Updated: December 8, 2020 9:59:41 pm
Morgan Freeman and Jessica Tandy in the Oscar-winning film ‘Driving Miss Daisy’ (1989). The rear seat diagonally opposite the driver provides the greatest possible distance between him and the passenger. In an Indian environment, the driver would be seated in the front right and the passenger in the rear left.
Suppose a person drives a car with a passenger. Due to the pandemic, the two of them sit as far apart as possible inside a car: the passenger in the back seat diagonally opposite the driver. How much of the air exhaled by either of them (whether they carry the coronavirus or not) can be inhaled by the other?
It depends on how many windows in the car are open, which ones are open, and the direction the air is flowing. These airflow dynamics are subject to a new study published in the journal Science Advances.
What is the point?
Scientists already understand that keeping all four windows open in a moving car would offer the best protection against the transmission of airborne pathogens. And keeping all four closed would carry the greatest risk, even with the air conditioning on. But it is not always practical to keep all windows open; it may be raining or very cold outside.
So Varghese Mathai from the University of Massachusetts-Amherst, who led the new study and co-director Asimanshu Das from Brown University, USA, examined the airflow inside such a car under various settings of windows open and closed, all windows open, all four closed, or two open, or three open.
The multi-author research was based on a Toyota Prius traveling at 50 miles per hour (approximately 80 km / h), with the passenger sitting diagonally behind the driver. 📣 Follow Express explained on Telegram
The graph shows six configurations examined in the study, ordered from highest risk of aerosol transmission to highest risk. The lowest-risk setting (all windows open) causes nearly four times the rate of air exchange compared to the highest-risk setting that was simulated (all windows closed, air conditioning on), Mathai said. The Indian Express.
This graph shows six configurations examined in the study
A higher air change rate (the number of times the air changes per hour inside the car) helps reduce the overall concentration of aerosols. But “the rate of air change alone is not the important factor, but the airflow directions are also important,” Mathai said by email.
A surprise between findings
Instinct might tell the passenger to open the window next to her and the driver to lower his. It turns out that this is not the best setting for two open and two closed windows. While keeping all windows open is always better than keeping just two open, the study found that if only two windows were kept open, the ideal pair would be the windows opposite the driver and passenger in the front and rear, respectively. In an Indian environment where the driver sits front right, this would mean keeping the front left and rear right windows open (the passenger would be sitting behind on the left).
The illustration, adapted by Das for an Indian setting (right hand drive) and provided by Mathai to The Indian Express, shows how the air flows when these two windows are open.
This illustration shows how air flows when these two windows are open.
Higher risk driver
Overall, the driver appears to be at slightly higher risk. This is because in a moving car, most of the air tends to enter the cabin through the rear windows and exit through 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 passenger on opposite sides, very few particles are transferred between the two.
“Note that once we have two or more windows open, the concentration of particulate matter in the air does not increase much, as there is good cross ventilation and an established air dilution,” said Mathai.
Keeping three windows open is obviously better than just two windows open, but again, choosing which window to close makes a difference. Two scenarios simulate an infected driver or an infected passenger. In such cases, a relatively safe option is to close only the window closest to the uninfected person. The only scenario that offers better protection is keeping all four windows open, the study found.
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