Heat Wave and Mortality: A Multicountry, Multicommunity Study

Abstract

Background: Few studies have examined variation in the associations between heat waves and mortality in an international context.

Objectives: We aimed to systematically examine the impacts of heat waves on mortality with lag effects internationally.

Methods: We collected daily data of temperature and mortality from 400 communities in 18 countries/regions and defined 12 types of heat waves by combining community-specific daily mean temperature ≥90th, 92.5th, 95th, and 97.5th percentiles of temperature with duration ≥2, 3, and 4 d. We used time-series analyses to estimate the community-specific heat wave-mortality relation over lags of 0-10 d. Then, we applied meta-analysis to pool heat wave effects at the country level for cumulative and lag effects for each type of heat wave definition.

Results: Heat waves of all definitions had significant cumulative associations with mortality in all countries, but varied by community. The higher the temperature threshold used to define heat waves, the higher heat wave associations on mortality. However, heat wave duration did not modify the impacts. The association between heat waves and mortality appeared acutely and lasted for 3 and 4 d. Heat waves had higher associations with mortality in moderate cold and moderate hot areas than cold and hot areas. There were no added effects of heat waves on mortality in all countries/regions, except for Brazil, Moldova, and Taiwan. Heat waves defined by daily mean and maximum temperatures produced similar heat wave-mortality associations, but not daily minimum temperature.

Conclusions: Results indicate that high temperatures create a substantial health burden, and effects of high temperatures over consecutive days are similar to what would be experienced if high temperature days occurred independently. People living in moderate cold and moderate hot areas are more sensitive to heat waves than those living in cold and hot areas. Daily mean and maximum temperatures had similar ability to define heat waves rather than minimum temperature. https://doi.org/10.1289/EHP1026.

Figure 1.

Locations of the study communities and annual mean heat wave days. Heat wave was defined by daily mean temperature $\ge 95\text{th}$ percentile of temperature with duration $\ge 2\mathrm{d}$.

Figure 2.

Cumulative effects of heat waves on mortality (A. overall effects of heat waves and B. added effects of heat waves after controlling for effects of daily mean temperature) over lag 0–10 d in 18 countries/regions for 12 types of heat wave definitions. Please refer Table 1 for heat wave definitions.

Figure 3.

Lag effects of heat waves on mortality along lag 0–10 d in 18 countries/regions for 4 types of heat wave definitions. Please refer to Table 1 for heat wave definitions.

Figure 4.

Cumulative effects of heat waves on mortality over lag 0–10 d in 4 climatic areas (cold, moderate cold, moderate hot, and hot areas) for 12 types of heat wave definitions. Cold areas: mean temperature of hot season: $\le 20.7°\mathrm{C}$; moderate cold areas: mean temperature of hot season: $20.7–24.1°\mathrm{C}$; moderate hot areas: mean temperature of hot season: $24.1–27.6°\mathrm{C}$; and hot areas: mean temperature of hot season: $>27.6°\mathrm{C}$. Please refer to Table 1 for heat wave definitions.

Figure 5.

Lag effects of heat waves on mortality along lag 0–10 d in 4 climatic areas (cold, moderate cold, moderate hot, and hot areas) for 12 types of heat wave definitions. Cold areas: mean temperature of hot season: $\le 20.7°\mathrm{C}$; moderate cold areas: mean temperature of hot season: $20.7–24.1°\mathrm{C}$; moderate hot areas: mean temperature of hot season: $24.1–27.6°\mathrm{C}$; and hot areas: mean temperature of hot season: $>27.6°\mathrm{C}$. Please refer to Table 1 for heat wave definitions.

Figure 6.

Comparison for cumulative effects of heat waves on mortality over lag 0–10 d in 16 countries/regions for 12 types of heat wave definitions using daily mean and maximum temperatures. Please refer to Table 1 for heat wave definitions. a, B, and $r^2$ are intercept, coefficient, and r-squared, respectively, derived from linear regression model for effect estimates modeled by heat waves defined by daily mean and maximum temperatures. $p-\text{Value}$ for difference is modeled by the t-test between effect estimates modelled by heat waves defined by daily mean and maximum temperatures.