Suppose a person is driving a car with a passenger. Because of the pandemic, the two sit as far apart as can be possible inside a car — the passenger in the rear seat diagonally opposite the driver. How much of the air exhaled by any one of them (whether carrying coronavirus or not) can be inhaled by the other?
It depends on how many of the car windows are open, which ones are open, and the direction in which the air is flowing. These air flow dynamics are the subject of a new study published in the journal Science Advances.
What’s the point?
Scientists already understand that keeping all four windows open in a moving car would offer the best protection from airborne transmission of pathogens. And keeping all four shut would carry the highest risk, even with air conditioning on. But it is not always practical to keep all windows open —it might be raining, or very cold outside.
So, Varghese Mathai from the University of Massachusetts–Amherst, who led the new study, and co-lead Asimanshu Das of Brown University, US, examined the flow of air inside such a car under various configurations of open and closed windows — all windows open, all four closed, or two open, or three open.
The multiple-author research was modelled on a Toyota Prius travelling at 50 miles per hour (roughly 80 km/hr), with the passenger sitting diagonally behind the driver.
The chart shows six configurations examined in the study, arranged in order from highest risk of aerosol transmission to highest risk. The lowest-risk configuration (all windows open) causes nearly four times the air exchange rate as compared to the highest-risk configuration that was simulated (all windows shut, with AC on), Mathai told The Indian Express.
A higher air change rate — the number of time the air changes per hour inside the car — helps to reduce the overall concentration of aerosols. But “the air change rate alone is not the important factor, but the air flow directions are also important,” Mathai said, by email. 📣 Follow Express Explained on Telegram
A surprise among findings
Instinct might tell the passenger to open the window next to her, and the driver to roll down his own window. As it turns out, this is not the best configuration for two-windows-open-and-two-shut. While keeping all windows open is always better than keeping only two open, the study found that if only two windows are to be kept open, the ideal pair would be the windows opposite the driver and the passenger in the front and the rear respectively. In an Indian setting in which the driver sits front right, this would mean keeping open the front left and the rear right windows (the passenger would be sitting rear left).
The illustration, adapted by Das for an Indian setting (right-hand drive) and provided by Mathai to The Indian Express, shows how the direction of air flows when these two windows are open.
Driver at higher risk
In general, the driver appears to be at a slightly higher risk. This is because in a moving car, most of the air tends to enter the cabin from the rear windows, and exit from the front windows. But when all the windows are open, this tendency creates two more-or-less independent flows on the left and right sides of the car. Since the simulations have seated the driver and the passenger on opposite sides, very few particles are transferred between the two.
“Note that once we have two or more windows opened, the concentration of airborne particles does not build up much, as there is good cross-ventilation and dilution of air established,” Mathai said.
Keeping three open windows is obviously better than only two open windows, but again, choosing which window to close makes a difference. Two scenarios simulate either an infected driver or an infected passenger. In such cases, a relatively safe option is to close only the window closest to the non-infected person. The only scenario that offers better protection is keeping all four windows open, the study found.
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