Kids who breathe secondhand smoke are more likely to struggle with mental health problems, suggests a large new study of British children.

The findings add urgency in the push for parents to put away their cigarettes for good, or at least smoke outside of the home, researchers say. However, it's still unclear if tobacco fumes actually take a toll on children's brains, or if something else is at play.

"We know that exposure to secondhand smoke is associated with a lot of physical health problems in children, although the mental health side has not been explored," lead researcher Mark Hamer, of University College London, told Reuters Health in an e-mail.

Two of every three kids between the ages of three and 11 are exposed to secondhand smoke in the U.S. Meanwhile, of children aged nine to 17, one in five have been diagnosed with some kind of mental or addictive disorder, according to the U.S. Department of Health and Human Services.

To see if the two statistics are linked, Hamer and his colleagues studied 901 nonsmoking kids who were between four and eight years old. They measured levels of a byproduct of cigarette fumes in the kids' saliva to gauge smoke exposure and had parents fill out a questionnaire about the kids' emotional, behavioral and social problems.

The more secondhand smoke a child took in, on average, the poorer his or her mental health. This was particularly true for hyperactivity and "bad" behavior, report the researchers in the Archives of Pediatrics and Adolescent Medicine.

Overall, about three percent of all kids received "abnormal" scores of 20 or more on the Strengths and Difficulties Questionnaire, a 40-point scale with the highest scores representing the poorest mental health.

Compared to the 101 kids who breathed in the least secondhand smoke, the 361 with the most exposure scored an average of 44 percent higher on the questionnaire, 9.2 versus 6.4.

The gap remained after the researchers accounted for other factors that could affect mental health such as asthma, physical activity and the families' income and housing situations. Still, it can't be ruled out that some unmeasured factor played a role.

Children were most likely to breathe secondhand smoke in their own homes.

It is not yet clear how secondhand smoke would trigger mental troubles. The researchers suggest it could be related to smoke's effects on chemicals in the brain such as dopamine. Genetics could also be at play, or simply the knowledge that smoke is harmful could be a downer for kids forced to breathe a lot of it every day.

While Hamer noted that further research is needed to confirm the findings, Dr. Michael Weitzman of New York University Medical Center, who was not involved in the study, said the results strengthen the evidence that secondhand smoke, and possibly prenatal exposure to tobacco, causes mental health problems in children.

"Many people now recognize that children's secondhand smoke exposure increases their risk for Sudden Infant Death Syndrome, ear infections and asthma," Weitzman told Reuters Health in an e-mail. "But secondhand smoke also poses a huge burden on the quality of life of children, their families and the larger society due to increased child mental health problems."

He recommends public education about these consequences, as well as more efforts to help parents quit smoking.

Until they kick the habit, Hamer suggests parents should "try and avoid smoking in their home when children are around since it's harmful for them -- both physically and mentally."