A team of scientists from Brown University has attempted to understand the potential of COVID-19 transmission within the microclimate of cars, shedding insight on the dos and don’ts of ride-sharing during the coronavirus era. Their findings are published in the journal Science Advances.
In order to study the airflow inside a car, the researchers, led by Varghese Mathai and Asimanshu Das, used computer simulations to analyze the patterns of airflow created under certain scenarios: windows open, closed, AC/heat on or off, etc. They conducted their simulations with the assumption of a driver and a single passenger, with the passenger sitting in the backseat on the opposite side of the car from the driver, ensuring the most space possible between them. The simulations were roughly based on a Toyota Prius moving at 50 mph, meaning that even at such an orientation, there would be fewer than 6 feet between the two riders.
Their findings discovered that when sharing a car, the safest way to ride is with the windows open (the more the better). Doing so was shown to reduce the airborne particles exchanged between the two people. Unfortunately, though perhaps unsurprisingly, turning off the heat/AC did not have the same effect.
"Driving around with the windows up and the air conditioning or heat on is definitely the worst scenario, according to our computer simulations," said co-author Das, who is a graduate student in Brown's School of Engineering. "The best scenario we found was having all four windows open, but even having one or two open was far better than having them all closed."
That being said, the team acknowledges that even in the best of cases, with all windows completely open, there is still risk in ride-sharing and all riders should still be wearing masks. Additionally, they note that different combinations of windows open, and to different amounts, affect the movement and concentration of aerosols in the air currents in the car. Aerosols are small particles that hang out in the air and are thought to be how the SARS-CoV-2 virus is transmitted in enclosed spaces.
For example, in the scenario of all windows open, the air pressure outside the windows creates an air current in which air enters the rear windows and leaves through the front windows in two almost-independent flows on each side of the car. In this case, the driver has a somewhat higher risk than the passenger because of the direction of airflow, but particle exchange is still limited.
On the other hand, with only some of the windows open, different air flows are generated. Senior author Kenny Breuer explains, "When the windows opposite the occupants are open, you get a flow that enters the car behind the driver, sweeps across the cabin behind the passenger, and then goes out the passenger-side front window. That pattern helps to reduce cross-contamination between the driver and passenger."
The authors note that their models only included the simulations for aerosol particles, not larger respiratory droplets.