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How much air pollution reaches people who are driving their vehicles with their windows shut? Researchers at USC's Keck School of Medicine have published their findings.

They say the single best thing you can do to protect yourself is roll up the windows and set your vehicle’s ventilation system to ‘recirculate.’

If you’re sitting in traffic on the freeway, they say you’re in a zone where air pollution can be five to 10 times higher than in surrounding areas. Even inside your vehicle, you’re probably breathing in pollutants through the windows or the air vents. After an hour-long commute, you’ve likely doubled your daily exposure to the harmful particles in vehicle exhaust.

Using that setting -- typically a button that shows a car with an arrow inside -- can cut pollution concentrations inside a typical car to 20% of on-road levels, scientists found. When you use a ventilation setting that draws in outside air, they found, you are exposed to up to 80% of the levels of pollution found in traffic, even with the windows up.

The USC Keck School of Medicine researchers made the findings after testing dozens of vehicles, driving them on Los Angeles-area freeways and roads to find out how much air pollution reaches people inside their vehicles.

The study, published in the journal Environmental Science & Technology, measured exposure to air pollution by vehicle age, speed and pollutant type. It turns out that less pollution gets through from outside air in newer cars, at slower speeds and on roads with lighter traffic.

“As you drive faster and your car gets older, it tends to get leakier,” Scott Fruin, a co-author of the study told the LA Times. “But you always appear to benefit by having that ‘recirculate’ setting on.”

Co-author Neelakshi Hudda, a research associate at Keck, said the study was the first to systematically measure the air pollution to which people are exposed inside a large variety of vehicles types and conditions, including older models with high-mileage, newer compact cars and spacious SUVs.

Scientists also tested for a wide range of pollutants, from particulate matter -- also known as soot -- to ultrafine particles so tiny they can enter the bloodstream. Particle pollution is linked to respiratory illness, heart disease, cancer and premature death.

There's one downside to keeping the windows up and using recycled air, especially if you’re carpooling: It can get stuffy. That’s a product of carbon dioxide from breathing, which can build up on longer drives in tightly sealed new cars with several passengers, according to the study.

“To prevent this, outside air should be pulled in every 10 or 15 minutes for a minute or two, especially if there are two or more people in the vehicle,” Hudda said.