COVID-19: Airborne particles last longer in drier environments

A new study measuring how long airborne coronavirus particles can remain infectious at different humidity levels has found that drier environments may be more dangerous for spread. of the virus.

By filling a special sealed chamber the size of a bathroom with virus particles, the researchers were able to measure that at low humidity, airborne particles remained infectious for twice as long as at humidity. Relatively recommended from 40 to 60%.

The study, published in December in the peer-reviewed journal PNAS-Nexus, adds credibility to what scientists have been saying since the start of the pandemic: that ventilation and filtration systems are key. lock to minimize transmission.

The researchers also found that the protective ability of saliva plays a big role in keeping infectious particles alive for a long time.

The researchers suggest that these findings could help prepare us for future reductions in airborne viruses, as well as help guide COVID-19 interventions. that this data has huge implications for naturally dry climates as well as enclosed spaces such as airplanes where humidity can be extremely low.

“The physical properties of the air in our buildings and the climate in which we live affect what we do,” said Mark Hernandez, senior author and professor of civil and environmental engineering at SJ Archuleta. things that can make us sick and how long they last. . “We now have cautious indications of how long coronaviruses like the one that causes COVID-19 can stay airborne and become an infectious disease threat.”

Hernandez runs the Environmental Engineering Microbiology and Disinfection Laboratory at the University of Colorado, one of the few full-scale bioaerosol labs in the United States.

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

It is well established that ventilation is extremely important in preventing or even stopping airborne virus transmission. But the persistence of coronaviruses like COVID-19 in air-conditioned indoor air is still not fully understood.

Buildings in the US are largely designed to have a relative indoor humidity of about 40% to 60%, according to the press release. Health Canada recommends that Canadians keep the relative humidity in their homes between 30 and 55% in the winter and ensure that the humidity does not exceed 55% in the summer.

But these percentages can vary widely between real-world cities and buildings — the release notes that in Colorado, the average humidity is around 25%, significantly drier than 60%. of San Francisco.

So, to measure whether humidity affects how long infectious particles can circulate, the researchers flooded closed chambers with airborne particles filled with the virus.

The researchers chose to use a coronavirus similar to COVID-19, known as rat hepatitis virus (MHV), to evaluate how the coronavirus particles behave in these viruses. this condition. They dropped these infectious particles into three chambers with different relative humidity levels: 25%, 40% and 60%.

Half of these particles are covered in saliva, in the same way that infectious particles are expelled from the human body during exhalation, sneeze, or cough, and all are aerosolized with high distributions. similar in size to SARS-CoV-2 particles.

What the study found was that regardless of the humidity level, saliva was the main defense against the virus. Half of airborne MHVs remained infectious after one hour of circulation in a chamber at 40% or 60% relative humidity.

But at 25% humidity, the virus’s viability doubled – half of the particles were still infectious after two hours of circulation at that humidity.

Hernandez points out that this is longer than taking a class, or maybe hanging out in a restaurant or coffee shop.

“That shows that this virus can persist for quite a long time, even hours,” he said in the press release. “An occupant can enter, release the coronavirus in the air, and leave. Depending on the architectural elements, then someone else might step into that space with a strong dose still hanging around.”

These findings also indicate that ventilation with proper filters is necessary to properly clean the air from infectious particles, as they can linger in the air for longer depending on humidity.

Marina Nieto-Caballero, lead author of the study, said: “I hope this paper has a technical impact in our buildings, such as in schools and hospitals, so that we can minimize the transmission of these viruses in the air.” liberate, release, free.

In 2021, she earned her doctorate at the Hernandez bioaerosol laboratory and is currently doing postdoctoral research at Colorado State University.

For many areas of the world that are naturally low in humidity, creating a more humid indoor environment can be difficult and expensive, Hernandez admits.

He emphasized that for areas where it is difficult to increase indoor humidity to 40%, the use of filtration systems combined with ventilation systems will help solve the problem of infectious particles remaining in the air longer.

“We can add simple, inexpensive air filters to remove particles from the air faster. We can increase ventilation rates, open windows and make sure we get more fresh air,” says Hernandez. “We knew this from the start, but this study gives us a target.”


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