A new government study reports that while fewer kids and teens are getting exposed to secondhand smoke while riding in the car, rates of exposure are still high enough to warrant concern.
The authors recommend that more parts of the country ban smoking in cars carrying kids—laws that are on the books in four states.
In a survey of middle and high school students, close to one-third said they'd driven in a car with someone who was smoking in the past week.
Researchers said parents and other drivers may not realize that even when the windows are down, smoking in a vehicle can create toxic levels of circulating smoke.
"The concentrations just get very high—they get as high as in a very, very smoky bar," said Dr. Ana Navas-Acien, who has studied secondhand smoke in cars at the Johns Hopkins Bloomberg School of Public Health in Baltimore.
"It's important for children, definitely, but it's a problem for everybody," Navas-Acien, who wasn't involved in the new study, told Reuters Health.
Even for smokers' own health, she added, "It's really important for them to realize that they should not smoke in such a small, confined space."
Researchers from the Centers for Disease Control and Prevention report that four states— Arkansas, California, Louisiana and Maine—have bans on smoking in cars carrying kids younger than 13 to 18, depending on the law. Puerto Rico also bars the practice.
Navas-Acien agreed with the authors that extending those laws to more of the country is necessary to protect kids from health problems linked to secondhand smoke, such as asthma and respiratory and ear infections.
For the new study, Brian King of the CDC and his colleagues analyzed data from the National Youth Tobacco Survey, conducted nationwide in more than 20,000 kids in grades six through 12 every couple of years between 2000 and 2009.
Students were asked if they smoked themselves, as well as if they'd been in the car with someone who was smoking in the past week.
By 2009, almost nine in every ten youth said they didn't smoke.
During the study period, the number of participants who reported recently being exposed to secondhand smoke in the car dropped from 48 percent to 30 percent overall.
Among smokers, that rate fell from 82 percent to 76 percent, and in non-smokers, from 39 percent to 23 percent.
King's team speculated in its study, published Monday in the journal Pediatrics, that those declines may be due to more smoke-free laws and fewer people smoking in the United States in general, as well as a changing public attitude about the appropriateness of smoking near kids.
But more needs to be done, researchers agreed.
"The alarming fact of it is, there's about one in five (non-smoking kids) that are still exposed in this environment," King told Reuters Health.
"We have evidence that there's no safe level for exposure" to secondhand smoke, he added.
"People are recognizing the importance of protecting children in cars, and that the amount of tobacco smoke in cars reaches levels that are quite high," said Geoffrey Fong, a tobacco researcher from the University of Waterloo in Canada who didn't participate in the new research.
He said that most provinces in Canada have also passed laws prohibiting smoking while youth are in the car.
"I envision that we are going to be seeing these kinds of laws passed throughout North America... and throughout the world. It makes sense," he told Reuters Health.
"Smoking in cars constitutes a significant public health hazard—that's quite intuitive."