Children exposed to one or more hours of secondhand smoke per day are more likely to develop a mental health disorder, according for a new study published in Tobacco Control.

The study, which originates in Spain, is based on data from the 2011-2012 Spanish National Health Interview Survey. The data consists of surveys and questionnaires completed by parents of 2,357 children ages 4 to 12. The surveys asked parents to report how many hours per day their child was exposed to secondhand smoke. The parents were also asked about family history of mental health and the mental health of the children.

Out of the 2,357 children, 7 percent of them were exposed to an hour of secondhand smoke per day, while 4.5 percent were exposed to more than one hour of the smoke per day. These children were 50 percent more likely to have a mental health disorder, specifically Attention Deficit Hyperactivity Disorder (ADHD).  The likeliness of developing ADHD or other mental health disorders rises with each hour of secondhand smoke exposure per day.

Although this study links secondhand smoke exposure with mental health disorders, the authors on the study warn that this may not be all there is to it. There could be other factors not studied or mentioned that could be contributing to the likeliness of having a disorder.