A new study out of Brown University investigates the risks of Covid-19 aerosol transmission in a car. According to Science Daily, the study, published in the journal Science Advances, found that the risk was lowest when all four windows were open. The researchers did not look at the airflow of respiratory droplets or the risk of infection, only the flow of aerosol particles through a moving car.
The researchers simulated airflow within a compact car using simulated models, in which the car’s driver was accompanied by a single passenger sitting in the rear passenger seat. One of the study’s lead authors told Science Daily that they found opening windows was a much better means of circulating air than turning on the vehicle’s own ventilation system. “Driving around with the windows up and the air conditioning or heat on is definitely the worst scenario, according to our computer simulations,” he said. “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.”
The researchers used a simulated car “loosely based on a Toyota Prius” moving at 50 miles per hour, simulating the flow of air in and around it, “as well as the movement and concentration of aerosols coming from both driver and passenger.” They found that opened windows are a better way to reduce the risk of transmission than turning on the AC because “it increases the number of air changes per hour (ACH) inside the car, which helps to reduce the overall concentration of aerosols.” Still, they found, opening different combinations of windows creates different risks of exposure. Opening all four windows did the most to reduce exposure because it “creates two more-or-less independent flows on either side of the cabin,” because “air tends to enter the car through the back windows and exit through the front windows.” In simulations with all four windows open, there was very little particle transfer between the occupants, who again were sitting on opposite sides of the car. As Science Daily notes, the driver had a marginally increased risk of exposure than the passenger, simply because of the direction of airflow.
The study found additionally that opening the window next to each occupant carried a higher risk of exposure than opening the windows opposite them. As one of the study’s senior authors explained: “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 study’s authors emphasized that opening windows to manage a car’s airflow is not a substitute for wearing masks inside a car, that the risk of Covid-19 transmission cannot be completely eliminated, and that federal authorities recommend that people cancel travel plans and remain in their homes. More information on the study is available via Science Daily.