A study reveals the airflow patterns inside a passenger car and how open windows can increase or decrease COVID-19 transmission risk.
Researchers studied how airflow changes inside a car may worsen or reduce COVID-19 transmission risk in a first-of-its-kind study. They observed that opening windows – the more windows, the better – created airflow patterns that dramatically reduced the concentration of airborne particles exchanged between a driver and a single passenger. Blasting the car’s ventilation system or putting on the heat didn’t circulate air nearly as well as a few open windows.
The study results were published in the Journal Science Advances.
The team of scientists led by Dr. Varghese Mathai from Brown University in the United States conducted numerical simulation experiments for various open and closed window configurations to see how this would impact risk, and if this would reduce COVID-19 transmission risk.
The computer models used in the study simulated a car (valid for most four-windowed passenger vehicles) with two people inside – a driver and a passenger sitting in the back seat on the opposite side from the driver. The idea behind choosing this distance was to maximize the physical distance between the two people – even though it was not the six feet recommended by the Centre for Disease Control and Prevention (CDC).
The computational modelling simulated airflow around and inside a car moving at 50 miles per hour and the movement and concentration of aerosols coming from both driver and passenger.
Aerosols are tiny particles that can linger in the air for extended periods. They are thought to be one way the COVID-19 virus is transmitted, particularly in enclosed spaces, such as a car.
The scientists suggested that part of the reason that opening windows is better in terms of aerosol transmission is because it increases the number of air changes per hour inside the car, which reduces the overall concentration of aerosols.
But air changes per hour is just part of the whole story.
In a moving car, air pressure near the rear windows tends to be higher than the front windows’ pressure. As a result, air tends to enter the vehicle through the back windows and exit through the front windows.
With all the windows open, this tendency creates two more-or-less independent flows on either side of the cabin. Since the occupants were sitting on opposite sides of the cabin, very few particles were transferred between them.
However, the driver in this scenario is at a slightly higher risk of COVID-19 than the passenger because the car’s average airflow goes from back to front. Still, both occupants experience a dramatically lower transfer of particles compared to any other scenario.
The simulations for scenarios in which some but not all windows are down yielded some possibly counterintuitive results. For example, one might expect that opening windows directly beside each occupant might be the simplest way to reduce exposure. The simulations found that while this configuration is better than no windows down at all, it carries a higher exposure risk than putting down the window opposite each occupant.
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. This pattern helps to reduce cross-contamination between the driver and passenger.
The researchers highlighted that airflow adjustment is no substitute for mask-wearing – as recommended by the CDC – by both occupants inside the car. Moreover, the findings are limited to potential exposure to lingering aerosols that may contain pathogens.
The study did not model larger respiratory droplets or the risk of actually becoming infected by the virus.
Still, the researchers say the study provides valuable new insight into air circulation patterns inside a car’s passenger compartment – something that had received little attention before now.
Reference: Mathai V., Das A., Bailey JA., Breuer K. Airflow inside passenger cars and implications for airborne disease transmission. Science Advances. 2020 Dec 4; eabe0166. doi:10.1126/sciadv.abe0166.
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