The new study uses computer simulations to track airflows in the passenger compartment of a car, offering potential strategies – some of which are contradictory – to reduce the risk of airborne transmission.
A new study of airflow patterns in the car’s passenger compartment offers some recommendations to potentially reduce their risk Covid-19 broadcast by sharing trips with others.
The study, conducted by a team of researchers at Brown University, used computer models to simulate airflow in a compact car with different combinations of open or closed windows. Simulations showed that openable windows – the more windows, the better – created airflow patterns that dramatically reduced the concentration of airborne particles exchanged between the driver and one passenger. When the car’s ventilation system was blown up, the air was not circulated nearly as well as some open windows, the researchers found.
“According to our computer simulations, driving around with windows and air conditioning or on air temperature is definitely the worst case scenario,” said Asimanshu Das, a graduate student at Brown School of Engineering and co-author of the study. “The best case scenario we found was that all four windows were open, but even one or two open windows were much better than all.
Dass led the study with Varghese Mathai, a former Brown postdoctoral student who is now an assistant professor of physics at the University of Massachusetts in Amherst. The study is published in a journal Development of science.
Researchers stress that it is not possible to completely eliminate the risk – and, of course, the current guidelines of the US Centers for Disease Control (CDC) indicate that delaying and staying home is the best way to protect personal and public health. The aim of the study was simply to investigate how changes in airflow in a car can worsen or reduce the risk of pathogen transmission.
The computer models used in the study simulated a car based loosely on a Toyota Prius with two people, a driver and a passenger, sitting in the back seat on the opposite side of the driver. The researchers chose this seating arrangement because it maximizes the physical distance between two people (although still less than the 6 feet recommended by the CDC). The models simulated the flow of air around and inside the car at 50 mph, as well as the movement and concentration of aerosols generated by both the driver and passenger. Aerosols are tiny particles that can linger in the air for long periods of time. It is believed that they are one of the ways SARS-CoV-2 the virus is spreading, especially indoors.
Part of the reason why opening windows is better in terms of aerosol transfer is because it increases the number of air changes per hour (ACH) inside the car, which helps to reduce the total concentration of aerosols. But researchers say the ACH was only part of the story. The study showed that different combinations of open windows inside the car created different airflows that could either increase or decrease the exposure to the remaining aerosols.
Because the air flows outside the car, the air pressure at the rear windows tends to be higher than the pressure at the windshields. As a result, air tends to enter the car through the rear windows and exit through the front windows. When all windows are opened, this trend creates two more or less independent flows on both sides of the cabin. As the people sitting in the simulations sat on opposite sides of the cabin, very few particles get between the two. In this scenario, the driver is at a slightly higher risk than the passenger, as the average airflow in the car goes from the back to the front, but both passengers have dramatically less particle transfer compared to any other scenario.
Simulation scenarios in which some, but not all, windows are down yielded some potentially conflicting results. For example, it could be expected that opening windows for each passenger could be the easiest way to reduce exposure. The simulations found that although this configuration is better than without windows facing down, it poses a higher risk of exposure compared to placing the window in front of each passenger.
“When the windows in front of the passengers are open, you get a stream entering the car behind the driver, sweeping across the cabin behind the passenger and then passing through the front window of the passenger side,” said Kenyan Breuer, engineering professor Brown and senior author of the study. “This model helps reduce driver-passenger pollution.”
According to the researchers, it is important to note that adjusting the airflow does not replace the wearing of a mask by both passengers while in the car. The findings are limited to the potential for long-term exposure to aerosols that may contain pathogens. The study did not model larger breaths or the risk of actually becoming infected with the virus.
However, the researchers say the study provides valuable new insights into air circulation patterns in the car’s passenger compartment – something that had previously received little attention.
“This is the first study we know of that really looked at the car’s climate,” Breuer said. “Some studies have been carried out to check how much external pollution enters the car or how long the cigarette smoke stays in the car. But this is the first time someone has looked at airflow patterns in detail. “
The study emerged from the COVID-19 research team set up in Brown to gather expertise from across the university to address a wide range of aspects of the pandemic. The group is led by Associate Professor of Pathology and Laboratory Medicine Jeffrey Bailey and co-author of the airflow study. Bailey was amazed at how quickly the study came together, and Mathai suggested using computer simulations that could be performed while laboratory research in Brown was suspended during the pandemic.
“It’s really a great example of how different disciplines can quickly come together and make valuable discoveries,” Bailey said. “I’m talking to Kenya briefly about this idea, and in three or four days his team has already done an initial test. It’s one of the biggest things about being in places like Brown where people want to collaborate and work in different disciplines.”
Reference: Varghese Mathai, Asimanshu Das, Jeffrey A. Bailey and Kenneth Breuer, ‘Airflows inside cars and the impact on disease transmission’, 4 December 2020, Development of science.
DOI: 10.1126 / sciadv.abe0166