An Easy Trick for Optimizing Fresh Air Flow Inside a Car

Source: juanwa/Pixabay

Decades ago, as a college student, I made a cross-country trip during summer break in a beat-up jalopy without air-conditioning. Unfortunately, opening all the windows while driving on the freeway made it hard to hear music playing on the cassette deck without cranking up the volume to deafening levels.

One day, as I was driving through the Arizona desert near the Grand Canyon, I happened to close the window opposite me in the front seat to make it quieter inside the cabin. Vroosh! I instantly felt a rush of refreshing, cool air push against the back of my head and neck. Even though it was over 100º and I thought all the windows were opened, I'd accidentally left the window behind me in the back seat closed. Much to my surprise, having these two kitty-corner windows closed created ventilation effects inside the car that were more robust than having all four windows opened.

From then on, whenever I needed to cool off, I'd open the window next to me in the driver's seat and the window opposite me in the back seat while leaving the other two windows closed. For reasons that I never fully understood, opening two windows in a kitty-corner configuration while keeping the other windows shut created airflow patterns inside my car that felt great and reduced the sound of howling wind when driving faster than 45 miles per hour.

At left is a computer simulation, at right a graphic illustration of the same situation. Mathai and colleagues suggest that if you cannot open all the windows, this may be a good seating arrangement and window adjustment scenario.

Source: UMass Amherst/Mathai lab, labeled for reuse

This morning, I had an "Aha!" moment while looking at the diagram above from a new study (Mathai et al., 2020) that illustrates how open/closed window configurations affect air flow paths in ways that may have implications for reducing airborne disease transmission. This study by a team of physicists from UMass Amherst's Mathia Research Group and scientists from Brown University's Center for Fluid Mechanics and Department of Pathology and Laboratory Medicine was published on December 4 in Science Advances.

According to the authors: "Our findings reveal the complex fluid dynamics during everyday commutes, and non-intuitive ways in which open windows can either increase or suppress airborne transmission."

"Transmission of highly infectious respiratory diseases, including SARS-CoV-2, is facilitated by the transport of exhaled droplets and aerosols that can remain suspended in air for extended periods of time. A passenger car cabin represents one such situation with an elevated risk of pathogen transmission," the authors explain. "Here, we present results from numerical simulations to assess how the in-cabin microclimate of a car can potentially spread pathogenic species between occupants, for a variety of open and closed window configurations."

Of note: During the COVID-19 pandemic, the researchers stress that everyone should always wear a mask or face covering when traveling inside a passenger car with anyone from outside their immediate household. The point of this study isn't to eliminate the need for face masks or to give people a false sense of security; traveling inside a passenger vehicle with anyone who might be contagious and shedding an airborne virus will always have risks and should be avoided if possible.

That said, if for some reason you find yourself needing to take a Lyft or Uber somewhere and are alone in the car with a driver, your first instinct might be to open the window right next to you. However, the latest research by Varghese Mathai and colleagues suggests that "opening the car window closest to you isn't always the best option to protect yourself from coronavirus or any airborne infection."

Although it's counterintuitive, this research suggests that the best combination for optimizing airflow patterns and potentially reducing transmission risk when there is one person in the driver's seat and one passenger in the back seat is for each occupant to sit diagonally across from one another and to open the window on the opposite side of where he or she is sitting.

"One might imagine that people instinctively open windows right beside them while riding with a co-passenger during the pandemic. That may not be optimal—though it's better than opening no window," Mathai said in a news release.

"We designed this research with ride-sharing in mind, from a traditional taxi or Uber and Lyft to noncommercial commutes, assuming a driver and one passenger, seated in the back on the passenger side to provide the best possible spacing between the occupants," he added. "To our surprise, the simulations showed an air current that acts like a barrier between the driver and the passenger."

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In a four window passenger car going over 50 mph, having the kitty-corner front and back windows open (and the other two windows and moon-roof closed) causes fresh air to rush into the vehicle from the back window and exit via the window next to the front "shotgun" passenger seat.

"We expect fully open windows to be the most efficient at reducing the contamination of the cabin environment," the authors note. "The flow patterns resulting from partially open windows, which can be a common driving setting, will be the focus of a future investigation."

"While these measures are no substitute for wearing a face mask while inside a car, they can help reduce the pathogen load inside the very confined space of a car cabin," Mathai concluded.

UMass Amherst/Mathai lab image via EurekAlert