Only 12 percent said they had been advised by their children's doctors to have a smoke-free car.
"Mostly we see when pediatricians talk to parents, it's about smoke-free homes," Nabi-Burza said. "Even bars are smoke-free, but cars have been kind of forgotten. Now that we know the extent of the problem, pediatricians should talk to parents about how smoking in cars is not good for children."
Danny McGoldrick, vice president of research at the Campaign for Tobacco-Free Kids in Washington, D.C., noted that even tobacco smoke residue -- so-called "thirdhand smoke" -- in cars can be harmful to children, increasing the importance of smoke-free car policies even if youngsters aren't present while a parent smokes.
"Fabrics obviously absorb a lot of these toxic components. Just because no one's in there smoking doesn't mean all the harmful [components] disappear," McGoldrick said. "The best thing to do as a smoking parent is to quit smoking. If they're not ready to quit yet or not able to succeed, then adopt smoke-free policies for your home and car."
The American Cancer Society offers more information about secondhand smoke.
SOURCES: Emara Nabi-Burza, M.B.B.S., M.S., senior clinical research coordinator, Center for Child and Adolescent Health Research and Policy, Massachusetts General Hospital for Children, Boston; Danny McGoldrick, vice president, research, Campaign for Tobacco-Free Kids, Washington, D.C.; December 2012, Pediatrics
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