awoodward@businessinsider.com (Aylin Woodward)
- Droplets containing the coronavirus can survive for a time on surfaces before drying out.
- A new study found that higher temperatures and lower humidity shorten the lifespans of coronavirus droplets on surfaces.
- The research also showed that in cities where weather made the droplets’ drying time longer, coronavirus infection rates rose faster.
- This could in part explain why New York City was hit harder by the pandemic in March compared to, say, Singapore.
- Visit Business Insider’s homepage for more stories.
A city’s weather could be linked to how hard it got hit by the coronavirus.
According to a study published Tuesday, the amount of time coronavirus-laden droplets last on surfaces depends on the surrounding temperature and humidity, as well as type of surface they settle on.
The virus typically spreads via droplets when an infected person coughs, sneezes, or talks loudly. Because some of those microscopic droplets settle on surfaces, a team of researchers decided to measure how long it took the droplets to dry out, thereby killing the virus inside. The longer that process takes, the more likely it is that the droplets could infect someone new.
The scientists found that higher temperatures and lower humidity dry out droplets faster — which could in part explain why the coronavirus took hold in some geographic areas more than others.
The researchers compared droplets’ average drying time in six cities with differing temperatures and humidity levels. Their results showed that “a longer drying time correlated with a larger growth rate of the pandemic,” according to Raneesh Bhardwaj, one of the study co-authors.
“Certain outdoor weather is something that matters in the growth of infections,” he told Business Insider.
A woman wearing a protective face mask rides a scooter across a nearly empty 3rd Avenue in midtown Manhattan, New York, April 21, 2020.
Low temperatures and high humidity help the virus survive
For their investigation, the researchers looked at the most common size of droplet that people with COVID-19 spew out: about 5 nanoliters, or “the width of a human hair,” Bhardwaj said.
They did not look at the lifespan of aerosols, which are clouds of even tinier viral particles.
Once droplets evaporate, the coronavirus that’s left behind quickly dies. So the chances of transmission depend on the droplets’ lifespan.
The study authors compared drying time in two different temperatures: 25 degrees Celsius (77 degrees Fahrenheit) and 40 degrees Celsius (104 degrees Fahrenheit), both with 50% humidity. That 15-degree increase halved the time it took droplets to evaporate, they found.
They also observed that an increase in relative humidity from 10% to 90% increased the evaporation time of a droplet almost seven-fold (with a steady temperature of 25 degrees Celsius.
Nacion/NurPhoto via Getty Images
An April study found a similar link between the virus’ lifespan and the surrounding temperature. At 4 degrees Celsius (39 degrees Fahrenheit), the virus lasted up to two weeks in a test tube. When the temperature was turned up to 37 degrees Celsius (99 degrees Fahrenheit), that lifespan dropped to one day.
Longer drying times are linked to quicker outbreak growth
To investigate whether outdoor weather conditions were connected to the severity of outbreaks in different cities, the researchers compared the growth of infections in New York, Chicago, Los Angeles, Miami, Sydney, and Singapore between March 1 and April 10. The results showed a correlation between the average drying time of a droplet in those cities (based on humidity and temperatures during that time period) and the growth rate of the local outbreak.
“If the drying time is less, the growth rate of infection in that city is expected to be less,” Amit Agrawal, another co-author of the study, told Business Insider.
New York City, for example, had a growth rate of new infections per day that was 35 times higher than Singapore’s. On average, the drying time for coronavirus droplets in New York was close to a minute — nearly double the drying time in Singapore.
“While the humidity is comparable between the two cities, New York was cold compared to Singapore when the pandemic started,” Bhardwaj said.
Other factors could have influenced the differing severity of those cities’ outbreaks, however. Singapore, a small island nation, was quicker than New York (and the rest of the US) to implement widespread coronavirus testing and contact tracing, as well as social distancing measures.
The researchers suggest that their research could provide clues about which countries and geographic areas might experience a more severe second wave this summer and fall.
“Infection rates in the Northern Hemisphere are decreasing with the warmer and drier summer, but the monsoon season is coming to southeast Asia, and bringing with it higher humidity and a higher growth rate,” Bhardwaj said.
The coronavirus’ lifespan on surfaces
A person can get the coronavirus if they touch a surface or object that has virus-laden droplets on it and then touch their mouth, nose, or eyes. The Centers for Disease Control and Prevention says it “does not spread easily” that way, however.
The new study suggests that droplets take longer to dry on certain surfaces — like smartphones, cotton, and wood — than on surfaces like glass and steel.
That’s because those latter surfaces are hydrophilic, meaning they’re more easily wettable. When a droplet hits a hydrophilic surface, it spreads into a thin film, which then evaporates more quickly into the surrounding air.
Wood, cotton cloth, and smartphone screens, by contrast, are hydrophobic — they stop liquid from spreading.
“If you put water on your phone screen, it becomes a blob. That blob will take longer to dry out than that water spread out over a larger area,” Bhardwaj said.
The researchers found that the evaporation time can increase by 60% for a hydrophobic surface compared to a hydrophilic one. That finding could indicate which surfaces should be cleaned most frequently, Agrawal said.
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