Children who are exposed to secondhand smoke at home are up to three times more likely to have attention deficit hyperactivity disorder (ADHD), compared with their peers who don’t live with smokers, according to a new study.

The more smoke children were exposed to, the greater the chance they had ADHD, Reuters reports.

The study included 2,357 children ages 4 to 12 in Spain. Parents reported the amount of time their children were exposed daily to secondhand smoke. They also answered questions about their children’s mental health.

Overall, children who were exposed regularly to secondhand smoke for an hour or more every day were almost three times more likely to have some type of mental disorder. Children exposed to secondhand smoke for an hour or more each day were more than three times as likely to have ADHD.

The researcher caution that the study does not prove that secondhand smoke exposure causes mental health problems. The findings are published in Tobacco Control.

According to the Centers for Disease Control and Prevention, there is no risk-free level of secondhand smoke. In children, secondhand smoke causes ear infections; more frequent and severe asthma attacks; respiratory infections (such as bronchitis and pneumonia); respiratory symptoms such as coughing, sneezing and shortness of breath; and a greater risk for sudden infant death syndrome.