By now, we all know the basic coronavirus rules. Wear a mask, limit your exposure to other people and when you can't, keep your distance, and ... keep wearing your mask. We also learned at the outset of the pandemic how to properly keep surfaces clean, including in your car. But what we haven't known is whether we should keep the windows up or down.
A new study from Brown University seeks to answer that question. Using complex computational fluid dynamic simulations, Varghese Mathai, Asimanshu Das, Jeffrey Bailey and Kenneth Breuer studied how the aerosols that we breath (some of which do escape even properly worn masks) move about a car's cabin and how those flows change with various windows up and down. The results are very interesting, but in short, you're probably going to want the windows rolled down when possible.
The details of the study are worth noting. The vehicle used in the simulations was loosely based on a Toyota Prius, and likely apply to vehicles of similar shape and size but may not be applicable to larger or smaller cars, trucks or vans. The occupants sat diagonally from one another, which is a common arrangement in taxis and rideshares. The team of researchers found that opening the windows opposite of each occupant can create a flow that drastically reduces the collection of aerosols in a car's cabin. According to the New York Times, they also found that opening the windows even halfway can be very helpful but that just cracking them a bit doesn't generate enough airflow.
There's a lot more information in the study that you can read here. There's also some additional information from the Times that's worth checking out.