A new study uses computer simulations to track airflows in a car’s passenger cabin and offers possible strategies – some of which are counterintuitive – to reduce the risk of airborne disease transmission.
A new study of airflow patterns in the passenger cabin of a car offers some suggestions to reduce the risk of cancer COVID-19 transmission while sharing rides with others.
The study, conducted by a team of Brown University researchers, used computer models to simulate airflow in a compact car with different combinations of open or closed windows. The simulations showed that opening windows – the more windows the better – created airflow patterns that drastically reduced the concentration of airborne particles exchanged between a driver and a single passenger. By blowing the car’s ventilation system, the air didn’t circulate as well as a few open windows, the researchers found.
“Driving around with the windows open and the air conditioning or heating on is definitely the worst scenario, according to our computer simulations,” said Asimanshu Das, a graduate student at Brown’s School of Engineering and co-lead author of the study. “The best scenario we found was that all four windows were open, but even one or two open was much better than having them all closed.”
Das led the study along with Varghese Mathai, a former postdoctoral fellow at Brown who is now an assistant professor of physics at the University of Massachusetts, Amherst. The study is published in the journal Science Advances.
The researchers emphasize that there is no way to completely eliminate risk – and, of course, current guidelines from the U.S. Centers for Disease Control (CDC) note that delaying travel and staying home is the best way to maintain personal and community health. to protect. The purpose of the study was simply to study how changes in the airflow in a car can worsen or reduce the risk of transmission of pathogens.
The computer models used in the study simulated a car, loosely based on a Toyota Prius, with two people in it: a driver and a passenger in the back seat on the opposite side of the driver. The researchers chose that setup because it maximizes physical distance between the two people (although still less than the 6 feet recommended by the CDC). The models simulated the flow of air around and inside a car traveling at 80 kilometers per hour, as well as the movement and concentration of aerosols from both driver and passenger. Aerosols are small particles that can remain in the air for extended periods of time. They are thought to be a way in which the SARS-CoV-2 virus is transmitted, especially in confined spaces.
Part of the reason opening windows is better in terms of aerosol transmission is because it increases the number of air changes per hour (ACH) in the car, which helps reduce the overall concentration of aerosols. But ACH was only part of the story, the researchers say. The study found that different combinations of open windows created different air currents in the car that could increase or decrease exposure to residual aerosols.
Because of the way air flows around the outside of the car, the air pressure at the rear windows is usually higher than at the windshields. As a result, the air tends to enter the car through the rear windows and out through the windshields. With all windows open, this tendency creates two more or less independent streams on either side of the cabin. Because the occupants in the simulations were on either side of the cabin, very few particles are ultimately transferred between the two. The driver is at a slightly higher risk than the passenger in this scenario as the average airflow in the car is from rear to front, but both occupants experience significantly lower particle transfer compared to any other scenario.
The simulations for scenarios where some, but not all windows are disabled, may have produced counter-intuitive results. For example, you might expect that opening windows right next to each resident is the easiest way to reduce exposure. The simulations showed that, while this configuration is better than no windows down at all, it poses a higher risk of exposure compared to placing the window facing each resident.
“When the windows are open across from the occupants, you get a current that enters the car behind the driver, sweeps through the cabin behind the passenger and then exits through the front windshield on the passenger side,” says Kenny Breuer, a professor of engineering at Brown and a senior author of the study. “That pattern helps to reduce cross-contamination between driver and passenger.”
It’s important to note, the researchers say, that airflow adjustments are not a substitute for wearing a mask by both occupants of a car. And the findings are limited to potential exposure to sustained aerosols that may contain pathogens. The study did not model greater respiratory drops or the risk of actually being infected by the virus.
Still, the researchers say the study provides valuable new insights into air circulation patterns in a car’s passenger compartment – something that had received little attention until now.
“This is the first study that we are aware of, which really looked at the microclimate in a car,” said Breuer. “There were some studies that looked at how much external pollution gets into a car, or how long cigarette smoke lingers in a car. But this is the first time anyone has looked at airflow patterns in detail. “
The research emerged from a COVID-19 research task force set up at Brown to gather expertise from across the university to address very diverse aspects of the pandemic. Jeffrey Bailey, associate professor of pathology and laboratory medicine and co-author of the airflow study, leads the group. Bailey was impressed with how quickly the research came about, with Mathai suggesting using computer simulations that could be done while laboratory research at Brown was interrupted due to the pandemic.
“This is a really great example of how different disciplines can come together quickly and deliver valuable findings,” said Bailey. “I spoke briefly with Kenny about this idea and within three or four days his team was already conducting preliminary tests. That’s one of the great things about being in a place like Brown where people love to collaborate and work across disciplines. “
Reference: “Passenger Car Air Flows and Implications for Airborne Disease Transmission” by Varghese Mathai, Asimanshu Das, Jeffrey A. Bailey and Kenneth Breuer, December 4, 2020, Science Advances.
DOI: 10.1126 / sciadv.abe0166