Greenery exposure and suicide mortality later in life : A longitudinal register-based case-control study

Background
Exposure to residential greenery accumulates over people’s lifetimes, and possibly has a protective association with suicide later in life.

Objectives
To examine the associations between suicide mortality and long-term residential greenery exposure in male and female adults.

Methods
Our population-based nested case-control study used longitudinally georeferenced Dutch register data. Suicide cases aged 18–64 years between 2007 and 2016 were matched by gender, age, and date of suicide to 10 random controls. We measured long-term greenery exposure along people’s 10-year residential address histories through longitudinal normalized difference vegetation indices (NDVI) from Landsat satellite imagery between 1997 and 2016. We assigned accumulated greenery exposures, weighted by people’s exposure duration, within 300, 600, and 1,000 m concentric buffers around home addresses. To assess associations between suicide and greenery, we estimated gender-specific conditional logistic regressions without and with adjustment for individual-level and area-level confounders. Stratified models were fitted for areas with a high/low level of urbanicity and movers/non-movers.

Results
Our study population consisted of 9,757 suicide cases and 95,641 controls. In our models adjusted for age, gender, and date of suicide, the odds ratios decreased significantly with higher quartiles of accumulated NDVI scores. NDVI associations were attenuated and did not remain significant after adjustment for socioeconomics, urbanicity, air pollution, social fragmentation, etc. for either males or females. For females, but not males, our model with 300 m buffers for areas with a low level of urbanicity showed a significant suicide risk reduction with increasing levels of NDVI. Individual risk factors (e.g., lack of labor market participation) outweighed the contribution of greenery.

Conclusion
We found limited evidence that long-term greenery exposure over people’s lifetimes contributes to resilience against suicide mortality. Ensuring exposure to greenery may contribute to suicide prevention for specific population groups, but the effectiveness of such exposure should not be overstated.