Drier environments more dangerous for spread of airborne viruses: study

A new study measuring how long airborne coronavirus particles can remain infectious at different humidities has found that drier environments may be more dangerous for viral spread.

By filling a specially sealed chamber up to the size of a bathroom with viral particles, researchers were able to measure that at low humidities, airborne particles remained infectious for twice as long as at recommended relative humidities of 40 to 60 per cent.

The study, published in December in the peer-reviewed journal PNAS-Nexus, adds further credence to what scientists have been saying since the start of the pandemic: that ventilation and filtration is key to mitigating transmission.

Researchers also found that saliva’s protective powers play a huge role in keeping infectious particles alive for so long.

The findings could help prepare us for mitigation with future airborne viruses, as well as help direct COVID-19 interventions, researchers suggested, noting that this data has huge implications for naturally drier climates as well as enclosed spaces such as airplanes, where the humidity can be extremely low.

“The physics of the air in our buildings and the climate in which we live affect things that can make us sick and how long they persist,” Mark Hernandez, senior author and S. J. Archuleta professor of civil and environmental engineering, said in a news release. “Now we have conservative indications of how long coronaviruses like the one that causes COVID-19 can stick around in the air and be an infectious disease threat.”

Hernandez runs the Environmental Engineering Microbiology and Disinfection Lab at the University of Colorado, one of the few full-scale bioaerosol labs in the U.S.

At the start of the pandemic, he suspected that humidity levels and how saliva interacted with airborne particles would be key factors in the transmission of COVID-19.

It’s well-established that ventilation is hugely important in stalling or even stopping transmission of airborne viruses. But the persistence of coronaviruses like COVID-19 in conditioned indoor air is still not fully understood.

Buildings in the U.S. are designed largely to have a relative indoor humidity between around 40 per cent and 60 per cent, according to the release. Health Canada recommends that Canadians keep their homes’ relative humidity between 30 and 55 per cent in winter and make sure it doesn’t exceed 55 per cent in summer.

But these percentages can vary widely from city to city and building to building in the real world — the release noted that in Colorado, average humidity is around 25 per cent, which is significantly drier than the 60 per cent of San Fransisco.

So to measure if humidity had an impact on how long infectious particles could remain circulating, researchers flooded sealed chambers with virus-laden, airborne particles.

Researchers chose to utilize a coronavirus similar to COVID-19, called murine hepatitis virus (MHV), in order to judge the way that coronavirus particles function under these conditions. They released these infectious particles into three chambers with different relative humidity levels: 25 per cent, 40 per cent and 60 per cent.

Half of the particles were coated in saliva, the way that infectious particles are when expelled from the human body in an exhale, sneeze or cough, and all were aerosolized in size distributions similar to SARS-CoV-2 particles.

What the study found was that regardless of the level of humidity, the saliva was a key protective element for the virus. Half of the airborne MHV were still infectious after one hour of circulating in a chamber at 40 per cent or 60 per cent relative humidity.

But at 25 per cent humidity, the staying power of the virus was doubled — half of the particles were still infectious after two hours of circulation at that humidity.

Hernandez pointed out that this is longer than the time it takes to attend a class, or perhaps hang out in a restaurant or cafe.

“It shows this virus can hang around for quite a while—hours, even,” he said in the release. “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.”

These findings also indicate that ventilation with proper filtration is necessary in order to properly clean the air of infectious particles, as they are able to remain in the air longer depending on the humidity.

“I hope this paper has an engineering impact in our buildings, for example, in schools and hospitals, so that we can minimize the infectivity of these viruses in the air,” Marina Nieto-Caballero, lead author of the study, said in the release.

In 2021, she earned her doctorate in the Hernandez bioaerosol lab, and now does postdoctoral research at Colorado State University.

For many regions of the world where humidity is naturally low, creating a more humid indoor environment may be difficult and pricey, Hernandez acknowledged.

He emphasized that for areas where raising the indoor humidity to 40 per cent is a struggle, using filtration in concert with ventilation will help to tackle the problem of infectious particles lingering in the air longer.

“We can add simple, inexpensive air filters that will take particles out of the air faster. We can increase the ventilation rate, open windows, and make sure we get more fresh air through,” said Hernandez. “We've known this from the beginning, but this research gives us a target.”