Temporal Variation in Heat-Mortality Associations: A Multicountry Study

Abstract

Background: Recent investigations have reported a decline in the heat-related mortality risk during the last decades. However, these studies are frequently based on modeling approaches that do not fully characterize the complex temperature-mortality relationship, and are limited to single cities or countries.

Objectives: We assessed the temporal variation in heat-mortality associations in a multi-country data set using flexible modelling techniques.

Methods: We collected data for 272 locations in Australia, Canada, Japan, South Korea, Spain, the United Kingdom, and the United States, with a total 20,203,690 deaths occurring in summer months between 1985 and 2012. The analysis was based on two-stage time-series models. The temporal variation in heat-mortality relationships was estimated in each location with time-varying distributed lag nonlinear models, expressed through an interaction between the transformed temperature variables and time. The estimates were pooled by country through multivariate meta-analysis.

Results: Mortality risk due to heat appeared to decrease over time in several countries, with relative risks associated to high temperatures significantly lower in 2006 compared with 1993 in the United States, Japan, and Spain, and a nonsignificant decrease in Canada. Temporal changes are difficult to assess in Australia and South Korea due to low statistical power, and we found little evidence of variation in the United Kingdom. In the United States, the risk seems to be completely abated in 2006 for summer temperatures below their 99th percentile, but some significant excess persists for higher temperatures in all the countries.

Conclusions: We estimated a statistically significant decrease in the relative risk for heat-related mortality in 2006 compared with 1993 in the majority of countries included in the analysis.

Citation: Gasparrini A, Guo Y, Hashizume M, Kinney PL, Petkova EP, Lavigne E, Zanobetti A, Schwartz JD, Tobias A, Leone M, Tong S, Honda Y, Kim H, Armstrong BG. 2015. Temporal variation in heat-mortality associations: a multicountry study. Environ Health Perspect 123:1200-1207; http://dx.doi.org/10.1289/ehp.1409070.

Figure 1

Geographic distributions of the 272 locations within the 7 countries included in the analysis, and the corresponding average mean daily temperature (°C) during the summer.

Figure 2

Overall cumulative exposure–response relationships^a between heat and mortality predicted from the model with no interaction (interpreted as the average throughout the study period) in 7 countries, with 95% CIs. The vertical lines represent the percentile of minimum mortality temperature (dotted) and the 90th and 99th percentiles of the temperature distribution (dashed). ^aThe curves are represented on a relative scale of summer temperature percentiles, using country-specific distributions.

Figure 3

Overall cumulative exposure–response relationships^a between heat and mortality predicted for 1993 (black) and 2006 (blue) in 7 countries, with 95% CIs. The vertical lines represent the percentile of minimum mortality temperature^b (dotted) and the 90th and 99th percentiles of the temperature distribution (dashed). The y-axis is scaled to the country-specific range. ^aThe curves are represented on a relative scale of summer temperature percentiles, using country-specific distributions. ^bEstimated as the minimum of the overall cumulative exposure–response curve from the model without interaction (interpreted as the average across the whole study period).

Figure 4

Lag–response relationships between heat and mortality predicted for 1993 (black) and 2006 (blue) in 7 countries, with 95% CIs. These curves are computed for the temperature corresponding to the 99th percentile vs. the country-specific minimum mortality temperature.^a The y-axis is scaled to the country-specific range. ^aEstimated as the minimum of the overall cumulative exposure–response curve from the model without interaction (interpreted as the average across the whole study period).