are tiny virus fragments
that join the other aerosolized virus particles that are shed as part
of speaking, coughing, sneezing and breathing.

"These are very tiny particles, usually smaller than 10 microns,"
said lead author Lei Zhao, who is a postdoctoral researcher in the Zhu
Lab. "And they can suspend in the air for hours, so people can take in
those particles by simply breathing.

"So in summer, aerosol transmission may be more significant compared to
droplet contact, while in winter, droplet contact may be more dangerous,"
he continued.

"This means that depending on the local environment, people may need to
adopt different adaptive measures to prevent the transmission of this
disease." This could mean, for example, greater social distancing if
the room is cool and humid, or finer masks and air filters during hot,
dry spells.

Hot and humid environments, and cold and dry ones, did not differ
significantly between aerosol and droplet distribution, according to
the researchers.

The quantitative descriptions of virus propagation under varying local conditions could serve as useful guidance for decision-makers and the
general public alike in our efforts to keep the spread to a minimum.

"Combined with our study, we think we can maybe provide design guidelines
for the optimal filtering for facial masks," said Zhao, adding that the research could be used to quantify real exposure to the virus -- how much
virus could land on one's body over a certain period of exposure. This knowledge could, in turn, lead to better strategies for airflow and
ventilation to prevent virus accumulation. In addition, the insights,
according to the study, "may shed light on the course of development
of the current pandemic, when combined with systematic epidemiological studies." Research on this paper was conducted also by Yuhang Qi and
Prof. Paolo Luzzatto-Fegiz at UC Santa Barbara, and Prof. Yi Cui at
Stanford University.


========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by Sonia
Fernandez. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Lei Zhao, Yuhang Qi, Paolo Luzzatto-Fegiz, Yi Cui, Yangying
Zhu. COVID-
19: Effects of Environmental Conditions on the Propagation of
Respiratory Droplets. Nano Letters, 2020; 20 (10): 7744 DOI:
10.1021/ acs.nanolett.0c03331 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201014141026.htm

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