How to reduce COVID-19 risk in a car

A study shows the airflow patterns in a passenger car and how open windows can increase or decrease the risk of COVID-19 transmission.

In a unique study, researchers looked at how changes in airflow in a car can worsen or reduce the risk of COVID-19 transmission. They observed that opening windows – the more windows the better – created airflow patterns that drastically reduced the concentration of airborne particles exchanged between a driver and an individual passenger. Blasting the car's ventilation system or turning on the heat didn't circulate nearly as well as a couple of open windows.

The study results were published in the journal Science Advances.

The team of scientists led by Dr. Varghese Mathai of Brown University in the United States performed numerical simulation experiments on various open and closed window configurations to determine how this would affect the risk and whether it would reduce the risk of COVID-19 transmission.

The computer models used in the study simulated a car (applicable to most passenger cars with four windows) with two people inside – a driver and a front passenger, sitting in the back seat on the opposite side of the driver. The idea behind choosing this distance was to maximize the physical distance between the two people – even if it wasn't the six feet recommended by the Center for Disease Control and Prevention (CDC).

The computer modeling simulated the flow of air around and in a car moving at a speed of 50 miles per hour, as well as the movement and concentration of aerosols coming from both the driver and front passenger.

Aerosols are tiny particles that can remain in the air for a long time. This is believed to be one way of transmitting the COVID-19 virus, especially in an enclosed space like a car.

The scientists suggested that part of the reason that opening windows is better for aerosol transfer is because it increases the number of air changes per hour in the car, reducing the overall concentration of aerosols.

But air changes per hour are only part of the story.

In a moving car, the air pressure near the rear windows tends to be higher than the pressure in the front windows. As a result, air tends to enter the vehicle through the rear windows and exit through the front windows.

With the windows open, this tendency creates two more or less independent currents on either side of the cabin. Since the occupants were sitting on opposite sides of the cabin, very few particles were transferred between them.

In this scenario, however, the driver is at a slightly higher risk of COVID-19 than the passenger because the average airflow of the vehicle is from the back to the front. Nevertheless, both occupants experience a dramatically lower particle transfer compared to any other scenario.

The simulations for scenarios where some, but not all, of the windows shut down may have produced counter-intuitive results. For example, one might expect that opening windows right next to each occupant would be the easiest way to reduce exposure. The simulations showed that this configuration is better than no windows at all, but poses a higher risk of exposure than turning off the window for every occupant.

When the windows are open to the occupants, a current is created that enters the car behind the driver, sweeps over the cab behind the passenger and then exits the front window on the passenger side. This pattern helps reduce cross-contamination between the driver and passenger.

The researchers stressed that adjusting the airflow is not a substitute for wearing masks – as recommended by the CDC – that both occupants wear in the car. In addition, the results are limited to possible exposure to lingering aerosols that may contain pathogens.

The study did not model larger respiratory droplets or the risk of actual infection with the virus.

Still, the researchers say the study offers valuable new insights into airflow patterns in the passenger compartment of a car – something that has received little attention so far.

Reference: Mathai V., Das A., Bailey JA., Breuer K. Airflow in Passenger Cars and Effects on Airborne Disease Transmission. Advances in science. 2020 December 4th; eabe0166. doi: 10.1126 / sciadv.abe0166.

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