The Worst Time to Roll Down Your Car Windows

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When cruising the open road, by all means roll down your windows and feel the breeze. But when driving in cities or crawling in heavy highway traffic, keep them shut—and turn off your AC or heater, too. Do this, and you’ll slash the amount of toxic particles pouring into your car by a whopping 76 percent.

Thank researchers at the University of Surrey in England for this lung- and heart-saving tip. “Vehicles emit a number of gaseous pollutants, as well as particles of various sizes,” says lead investigator Prashant Kumar. “All of these are known to harm human health, including increasing risk of respiratory and cardiovascular diseases." In fact, the World Health Organization has placed outdoor air pollution among its top 10 risks faced by humans, believing it to be linked to 7 million premature deaths every year.

Last year, Kumar’s team determined that the concentration of harmful particles is 29 times higher near signaled intersections than on stretches of road where traffic flows freely. “When vehicles stop at red lights, they go through different driving cycles such as deceleration, idling, and acceleration,” all of which kick out toxic emissions, he explains. Compounding the problem, there are oftentimes dozens of vehicles queued up at red lights, creating a cloud of pollutants that takes a long time to disperse.

This is obviously bad news for pedestrians waiting for crosswalk signals to turn. But just how much pollution are drivers exposed to within their cars when idling at intersections or riding bumper-to-bumper? And how much do open windows and ventilation systems affect this? The researchers got behind the wheel to find out.

For their latest study, published in Environmental Science: Processes and Impacts, they completed 74 test-drives along a six-kilometer city loop. The route included 10 traffic stops, and at various points along the drive, they measured the concentration of particles polluting the car’s interior. Overall, they found that 40 percent more particles lingered inside the car while it idled at intersections compared to when it was cruising.

The researchers also tinkered with different ventilation settings from one test-drive to the next; they also did some drives with the windows up and others with them down. Open windows drew in more large particles, while ventilation systems filtered more of those out. However, filtration-only caused the in-car concentration of fine particles to increase, making neither option safe.

“When the windows were closed but the fan was on, the exposure to the more hazardous fine particles was usually the highest,” says Kumar. “Once the fan sucks dirty outside air into the vehicle, it takes some time to dilute or escape, resulting in a large accumulation of pollutants inside.” And because you’re essentially trapped in a box with polluted air until it dilutes, your exposure at intersections is actually six times greater than that of a pedestrian, the researchers found.

Kumar says the best way to protect yourself in dense traffic is to batten down the hatches. “Keeping windows shut and your fan and heat off can reduce the concentration of the smallest, most hazardous particles by up to 76 percent,” he explains. “If the fan or heater needs to be on because of weather, have the air recirculate within the car so you’re not drawing in dirty air from outdoors.”

Another way to reduce your risk — especially when you have no choice but to crank the AC or heat — is to avoid riding bumpers in heavy traffic. Don’t pull up right behind other cars at stoplights, either. “Keeping as much distance from other cars as possible can also help cut the emissions coming inside your vehicle,” Kumar says.  

This newest study about traffic-jam toxins adds to a body of science that proves sitting in traffic is bad for your health. Aside from backaches and stress, a study published in the American Journal of Preventative Medicine, for example, found commuters who drove longer distances to work were less likely to exercise, had a greater body mass index, bigger waists, and higher blood pressure. The bottom line: the less time you spend in your car — with windows up-or-down — the better.