Over the last decade exposure among youth to secondhand smoke in cars has dropped by slightly less than half but more than 1 in 5 students in middle school and high school are still exposed, according to a national estimate from the Centers for Disease Control and Prevention.

The data comes from research published online in the journal Pediatrics. Data on youth exposure to secondhand smoke was reviewed from the National Youth Tobacco Survey, which tracks responses of U.S. students in grades 6 through 12.

The study is based on surveys asking students how often they rode in a car where another person was smoking.

The study found that 22.8 percent of teens and pre-teens were exposed to smoke in cars in 2009, a figure which dropped from 40 percent in 2000.

The study “Secondhand Smoke Exposure in Cars Among Middle and High School Students – United States, 2000-2009, was led by Brian King, PhD, of the CDC.

Study authors said that little is known about youth exposure to SHS in motor vehicles.