Humidity can play a big role in how long airborne viruses can survive, according to a new study out of the University of Colorado Boulder.
Researchers released airborne particles of a coronavirus similar to the one that causes COVID-19 into chambers with different levels of humidity. They found the particles remained infectious for twice as long in dry environments like those in much of the Mountain West.
CU Boulder Professor Mark Hernandez, the senior author of published in PNAS-Nexus in December, said his hypothesis that humidity and saliva are important in the transmission of COVID-19 was a 鈥済ross hunch.鈥� After all, humans spew microscopic droplets of spit and snot with every breath and those droplets sometimes carry airborne viruses.
鈥淲e accept this if we have a cat or a dog. The dog slobbers and the cat licks itself. If it dries out, you gotta scrub [the slobber], right? It鈥檚 different than if it鈥檚 wet, then you just wipe it up,鈥� he said. 鈥淭hat physics has a lot to do with the survival of what鈥檚 in it.鈥�
Hernandez released some coronavirus airborne particles covered in fake spit and others without it into chambers with different levels of relative humidity 鈥� 25%, 40% and 60%.
They found that in the higher humidity chambers, half of the airborne particles were infectious after an hour, but at 25% humidity, half of the particles were still infectious after two hours.
鈥淓verything else is the same," Hernandez said. "I just drop the humidity 15% from 40 to 25 and it doubles its half-life. That鈥檚 remarkable.鈥�
He explained that the saliva dried around the particles in the low-humidity chamber and made a protective glass-like layer around the virus.
For reference, recent humidity levels in Albuquerque and Denver have been below 25% while Reno, Salt Lake City, Boise and Laramie all had recent humidity levels between 40% and 60%.
鈥淚t shows this virus can hang around for quite a while 鈥� hours, even," Hernandez said in a . "It's longer than a class, longer than the time you're in a restaurant, longer than the time you take to hang out in the cafe. An occupant may come in, spread coronavirus in the air, and leave. Depending on architectural factors, then someone else could walk into that space with potent doses still hanging around.鈥�
He added that the virus can survive for longer in arid environments than the average time it takes a building鈥檚 ventilation system to replace the stale air carrying the particles with fresh outdoor air.
His advice? To increase ventilation or open some windows.
鈥淚 hope this paper has an engineering impact in our buildings, for example, in schools and hospitals, so we can minimize the infectivity of these viruses in the air,鈥� Colorado State University's Marina Nieto-Caballero, the study's lead author, said.
The researchers say it could help explain why arid Colorado had such a high rate of flu-like illnesses last November.
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