Editor’s note: Varghese Mathai is a physicist at the University of Massachusetts Amherst studying the flow of liquids and gases. He conducted a study using simulations of calculating fluid dynamics to understand how air flows inside a car and its implications for COVID-19 transmission in the air. In this interview, he explains the optimal ways to ensure maximum airflow within a car.
What can be done to reduce the risk of air pollution inside a car?
It is important to have good ventilation. This means getting as much outdoor air as possible to mix with the air in the cabin and then flush it out.
You can do this in a few ways. One is by turning on the heating system, which absorbs fresh air from outside, and opening windows through which it can be flushed out. Another way is to just open the windows. The advantage of opening windows is that if you drive 20 km per hour or faster, a lot of air is flushed out just by the speed of the car.
By opening windows, more air can be flushed out than by simply turning on the heating or air conditioning.
Which windows should be kept open and closed to ensure optimal airflow?
We think the best setup is to open all windows and, if possible, open them completely. If it is not practical, it would be good to have two windows open. Preferably one at the back and one at the front.
What we found out from computer simulations is that the air enters through the rear window, turns behind the rear passenger and exits through the windshield. In this way, many of these aerosol particles can be flushed out of the cabin.
What about barriers and screens between the passenger and the driver?
Many taxi and ride-sharing services such as Uber and Lyft use a barrier or screen between the front and rear parts of the cabin. This helps to reduce the transmission through larger droplets. These are the kind of drops that are released by coughing, sneezing or talking loudly. The disinfection of surfaces helps against the transfer of fomite. However, the transmission of air will not be much reduced by these barriers, because there are always gaps and holes in barriers through which air can pass.
How did you conduct this study?
For this study, we used computer simulations, specifically simulations for calculating fluid dynamics, that are commonly used to study flows around cars and airplanes. We used it because of the fast turnaround time, so we could compare different open and closed window configurations and qualitatively predict what would be better to remove these air particles.
After this publication went out, we did a number of field tests to get some sort of validation of the simulated airflow. We released smoke in different places in the car and looked at the smoke lanes while it was released inside the car. It was more or less the same as what we found from the computer simulations.