Coronavirus may spread more via respiratory droplets in winter, say scientists

While transmission of the novel coronavirus as small aerosol particles is more significant in summer, direct contact with respiratory droplets may be more pronounced in the winter months, according to a new research.

The modelling study, published in the journal Nano Letters, also noted that the currently followed physical distancing guidelines are inadequate in curbing the transmission of Covid-19.

"We found that in most situations, respiratory droplets travel longer distances than the 6-foot social distance recommended by the CDC," said Yanying Zhu, a co-author of the study from the University of California (UC) Santa Barbara in the US.

In indoor environments 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, the scientists said this effect is increased with the droplets transmitting to distances of up to 6 metres (19.7 feet) before falling to the ground.

They said in such environments, the virus is particularly persistent, remaining "infectious from several minutes to longer than a day in various environments."

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

At the opposite extreme, in hot and dry places, the researchers said respiratory droplets more easily evaporate.

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In such conditions, they said the evaporated droplets leave behind tiny virus fragments that join the other aerosolised virus particles that are shed as part of speaking, coughing, sneezing and breathing.

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

In summer, the scientists said aerosol transmission may be more significant compared to droplet contact, while in winter, droplet contact may be more dangerous.

"This means that depending on the local environment, people may need to adopt different adaptive measures to prevent the transmission of this disease," Zhao said.

The scientists recommended greater social distancing if the room is cool and humid, and finer masks and air filters during hot, dry spells.

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

They believe the findings could serve as useful guidance for public health decision-makers in efforts to keep the Covid-19 spread to a minimum.

"Combined with our study, we think we can maybe provide design guidelines for the optimal filtering for facial masks," Zhao said.

He added 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.

According to the scientists, the insights, "may shed light on the course of development of the current pandemic, when combined with systematic epidemiological studies."