LOS ANGELES, CA (Catholic Online) - 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. The authors of the study 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," Dr. Ana Navas-Acien, who has studied secondhand smoke in cars at the Johns Hopkins Bloomberg School of Public Health in Baltimore says.

"It's important for children, definitely, but it's a problem for everybody," Navas-Acien reiterates. She was not involved in the study.

"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 agrees 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.

Brian King of the Centers for Disease Control 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.

In the study, 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 young people said they didn't smoke.

The number of participants who reported recently being exposed to secondhand smoke in the car dropped from 48 percent to 30 percent overall. Among those who smoke, 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 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.

While very encouraging, more remains 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 says.

"We have evidence that there's no safe level for exposure" to secondhand smoke, he added.

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