Children exposed to tobacco smoke at home are up to three times more likely to have attention deficit hyperactive disorder (ADHD) as unexposed kids, according to a new study from Spain.

The association was stronger for kids with one or more hours of secondhand smoke exposure every day, the authors found. And the results held when researchers accounted for parents’ mental health and other factors.

“We showed a significant and substantial dose–response association between (secondhand smoke) exposure in the home and a higher frequency of global mental problems,” the authors write in Tobacco Control.

According to the Centers for Disease Control and Prevention, two of every five children in the US are exposed to secondhand smoke regularly.

Alicia Padron of the University of Miami Miller School of Medicine in Florida and colleagues in Spain analyzed data from the 2011 to 2012 Spanish National Health Interview Survey, in which parents of 2,357 children ages four to 12 reported the amount of time their children were exposed to secondhand smoke every day.

The parents also filled out questionnaires designed to evaluate their children’s mental health. According to the results, about eight percent of the kids had a probable mental disorder.

About seven percent of the kids were exposed to secondhand smoke for less than one hour per day, and 4.5 percent were exposed for an hour or more each day.

After taking the parent’s mental health, family structure and socioeconomic status into consideration, children who were exposed to secondhand smoke for less than one hour per day were 50 percent more likely to have some mental disorder compared to kids not exposed at all.

And children who were habitually exposed to secondhand smoke for an hour or more each day were close to three times more likely to have a mental disorder.

In addition, kids exposed less than one hour per day were twice as likely to have ADHD as kids who weren’t exposed, and children exposed for an hour or more on a daily basis were over three times more likely to have ADHD.

“The association between secondhand smoke and global mental problems was mostly due to the impact of secondhand smoke on the attention-deficit and hyperactivity disorder,” the authors write.

The study looks at a single point in time and cannot prove that secondhand smoke exposure causes mental health problems, the study team cautions.

Frank Bandiera, a researcher with the University of Texas Health Science Center in Houston who was not involved in the study, liked that the researchers “controlled for parents’ mental health in the new study because that could be a confounder.”

But, he added, the study might be limited because, although the questionnaires are thought to be valid, the mental disorders were not actually diagnosed by physicians.

“We’re not sure if it’s causal or not,” Bandiera told Reuters Health. “I think (the research) is still in the early stages and the findings are inconclusive.”

But, he said, since secondhand hand smoke has been related to a lot of physical diseases, parents should avoid smoking around their kids.

“We need to sort it out more, so we’re not sure yet, but just as a precaution, I don’t think parents should smoke at home - they should keep their kids away from secondhand smoke,” Bandiera said.

Lucy Popova, from the Center for Tobacco Control Research and Education at the University of California, San Francisco, said there is a lot of evidence about the harms of secondhand smoke on physical wellbeing.

“But research on effects of secondhand smoke on mental health have been really just emerging and this study really contributes to this growing body of evidence that exposure to secondhand smoke in children might be responsible for cognitive and behavioral problems,” she said.

Popova, who wasn’t involved in the study, said no amount of secondhand smoke is safe – any exposure is bad.

“So parents should not expose their children – the best thing to do is quit,” she said. “And this will not only not expose their children to the secondhand smoke, but will also let them enjoy their life with their children longer.”