A new study of airflow patterns inside a car offers some suggestions for potentially reducing the risk of COVID-19 transmission while sharing rides with others.

The researchers used computer models to simulate the airflow inside a compact car with various combinations of windows open or closed.

The simulations showed that opening windows—the more windows the better—created airflow patterns that dramatically reduced the concentration of airborne particles exchanged between a driver and a single passenger.

Blasting the car’s ventilation system didn’t circulate air nearly as well as a few open windows, the researchers found.

“Driving around with the windows up and the air conditioning or heat on is definitely the worst scenario, according to our computer simulations,” says co-lead author Asimanshu Das, a graduate student in Brown University’s School of Engineering.

“The best scenario we found was having all four windows open, but even having one or two open was far better than having them all closed,” Das says.

The researchers stress that there’s no way to eliminate risk completely—and, of course, current guidance from the US Centers for Disease Control (CDC) notes that postponing travel and staying home is the best way to protect personal and community health. The goal of the study was simply to study how changes in airflow inside a car may worsen or reduce the risk of pathogen transmission.

The computer models used in the study simulated a car, loosely based on a Toyota Prius, with two people inside—a driver and a passenger sitting in the back seat on the opposite side from the driver. The researchers chose that seating arrangement because it maximizes the physical distance between the two people (though still less than the 6 feet recommended by the CDC). The models simulated airflow around and inside a car moving at 50 miles per hour, as well as the movement and concentration of aerosols coming from both driver and passenger.

Aerosols are tiny particles that can linger in the air for extended periods of time. They are thought to be one way in which the SARS-CoV2 virus is transmitted, particularly in enclosed spaces.

Part of the reason that opening windows is better in terms of aerosol transmission is that it increases the number of air changes per hour (ACH) inside the car, which helps to reduce the overall concentration of aerosols. But ACH was only part of the story, the researchers say.

The study showed that different combinations of open windows created different air currents inside the car that could either increase or decrease exposure to remaining aerosols.

Because of the way air flows across the outside of the car, air pressure near the rear windows tends to be higher than the pressure at the front windows. As a result, air tends to enter the car through the back windows and exit through the front windows. With all the windows open, this tendency creates two more-or-less independent flows on either side of the cabin. Since the occupants in the simulations were sitting on opposite sides of the cabin, very few particles end up being transferred between the two. The driver in this scenario is a slightly higher risk than the passenger because the average airflow in the car goes from back to front, but both occupants experience a dramatically lower transfer of particles compared to any other scenario.

The simulations for scenarios in which some but not all windows are down yielded some possibly counterintuitive results. For example, one might expect that opening windows directly beside each occupant might be the simplest way to reduce exposure. The simulations found that while this configuration is better than no windows down at all, it carries a higher exposure risk compared to putting down the window opposite each occupant.

“When the windows opposite the occupants are open, you get a flow that enters the car behind the driver, sweeps across the cabin behind the passenger, and then goes out the passenger-side front window,” says Kenny Breuer, a professor of engineering and senior author of the research. “That pattern helps to reduce cross-contamination between the driver and passenger.”

It’s important to note, the researchers say, that airflow adjustment are no substitute for mask-wearing by both occupants when inside a car. And the findings are limited to potential exposure to lingering aerosols that may contain pathogens. The study did not model larger respiratory droplets or the risk of actually becoming infected by the virus.

Still, the researchers say the study provides valuable new insights into air circulation patterns inside a car’s passenger compartment—something that had received little attention before now.

“This is the first study we’re aware of that really looked at the microclimate inside a car,” Breuer says. “There had been some studies that looked at how much external pollution gets into a car, or how long cigarette smoke lingers in a car. But this is the first time anyone has looked at airflow patterns in detail.”

The study appears in the journal of Science Advances. Das co-led the research with Varghese Mathai, a former postdoctoral researcher at Brown who is now an assistant professor of physics at the University of Massachusetts, Amherst.

AstraZeneca and Oxford University have more work to do to confirm whether their COVID-19 vaccine can be 90 percent effective, peer-reviewed data published in The Lancet showed on Tuesday, potentially slowing its eventual rollout in the fight against the pandemic.

Once seen as the frontrunner in the development of a vaccine against the coronavirus crisis, the British team was overtaken by U.S. drugmaker Pfizer, whose shots — with a success rate of around 95 percent — were administered to the U.K. pensioners on Tuesday in a world-first hailed as V-Day.

Detailed results from the AstraZeneca/Oxford trials have been eagerly awaited after some scientists criticized a lack of information in their initial announcement last month.

However, the Lancet study gave few extra clues about why efficacy was 62 percent for trial participants given two full doses, but 90 percent for a smaller sub-group given a half, then a full dose.

“[This] will require further research as more data becomes available from the trial,” the study said.

Less than six percent of U.K. trial participants were given the lower dose regimen and none of them were over 55, meaning more research will be needed to investigate the vaccine’s efficacy in older people who are particularly susceptible to COVID-19.

Pooling the results, overall efficacy was 70.4 percent, the data on Tuesday showed. That is above the 50 percent minimum set by the U.S. Food and Drug Administration.

Efficacy reflects how well a vaccine works in clinical trials. Effectiveness is how well a vaccine or a product will work in the real world rather than ideal settings.

Vaccine efficacy levels vary

COVID-19 vaccines from Pfizer and German partner BioNTech and from Moderna have reported efficacy levels of more than 90 percent in late-stage trials.

But the AstraZeneca/Oxford vaccine is seen as particularly important to tackling the pandemic in the developing world, as it would be cheaper and easier to distribute.

“The basic message: that the overall efficacy across the trials that are reported here is about 70 percent but with a clear description of its uncertainty,” said Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.

“The statistical uncertainty is that the efficacy could be as low as 55 percent or as high as 80 percent. The Pfizer/BioNTech and Moderna vaccines both have efficacies above 90 percent and are clearly more efficacious under trial conditions.”

Asked whether the half, then full dose regimen had been a mistake, Andrew Pollard, director of the Oxford Vaccine Group and chief investigator into the trials, told a news conference it had been “unplanned.”