COVID-19 has completely changed the face of travel as we know it, with massively reduced numbers of people taking flights and public transportation—but our cars have remained a mystery. How safe are we in our vehicles? What are our risks?

A study published by Science Advances in early January has begun to answer some of our most pressing questions about COVID-19 transmission in our vehicles. Four scientists from the University of Massachusetts, Amherst and Brown University used computational fluid dynamics to evaluate the risks posed by the virus within a vehicle’s cabin and have also suggested ways to mitigate risk.

If you’re familiar with the design process of a race car or airplane, then you’ve likely encountered computational fluid dynamics before. Essentially, these computer simulations model how gases and liquids move over and through different surfaces. In this particular case, our scientists used CFD to model the way air moves inside a car.

The simulated vehicle used in the study was loosely based on a Toyota Prius traveling at 50 mph carrying two passengers: a driver in the front left of the car and a passenger in the back right. Interestingly, the air flow outside the moving car creates a pressure gradient inside the car that causes air to circulate from the back of the car to the front. Then, they started modeling the interior air flow with different combinations of the windows being open or closed. It’s important to note here that, no matter the combination, the air conditioning was on.

The results probably aren’t going to be surprising. When all four windows were closed, the car was at its most poorly ventilated, so eight to 10 percent of aerosols—on which COVID-19 travels—exhaled by one person in the car traveled to the other. When all the windows were open, the car was at its best ventilated, with just 0.2 to two percent of aerosols swapping between passengers.

Of course, wide open windows aren’t always practical when you’re driving. Up north, you’ll freeze in the winter. Down south, someone with a delicate constitution will melt in the summer. A heavy rain will make things twice as miserable. So, having both the driver and the passenger roll down their windows was found to be better than keeping everything shut tight. That diagonal configuration allows air to flow in and then right back out. It might not be comfortable, but it could save lives.

A later study that hasn’t yet been published found that cracking windows halfway was also a good idea, but only rolling them a quarter of the way down was significantly more dangerous, the . For larger vehicles like minivans or for vehicles transporting more people, the recommendation is to keep everything open.

Opening windows has been recommended since the onset of the virus. The increased ventilation allows virus particles to be whisked away rather than recirculated. And we also know that the smaller the space we share, the more likely we are to swap aerosol particles. This study basically just used science to give us the ideal strategy for, say, rideshares or short jaunts outside your bubble.

Of course, there are still dangers, even when opening your windows. In fact, driving with your windows open by 80 percent, which thus increases your likelihood of dying as a result of air pollution.

The very best option is, of course, to stay home unless absolutely necessary and, when traveling, to do so in off-peak hours.