As the traffic jam grows longer, you fuss, fume and complain about the inconvenience. But traffic jams pose a threat to more than just your blood pressure and sanity: As your car sits, unhealthy air builds up inside.

Researchers in England found that the air inside a traffic-bound car is substantially dirtier than in a moving car — up to 29 times more polluted. So, what’s behind the problem? The simple structure of a traffic jam. Vehicles bunch up, leading to a concentration of exhaust fumes. Then traffic inches along, increasing drivers’ exposure to the chemicals created by combustion. The findings were published in the journal Environmental Science: Processes & Impacts.

To measure pollution inside vehicles, researchers tested particulate levels as a car traveled nearly 4 miles through 10 traffic lights. Air samples were taken at three-way and four-way intersections with various settings on the car’s ventilation system.

The highest levels of pollution were found when the car’s windows were closed at the traffic lights and the fan was on. Researchers said that’s because the vent fan pulls in dirty air from the outside, where it collects in the car’s sealed interior.

Fortunately, there’s an easy fix. The researchers suggest closing your car windows and turning off your fan during a traffic jam. That alone can reduce the chance of inhaling polluted air by 76%. If a closed car with no fan makes it just too hot, keep the windows closed and set your ventilation to recirculate air internally. That keeps more pollution out.

So, when traffic starts backing up, relax and take a deep breath — but close your windows first.