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.