Introduction:
Secondhand smoke is a well-established cause of physical disorders such as lung cancer, respiratory disease and heart disease. In the past decade, studies have also started linking secondhand smoke exposure to various mental health disorders. However, relatively little attention is paid to this research in the promotion of smokefree policies or mental wellbeing. We review the current evidence on secondhand smoke and mental health outcomes, and discuss its potential policy implications.

Material and Methods:
Systematic literature review of studies examining the link between secondhand smoke and mental health. After screening 213 articles, we gathered and analysed data from 82 articles which met our final inclusion criteria.

Results:
Secondhand smoke exposure is associated with Attention Deficit Hyperactivity Disorder, depressive and anxiety disorders, suicide, stress, sleeping disorders, cognitive and behavioural issues in children, and dementia in older adults. Associations were generally more consistent and showing dose-response effects for Attention Deficit Hyperactivity Disorder, depression, anxiety and dementia, with higher odds reported in people exposed to secondhand smoke at high levels, frequently, or in the home environment. Most studies were cross-sectional albeit in large, nationally representative samples from various countries with a smaller number of longitudinal studies. More research investment is needed in this area to determine whether secondhand smoke exposure directly causes adverse mental health outcomes via biological mechanisms, or is more a sign of living in an environment that contributes to poor mental health.

Conclusions:
More emphasis on the link between secondhand smoke exposure and adverse mental health outcomes could help to promote smokefree environments as part of a broader effort to promote mental wellbeing, especially in the home environment.