Survival Time of Covid-19 Droplets Linked to Climate and Location

Electron microscope image of the novel coronavirus, in yellow. (Credit: NIAID-RML)

(CN) — New research revealed Tuesday shows that heat and humidity levels may play an important part in the survival and transmission of the Covid-19 virus.

In a paper published in the journal Physics of Fluids, researchers examined the drying time of respiratory droplets from Covid-19-infected subjects on various surfaces in six cities around the world.

Using 5-nanoliter droplets, with a diameter roughly as wide as a human hair, researchers examined frequently touched surfaces, such as door handles and smartphone touchscreens and measured the time it took for the droplets to dry.

Once the droplets carrying the virus evaporate, the residual virus dies quickly, so the survival and transmission of the virus are directly impacted by how long the droplets remain intact.

By comparing the drying times of the droplets in the cities of New York, Chicago, Los Angeles, Miami, Sydney and Singapore, a mathematical model showed that ambient temperature, type of surface and relative humidity play critical roles in how long the droplets lingered.

By plotting the data against the growth rate of the pandemic, researchers Rajneesh Bhardwaj and Amit Agrawal found that in cities with a larger growth rate of infections, the drying time of an infected droplet was longer.

“In a way, that could explain a slow or fast growth of the infection in a particular city. This may not be the sole factor, but definitely, the outdoor weather matters in the growth rate of the infection,” Bhardwaj said.

It is important to note that while there have been speculations that the warmer summer weather will slow or halt the spread of Covid-19, recent research and modeling suggests that humans’ lack of immunity to the virus, rather than the weather, will likely be a primary factor driving the continued, rapid spread of the novel coronavirus this summer and into the fall.

An article published by the Director of the National Institute of Health cited a pair of studies which used mathematical models to simulate how seasonal temperature changes might influence the trajectory of the virus.

“In all three scenarios,” the blog article penned by Dr. Francis Collins explained, “their models showed that climate only would become an important seasonal factor in controlling COVID-19 once a large proportion of people within a given community are immune or resistant to infection.

“In fact, the team found that, even if one assumes that SARS-CoV-2 is as sensitive to climate as other seasonal viruses, summer heat still would not be enough of a mitigator right now to slow its initial, rapid spread through the human population. That’s also clear from the rapid spread of COVID-19 that’s currently occurring in Brazil, Ecuador, and some other tropical nations.”

However, whether or not the warming weather has a direct effect on the survival of the virus in our environment, the study of droplets and their drying time in various cities provides insight into this and other potential pandemics.

“Understanding virus survival in a drying droplet,” Agrawal explained, “could be helpful for other transmissible diseases that spread through respiratory droplets, such as influenza A.”

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