Heat Exposure and Mortality in Aotearoa New Zealand: A Time-Stratified Case-Crossover Study

Extreme heat is increasing in frequency and intensity, with disproportionate risks for clinically and socially vulnerable groups. This paper identifies the relationship between acute heat and mortality in Aotearoa New Zealand. To estimate same-day heat-mortality associations, we link weather-station observation data to national administrative mortality records for all deceased individuals. Both wet bulb globe temperature (WBGT) and air temperature (daily mean, maximum, minimum) were used to estimate same-day effects using time-stratified case-crossover models with month×weekday strata for four samples: all-cause or only non-injury mortality, and full year or only warm season mortality. We additionally examine discrete exposure bins, distributed lag non-linear curves (lags 0–7), and heterogeneity by location, sex, and age. Across daily summaries, the mean series of both variables yields the largest effect: over the full year, all-cause mortality rises by 0.44% per +1 °C for WBGT (mean) versus 0.39% per +1 °C for Temperature (mean). Similar results are identified for non-injury mortality. Warm season effects are larger: all-cause mortality increases by about 0.57% per 1 °C in mean WBGT, versus 0.51% per 1 °C in mean air temperature. Across samples, WBGT estimates are generally larger than temperature-only measures. A simple scaling using average death counts suggests that a uniform +1 °C increase in mean WBGT would correspond to approximately 140 additional all-cause deaths and 110 non-injury deaths per year, and about 85 and 70 extra deaths per warm season, respectively.