Ambient heat and emergency department visits for mental health in Canada: assessing temporal variations by greenspace, urbanicity and socioeconomic status

BACKGROUND AND AIM Although previous research has suggested that greenspace might attenuate health risks associated with ambient heat, there is limited research on mental health disorders. Thus, we evaluated the effect modification of greenspace on heat-related mental health emergency department (ED) visits and whether these effects changed over time. METHODS We conducted a case-crossover study for mental health-related ED visits in Canada between 2004 and 2021 during warm-season months (May to August). ED visits for all Canadians living in 111 health regions were included. Daily average temperature as well as greenspace exposure, estimated by the Normalized Difference Vegetation Index (NDVI), were assigned to individuals at their residential location. Extreme heat was defined as the 95th percentile of the health region-specific warm-season temperature distribution. Conditional logistic regression was used to estimate associations between heat and mental health-related ED visits. RESULTS A total of 10,638,372 ED visits for any mental health condition were included. Days of extreme heat were associated with an increased risk of ED visits for any mental health condition (OR = 1.07; 95% CI: 1.05-1.09). In the earlier part of the study period (2004–2009), the associations between ambient heat and mental health ED visits were stronger among individuals exposed to the lowest quartile of NDVI exposure (OR = 1.20; 95% CI: 1.15-1.25), compared to individuals in the highest quartile of NDVI (OR = 1.01; 95% CI: 0.96-1.07). This difference, however, accentuated in the later part of the study (2016-2021) with the ORs for regions in the lowest quartile of NDVI increasing to 1.29 (95% CI: 1.25-1.34) and remaining stable for regions in the highest quartile. Effects appeared strongest in the most deprived socioeconomic areas. CONCLUSIONS Potential health benefits of greenspaces should be considered in mitigation strategies regarding the impacts of climate change on mental health ED visits.