Airflow patterns affect droplet travel
One of the key messages of physical distancing is that if you’re outdoors, your risk of contracting the coronavirus is generally lower than if you’re indoors. This is because the virus is more quickly diluted — which means you’re exposed to fewer particles.
“If you’re in a room and someone coughs, sneezes, or speaks, the droplets they expel can hang around for a long time,” said Capecelatro. “It doesn’t matter where you are in the room. You are going to be breathing in some of those droplets, especially if there’s poor ventilation.”
A preprint paper by Japanese researchers found that the risk of transmission indoors is 18.7 times higher than the outdoor risk. This paper has not been peer-reviewed, so the results should be viewed with some caution.
However, not all indoor settings are created equal. Airflow patterns matter and sometimes can make things worse.
“What the airflow can do, especially if it’s turbulent, is cause these [virus] particles to cluster, and that will increase the number of particles that you could breathe in,” said Capecelatro.
He and his colleagues recently simulated how virus particles exhaled by passengers would spread throughout a bus, in order to reduce the risk of people riding the University of Michigan buses.
Some studies support the idea that poorly ventilated spaces are riskier.
In one case in China, 10 people from three families who visited a restaurant contracted the coronavirus over 1 hour. None of them had direct physical contact with the person with the virus — some were sitting up to 15 feet away.
Outbreaks have also been reported at other indoor settings, including gyms, call centers, and churches.
Many studies, though, have only looked at average airflow rates, not fluctuations in the movement of air within a space.
Another preprint study from researchers at the University of Minnesota College of Science and Engineering looked in more detail at how the coronavirus spreads indoors when exhaled by people. They analyzed three specific settings — an elevator, a small classroom, and a supermarket.
They found that good ventilation can remove some of the virus particles from the air, but many will end up on surfaces in the room.
Study author Jiarong Hong, PhD, an associate professor of mechanical engineering at the University of Minnesota, said if those surfaces aren’t cleaned frequently, people may pick up the particles when they touch the surfaces. The particles can also be resuspended in the air and breathed in.
In some cases, poor ventilation can cause “hot spots,” locations where the virus particles congregate. Hong’s research shows how to fix some of these problems.
“Our tools can predict the presence of hot spots [in a setting] and how to potentially change the ventilation system in those spaces to minimize risks,” said Hong.
Although each space is unique and requires a different approach to reducing the spread of coronavirus, Hong said there are some general approaches that businesses can take to improving their spaces.
This includes putting ventilation sources near the major virus emitters — such as where a teacher stands in a classroom — in order to capture as many of the particles as possible. Opening windows is another option that most spaces can benefit from, he says.
Hong’s research on the spread of coronavirus within different settings also provides some insights into how people can reduce their risk in different settings.
“When you enter a space, you can evaluate the potential [coronavirus] risk,” he said, looking at things such as how crowded the space is, whether people are wearing masks and the airflow.
“For example, if you have to go into a small space that is poorly ventilated, you can decide whether you want to go in there or if you want to shorten your time in that space. These are both things you can do to reduce your risk.”