Pollution Control: Shut Windows, Crank The AC For Healthiest Commute, Study Finds

ST. LOUIS — You may assume keeping your windows open when driving will circulate your car with clean air and filter out as much air pollution as possible, but a new study finds you’re actually better off turning on the AC instead.

Engineers from Washington University in St. Louis sought to gauge pollution levels in their car during drives to and from the school during a four-month testing period in 2014. Measurements would be taken in various scenarios where pollutants may be more concentrated, such as when being stuck in heavy highway traffic, traveling behind buses or big rigs, passing construction zones, or idling at traffic lights.

“We know that traffic generates a lot of pollution, and therefore it’s the time when you’re traveling in traffic that you can have a disproportionately high amount of your daily exposure to many harmful pollutants,” explains co-author Anna Leavey, a research scientist at the university’s engineering school, in news release. “What we wanted to see was: When and where are our highest exposures occurring, and how should one be driving to mitigate the risk?”

In 2009, The U.S. Census Bureau calculated that it takes the average American nearly about 27 minutes to get to work, rounding out to an hour of travel each day. With such a lengthy commute, it’s vital that drivers avoid as much toxic air as possible. Air pollutants have been connected to cardiovascular disease, respiratory issues, and lung cancer.

Researchers used portable instruments, sensors, and dashcams to keep track of the pollutant levels of their car’s indoor cabin air and the direct air that from outside of the car during their test commutes. Measurements were taken when the windows were both up and down, and when the air conditioner or fan was on and off.

“As aerosol scientists, we had access to state-of-the-art air monitoring equipment,” says co-author Nathan Reed.

The researchers found that anywhere from 20% to 34% of air pollution was reduced by using the air conditioner during a commute, depending on the various metrics, concentration levels of pollution outside, weather, and road conditions. In fact, there were no measurements of carbon dioxide concentrations during 75 percent of the rides in which AC was used.

Taking in all factors, drivers can expect an 8% to 44% percent “protective boost” against polluted air with closed windows.

“The vehicle cabin can be viewed as a buffer, protecting us from the outside air,” says Leavey. “While driving with your air conditioning on and windows closed is the most protective thing that you can do, running the AC can decrease your fuel economy.”

Leavey suggests keeping the AC on and windows shut when on the highway or if you’re stuck behind larger vehicles that expel more pollutants.

“Once you have left the polluted environment, we recommend opening your windows to remove any pollutant build-up from your car,” she says.

This study’s findings were published in October in the journal Atmospheric Environment.