Pollution levels inside cars at red lights or in traffic jams are up to 40 percent higher than when traffic is moving, according to a new study that raises questions about indoor air quality dangers to motorists.

For the study, published in the journal Environmental Science: Processes & Impacts, researchers at the University of Surrey in England examined the effects of traffic on concentrations of polluting particles, The New York Times reports.

They also tracked how ventilation settings changed those concentrations inside of cars.
In a car stuck in traffic, the researchers found pollution exposure was more than six times greater in cars with open windows than for pedestrians at three- or four-way intersections.

The results also showed an increase in smaller, more dangerous particles inside the vehicle compared with larger ones when the heat was off and fans were on full blast, drawing in air from outside.

But shutting all the windows and turning off the fan or heat reduced concentration doses of the smallest, most hazardous particles by up to 76 percent.

More than 80 percent of people living in cities where pollution is tracked are exposed to toxic levels above those considered safe by the World Health Organization.

The WHO estimates poor air quality is responsible for the deaths of 3.7 million people younger than 60 in 2012.