How the impact of heat and cold on mortality has changed in Switzerland during the last 50 years: a nationwide analysis.

Abstract

Background: Although previous studies quantified the effect of non-optimal ambient temperatures on mortality, many were limited to urban areas and short periods of time. By applying state-of-the-art methodologies and high-resolution nationwide data available in Switzerland, we aimed to assess trends in mortality attributable to heat and cold during the last five decades across the full Swiss geography. Methods: We collected daily mortality and derived population-weighted daily mean temperature from 2.2km-grid maps for each Swiss municipality between 1969 and 2017. We performed separate time series analyses with conditional quasi-Poisson regression and distributed lag non-linear models to obtain the corresponding temperature-mortality associations for each Canton and decade. We then pooled them through multivariate longitudinal meta-regression and calculated the corresponding excess mortality attributable to non-optimal temperatures. The analysis was repeated across categories of sex, age and main causes. Results: Between 1969 and 2017, overall all-cause excess mortality associated to non-optimal temperatures was 6.24%(95%CI,5.58-6.78) which translates into 3,714 annual excess deaths. Cold-related mortality represented a larger fraction compared to heat (5.89%(95%CI,5.26-6.42), versus 0.36%(95%CI,0.30-0.40)), amounting to 3,588 and 218 annual deaths, respectively. Cold-related mortality increased over the last five decades (5.17% to 6.43%) while for heat an increasing trend was observed until 1998 (0.23% to 0.40%), when it reached a plateau to present years. Similar trends were observed for sex, age and main causes. Conclusion: This first Swiss-wide study found a substantial mortality burden attributed to non-optimal temperatures. Despite the progressive warming of climate, our findings suggest that heat-related mortality remained constant during the last two decades possibly due to the recent implementation of public health measures, while cold-related mortality, which represents a larger burden, increased during the last 50 years. Future analyses will seek to identify potential vulnerability factors and adaptive mechanisms to non-optimal temperatures in Switzerland.