About 1 in 5 kids in middle school or high school is exposed to secondhand smoke in cars.
Sitting in a car with a smoker is about as close to lighting up as a nonsmoker can get.
And quite a few schoolchildren get exposed to secondhand smoke this way, according to an estimate by researchers at the Centers for Disease Control and Prevention.
About 1 in 5 nonsmoking kids in middle and high school reported sharing a car with a smoker who had lit up within a week of answering a survey in 2009. The researchers say the survey, which included responses from thousands of students, gives an accurate snapshot of what's happening across the country.
"The car is the only source of exposure for some of these children, so if you can reduce that exposure, it's definitely advantageous for health," CDC researcher Brian King told The Associated Press. The findings appear in the latest issue of Pediatrics.
The American Academy of Pediatrics says that any exposure to secondhand smoke is unsafe for kids.
And the latest report does find that nonsmoking kids' exposure to secondhand smoke in cars declined to 22.9 percent in 2009 from 39 percent in 2000. The researchers figure that laws barring smoking in many public places may have been a factor.
A decline in smoking prevalence and a hardening of attitudes against secondhand smoke also could be helping.
Still, researchers say more should be done. They recommend a ban on smoking in cars when children are present. That's already the law in a few places, they note, including California and Arkansas (for children under 14).