A new series of computational fluid dynamics simulations suggests that, for two people who must travel together in the same passenger car, the safest way to prevent possible transmission of COVID-19 in such a risky, enclosed environment is to do so with all four windows down and the passenger seated as far as possible from the driver, in the rear seat on the opposite side. Varghese Mathai and colleagues found that this modeled configuration created two distinct flows of air in the car’s cabin, separated along the midline of the car and moving – perhaps counterintuitively – from the rear towards the front of the car, due to exterior pressure differentials dictated by the car’s aerodynamics. This separated airflow configuration was the most effective at reducing the transmission of simulated infectious droplets from either driver to passenger, or vice versa. However, recognizing that such a breezy configuration may be less desirable for many travelers, the researchers also tested the opposite scenario – all four windows up – as well as four other scenarios with either one or two windows closed. The fully enclosed scenario, which relied only on simulated, non-recirculated airflow from the car’s air conditioning system, was the riskiest of all six simulated scenarios, conferring the highest risk of droplet transmission. Traveling with three open windows fared better than only two open windows, but the researchers found that choosing which window to close may in fact matter a great deal. In scenarios that simulated either an infected driver or an infected passenger, closing only the window closest to the non-infected person conferred the greatest protection, second only to the scenario with all four windows open. Mathai et al. note that their simulations, based on an idealized model sedan patterned after the body shape of a Toyota Prius, may not accurately reflect airflow dynamics in other vehicles such as trucks, minivans, and cars with an open sunroof. They also note that their models may also miss some other nuances of airflow and particle residence times that may result from, for example, strong crosswinds or otherwise exceptionally windy conditions. All the same, the authors conclude that “these results will have a strong bearing on infection mitigation measures for the hundreds of millions of people driving in passenger cars and taxis worldwide, and potentially yield to safer and lower-risk approaches to personal transportation.”
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