Exposure to air pollution and tobacco smoking and their combined effects on depression in six low- and middle-income countries

Background: Little is known about the joint mental health effects of air pollution and tobacco smoking in low- and middle-income countries. Aims: To investigate the effects of exposure to ambient fine particulate matter pollution (PM_2.5) and smoking and their combined (interactive) effects on depression. Method: Multilevel logistic regression analysis of baseline data of a prospective cohort study (n=41 785). The 3-year average concentrations of PM_2.5 were estimated using US National Aeronautics and Space Administration satellite data, and depression was diagnosed using a standardised questionnaire. Three-level logistic regression models were applied to examine the associations with depression. Results: The odds ratio (OR) for depression was 1.09 (95% Cl 1.01-1.17) per 10μg/m³ increase in ambient PM_2.5, and the association remained after adjusting for potential confounding factors (adjusted OR=1.10, 95% CI 1.02-1.19). Tobacco smoking (smoking status, frequency, duration and amount) was also significantly associated with depression. There appeared to be a synergistic interaction between ambient PM_2.5 and smoking on depression in the additive model, but the interaction was not statistically significant in the multiplicative model. Conclusions: Our study suggests that exposure to ambient PM_2.5 may increase the risk of depression, and smoking may enhance this effect.