Investigating the independent and synergistic associations between neighbourhood greenness and physical activity in relation to perceived mental health among adults in Canada

The relationships among neighbourhood greenness, physical activity, and mental health are unclear; therefore, we examined the independent and synergistic associations between neighbourhood greenness and self-rated mental health among a nationally representative sample of urban-dwelling adults in Canada (18-79 years) from the 2007-2019 Canadian Health Measures Survey (n = 12,531). We assessed neighbourhood greenness using the Normalized Difference Vegetation Index within a 500-meter radius of participants' residential postal codes. We measured physical activity using accelerometers and determined adherence to the recommended 150-minutes of moderate-to-vigorous intensity physical activity (MVPA) per week. We used weighted logistic regression models to test whether MVPA guideline adherence was an effect modifier in the association between neighbourhood greenness and self-rated mental health, adjusting for individual and neighbourhood characteristics. Neighbourhood greenness (aOR = 0.89 [0.62, 1.29]) and MVPA adherence (aOR = 1.22 [0.89, 1.69]) were not associated with self-rated mental health, and no interaction were found on the additive (Relative Excess Risk Due to Interaction = -0.45 [-1.24, 0.35], Attributable Proportion = -0.38 [-1.02, 0.26], Synergy Index = 0.28 [0.02, 3.20]) or multiplicative (OR = 0.7 [0.4, 1.3]) scales. Engaging in the recommended amount of MVPA did not change the finding that Canadian adults had similar self-rated mental health regardless of their neighbourhood greenness.