Exploring the local impacts of heat on mortality: A small-area spatial analysis of Montreal, Toronto, and Vancouver, Canada.

Abstract

Objectives: The objectives of this study were to quantify the small-area associations between heat and mortality and to characterize the spatial patterns of mortality risks at hot temperatures.

Methods: Daily mortality and temperature data were retrieved for the cities of Montreal, Toronto, and Vancouver during the summer months between 2018 and 2022. Spatial distributed lag non-linear models quantified the associations between temperature and mortality at the small-area scale. Heat-mortality hotspots were identified based on the relative risks of mortality at hot temperatures. The spatial patterns of mortality at hot temperatures were described using small-area sociodemographic and environmental characteristics.

Results: Hot temperatures were associated with elevated relative risks of mortality in Montreal and Vancouver compared to median summer temperatures. At 95th percentile temperatures, 38% and 18% of areas in Montreal and Vancouver were classified as heat-mortality hotspots, respectively. In Toronto, 95th and 99th percentile temperatures were not associated with elevated relative risks of mortality and no heat-mortality hotspots were identified. In all cities, the relative risks of mortality at hot temperatures were positively correlated with population density and residential instability. Areas with high levels of greenness had lower relative risks in Montreal and Toronto.

Conclusion: Understanding the small-area associations between heat and mortality is important for public health programs that aim to reduce the health impacts of extreme heat. The impacts of heat on mortality exhibited considerable spatial variability, and small-area vulnerability was found to be characterized by high population density and high residential instability.