Fewer US children and teens are being exposed to secondhand smoke while riding in cars, but rates of exposure are still high enough to warrant concern and more states should ban smoking in cars carrying children, according to a US study.

In a survey of middle and high school students used as the basis for the study, published in Pediatrics, close to one-third said they'd driven in a car with someone who was smoking in the previous week.  

"The alarming fact of it is, there's about one in five (non-smoking children) that are still exposed in this environment," said Brian King at the Centers for Disease Control (CDC), who led the study.

"We have evidence that there's no safe level of exposure."

King and his colleagues analyzed data from the National Youth Tobacco Survey, covering more than 20,000 children in grades six through twelve every few 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 youths said they didn't smoke. And 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.

"Jurisdictions should expand comprehensive smoke-free policies that prohibit smoking in worksites and public places to also prohibit smoking in motor vehicles occupied by youths," King and his colleagues wrote.  

King's team speculated in its study that the declines may be due to more smoke-free laws and fewer people smoking in the United States in general, as well as changing public attitudes about the appropriateness of smoking near children.

Four states -- Arkansas, California, Louisiana and Maine -- have bans on smoking in cars carrying children younger than 13 to 18 years-old, depending on the law. Puerto Rico also bars the practice.

Experts said that 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 Ana Navas-Acien, who has studied secondhand smoke in cars at the Johns Hopkins Bloomberg School of Publich Health in Baltimore but was not involved in the study.

"It's important for children, definitely, but it's a problem for everybody."