While many Americans are avoiding crowded spaces amid the novel coronavirus epidemic, it's harder for some to avoid riding in a car with others—particularly if they rely on taxis or services such as Uber or Lyft for transportation. But how risky is coronavirus transmission in a car? A new study sheds light—and some insight on how drivers and passengers can protect themselves against the pathogen.

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Aerosols that could spread the coronavirus can accumulate in cars, researchers warn

Citing their own modeling, Joseph Allen, a ventilation expert at the Harvard T.H. Chan School of Public Health; Richard Corsi, an air quality expert at Portland State University; and Jack Spengler, a professor at the Chan School and director of JPB Environmental Health Fellows Program in a 2020 op-ed warned that "fine aerosol particles" that can spread the novel coronavirus could accumulate in cars' cabins if people were in the cars with the windows closed. As such, they urged people to exercise caution when riding in cars with others, such as increasing ventilation in the vehicles by opening windows.

But to better understand how respiratory droplets and aerosols that can spread the novel coronavirus can accumulate inside a car, Varghese Mathai, a physicist at the University of Massachusetts-Amherst, and three of her colleagues at Brown University—Jeffrey Bailey, Kenneth Breuer, and Asimanshu Das—used computer stimulations to map the spread of airborne particles inside a vehicle. For the study, published earlier this month Science Advances, the researchers modeled air flow in a car that they loosely based on a Toyota Prius, driving at 50 miles per hour with a driver in the front left seat, a passenger in the back right seat, and the car's air conditioning turned on—a scenario that reflects common use of taxis and ride-services.

According to the researchers, their models showed that the air flow around the outside of the moving car created a pressure gradient inside the car, with slightly lower air pressure in the front of the car than in the back. That meant air circulating inside the car tended to flow from the back to the front of the car, the researchers found.

The researchers also modeled the air flow and movement of simulated aerosols with different combinations of closed and open windows in the car. For example, the researchers modeled the ventilation rate when the car had all four windows closed and found that, in this scenario, about 8% to 10% of aerosols exhaled by one of the car's occupants could reach the other person in the car. According to the researchers, that was the highest rate of all of the scenarios studied.

Increasing ventilation is key, researchers find

But in contrast, the researchers found that ventilation increased significantly when the car had all four windows open. According to the study, only 0.2% to 2% of the simulated aerosols moved between the driver and passenger when all the windows in the car were completely open, which was the lowest rate of all of the scenarios studied. Based on those findings, the researchers wrote that "the most effective way to minimize cross-contamination between the occupants is to have all the windows open."

But the researchers acknowledged in the study that "driving with all windows open might not always be a viable or desirable option, and, in these situations, there are some nonintuitive results that are revealed by the calculations.

According to the researchers, when people cannot roll down all of the windows in a car, they can best increase ventilation by having the driver and passenger open the windows opposite them, as opposed to the windows directly next to them. Having the opposite windows open in both the front and back allows air to flow through the back left window and out the right front window, which essentially creates a type of "air curtain" between the driver and passenger, Mathai said. "It flushes out all the air that's released by the passenger, and it also creates a strong wind region in between the driver and the passenger."

Experts raise questions, urge caution

Corsi applauded the new study, calling it "sophisticated." However, he cautioned that changing the number of passengers inside a car or the driving speed of a car could affect the airflow inside a vehicle. For instance, Corsi said that in a study that hasn't yet been published, he found a 20-minute car ride with a person who's emitting coronavirus particles can be riskier than being in a classroom or restaurant with an infected person for more than an hour, the New York Times reports.

Mathai said his study's findings most likely would hold true for many four-door, five-seat vehicles. And "[f]or minivans and pickups, I would still say that opening all windows or opening at least two windows can be beneficial," he said. "Beyond that, I would be extrapolating too much," Mathai added.

Mathai said he sent a copy of his study to Uber and Lyft, and he believes those companies should be encouraging their drivers and passengers to apply the findings. However, he hasn't yet received responses from the companies, the Times reports (Allen/Corsi, USA Today, 4/22/2020; Anthes, New York Times, 1/17; Mathai et al., Science Advances, 1/1).