Study Shows Which Car Windows to Roll Down to Keep Covid at Bay

By Kurt Stolz on 16 January 2021
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Interior of 2019 Toyota Prius

The interior of a passenger car represents an elevated risk of coronavirus transmission and drivers and passengers have long since been told to roll down the windows to keep aerosols and droplets at bay.

But which windows should one lower, especially if one doesn’t wish to have all four down?

A new study by researchers at the University of Massachusetts and Brown University has the answer. 

Noting that, even with proper mask wearing, “the in-cabin microclimate during these rides falls short on a variety of epidemiological guidelines,” they found that a solution that might be counter-intuitive to some creates “an airflow pattern that travels across the cabin, farthest from the occupants” will reduce the risk of virus transmission.

The researcher team, which included Varghese Marthai, a professor of physics at the University of Massachusetts who focuses on dispersed multiphase fluids and soft matter, and Kenneth Breuer, a professor at Brown University who heads the Breuer Lab there that is focused on experimental fluid mechanics, found that the safest solution where it is undesirable to open all four is to open the windows opposite each occupant in order to accomplish create such an airflow pattern, coincidentally a pattern that FBT Editorial Director Jonathan Spira wrote about in the spring, although he admittedly did not conduct extensive research on it.

To solve this riddle, the researchers simulated the cabin of a vehicle based on a recent model Toyota Prius that was traveling at 50 mph (80 km/h) with two occupants, a driver in the front left seat and a passenger in the right rear seat.  This is a seating arrangement that is common in many situations including taxi rides and trips made with ride-hailing services such as Uber and Lyft.  Sitting in such a manner also maximizes social distancing within a vehicle, even in those with a partition.

The team found that the optimal ventilation pattern to flush droplets and aerosols out of the vehicle was to have the windows opposite each occupant open, namely having the front right and left rear ones down.  Not only did this dramatically increase the airflow in the vehicle, but the stream of air acts as an air curtain, the researchers said in the study.

This configuration allowed air to flow in through the back left window and out through the front right window, plus it also created a barrier between driver and passenger.

The configuration with all windows closed was, predictably, the worst in terms of minimizing cross-contamination between driver and passenger and the configuration with all windows open was the best as the streamlines obey left-right symmetry. 

It is the compromise configuration of having the windows opposite each occupant open that creates airflows that minimize the risk of cross-contamination, the researchers concluded, noting that this configuration does not cause the noise and drafts associated with having all four windows open.

The research is applicable to right-wheel drive vehicles in a mirrored configuration but it may not be applicable, the scientists said, to SUVs, minivans, and vehicles with an open sunroof.  In addition, as the simulation was conducted at speeds of 50 mph (80 km/h), the results at other speeds will likely vary.

(Photo: Accura Media Group)