30 Kim NH

Park JH

Choi DP

Lee JY

Kim HC Secondhand smoke exposure and depressive symptoms among Korean adolescents: JS High School Study.

24 Vancampfort D

Stubbs B

Firth J

Van Damme T

Koyanagi A Sedentary behavior and depressive symptoms among 67,077 adolescents aged 12‒15 years from 30 low- and middle-income countries.

49 Ranasinghe S

Ramesh S

Jacobsen KH Hygiene and mental health among middle school students in India and 11 other countries.

50 Xi B

Liang Y

Liu Y

et al. Tobacco use and second-hand smoke exposure in young adolescents aged 12–15 years: data from 68 low-income and middle-income countries.

51 Lange S, Koyanagi A, Rehm J, Roerecke M, Carvalho AF. Association of tobacco use and exposure to second-hand smoke with suicide attempts among adolescents–findings from 33 countries. Nicotine Tob Res. In press. Online September 5, 2019. https://doi.org/10.1093/ntr/ntz172.

52 Bird Y

Staines-Orozco H

Moraros J Adolescents’ smoking experiences, family structure, parental smoking and socio-economic status in Ciudad Juárez, Mexico.

53 Culpin I

Heron J

Araya R

Melotti R

Joinson C Father absence and depressive symptoms in adolescence: findings from a UK cohort.

54 Sustainable Development Goals. New Education Data for SDG4: Focus on Out-Of-School Children. www.sdg4education2030.org/new-education-data-sdg4-focus-out-school-children-27-september-2018. Published 2018. Accessed December 18, 2019.

The findings should be interpreted in the light of several limitations. There was a lack of detailed data on SHS (e.g., type of smoke, place, circumstances, and intensity), and these data could have provided more insight into the association between SHS and depressive symptoms. In particular, future studies should take place of SHS exposure into account, as a study including Korean adolescents showed that the SHS–depressive symptom relationship was significant when SHS exposure occurred at home but not at school.Relatedly, the question on SHS was based on the number of days people smoked in the presence of the student. Given that the level of exposure within a day can vary widely between students, it may not accurately reflect the level of SHS exposure. Moreover, SHS and depression were assessed with a single question based on self-report for which validity and reliability have not been established. Although these measures have been used in numerous previous publications,self-reported data are subject to biases (e.g., social desirability bias and recall bias) and misclassification is possible. It is also possible that these questions were interpreted differently across various cultures and languages. The use of 3 days/week as the cut off for SHS exposure in the country-wise analysis is likely to have improved specificity; the question on depressive symptoms referred to core symptoms of depression, but future studies with biochemical verification of SHS (e.g., salivary cotinine) and clinical assessments of depressive symptoms are warranted. Although it is unlikely that exposure to SHS changes within a timeframe of a year, variables on SHS (past week) and depressive symptoms (past year) used different timeframes. Also, food insecurity may not completely capture differences in SES playing a significant role in the SHS–depression relationship, and thus residual confounding by SES remains possible. For example, as family structure has been reported to be associated with both SHS exposureand depression in adolescents,residual confounding because of this factor may exist. Data on chronic physical conditions (e.g., asthma) were also not available in the data set. Thus, their mediating role in the association between SHS exposure and depression could not be assessed. In addition, given that 1 in 3 children and adolescents is out of school in LMICs,these results may not be generalizable to adolescents not attending school. Furthermore, parental consent may have been more difficult to obtain for adolescents with a low SES who were at a potentially increased risk for both SHS exposure and depressive symptoms. This may have led to an underestimation of the prevalence of SHS and depressive symptoms among adolescents from LMICs, as well as an underestimation of the association of the 2 variables. Finally, as this was a cross-sectional study, one cannot draw any conclusions regarding causality or temporality of the SHS–depressive symptom relationship. However, the mere fact that depressive symptoms are more common in adolescents who are more highly exposed to SHS is a concern given the devastating consequences of adolescent depression and the higher risk for noncommunicable diseases associated with SHS exposure.