"Short of driving less, putting your ventilation to 'recirculate' is the best way to reduce exposure to all types of vehicle-related particulate pollution," says Scott Fruin. (Credit: Wayne Williamson/Flickr)

Car vents set to ‘recirculate’ keep air cleaner

Setting a car’s ventilation system to “recirculate” is the best way to reduce exposure to harmful traffic pollution.

That’s the advice researchers have for parents as millions of children return to school, many of them driven in cars on the highway.

Environmental health researchers recently conducted the first systematic measurements of in-vehicle exposure that included a full range of car types and operating conditions, and for all types of particulate pollution.

“Short of driving less, putting your ventilation to ‘recirculate’ is the best way to reduce exposure to all types of vehicle-related particulate pollution,” says Scott Fruin, assistant professor of preventive medicine at the University of Southern California.

“Otherwise, an hour-long commute to work or school can double your daily exposure to traffic-related particulate air pollutants.”

The study, published in Environmental Science & Technology, quantifies the extent of exposure reduction according to car age, speed, and pollutant type. In addition to the benefits of recirculation settings, exposures are lower in newer cars, at slower speeds, and on arterial roads, where pollutant concentrations are lower than on freeways.

Concentrations of particle pollutants on freeways are often five to 10 times higher than elsewhere.

To put the results in perspective, measurements were turned into predictive models, then the models were applied to the  national fleet of car models and ages and Los Angeles driving conditions.

For a typical car (seven years old, the national average), recirculation settings reduce in-vehicle particle pollution for very small particles from 80 percent (of on-road levels) to 20 percent, and from 70 percent to 30 percent for larger particles, compared to air ventilation settings which bring in outside air. (Windows were always closed in this study. Keeping windows open while driving quickly raises inside pollutant concentrations to the same levels as on-road levels.)

Like a stuffy meeting room

“Until this comprehensive study, measurements have been based on only a few cars and usually only one pollutant,” says Neelakshi Hudda, PhD research associate in the environmental health department.

“We showed that recirculation settings produce large exposure reductions across all car types and for all particulate pollutants.”

The researchers also found that leaving the windows closed over 30-minute or longer drives with several passengers raised carbon dioxide levels in tight new cars to those of stuffy meeting rooms.

“Some people are sensitive to high CO2 concentrations. 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 says.

Traffic-related particulate pollutants studied included ultrafine particles (the very smallest that can enter the bloodstream) to the larger particulate matter (PM) sizes such as PM2.5 (invisible particles smaller than 2.5 microns) that are linked to heart disease and premature mortality.

Pollutants also included black carbon and particle-bound polycyclic aromatic hydrocarbons, both high in diesel emissions, considered by the State of California to produce cancer.

Source: University of Southern California

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  1. Bruce W. Fowler

    Do the windows on automobiles in California not “roll” down?

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