Heat warnings, mortality, and hospital admissions among older adults in the United States

Abstract

Background: Heat warnings are issued in advance of forecast extreme heat events, yet little evidence is available regarding their effectiveness in reducing heat-related illness and death. We estimated the association of heat warnings and advisories (collectively, "alerts") issued by the United States National Weather Service with all-cause mortality and cause-specific hospitalizations among Medicare beneficiaries aged 65 years and older in 2,817 counties, 2006-2016.

Methods: In each county, we compared days with heat alerts to days without heat alerts, matched on daily maximum heat index and month. We used conditional Poisson regression models stratified on county, adjusting for year, day of week, federal holidays, and lagged daily maximum heat index.

Results: We identified a matched non-heat alert day for 92,029 heat alert days in 2,817 counties, or 54.6% of all heat alert days during the study period. Contrary to expectations, heat alerts were not associated with lower risk of mortality (RR: 1.005 [95% CI: 0.997, 1.013]). However, heat alerts were associated with higher risk of hospitalization for fluid and electrolyte disorders (RR: 1.040 [95% CI: 1.015, 1.065]) and heat stroke (RR: 1.094 [95% CI: 1.038, 1.152]). Results were similar in sensitivity analyses additionally adjusting for same-day heat index, ozone, and PM_2.5.

Conclusions: Our results suggest that heat alerts are not associated with lower risk of mortality but may be associated with higher rates of hospitalization for fluid and electrolyte disorders and heat stroke, potentially suggesting that heat alerts lead more individuals to seek or access care.

Keywords: Early warning systems; Extreme heat; Hospitalization; Medicare; Mortality; United States.

Fig. 1.

Maps of (A) the total number of heat alert days in the contiguous US, 2006–2016, and (B) the percent of heat alert days for which we were able to identify a matched non-heat alert day. Gray shading indicates a value of 0.

Fig. 2.

Sensitivity analyses of the association between heat alerts and health outcomes among Medicare beneficiaries aged 65 and older, 2006–2016. Results are shown for the main model; the main model additionally adjusted for same day PM_2.5, ozone, and heat index; the main model restricted to heat alert days with a maximum heat index of at least 100°F; and the main model restricted to days with excessive heat warnings only.

Fig. 3.

Causal diagram of hypothesized impacts of heat on hospitalizations and deaths and the potential mitigating influences of heat alerts. In this diagram, the term “illness” refers to pathophysiologic changes that may or may not be symptomatic. For simplicity, we elect not to explicitly show forecast heat, which is a determinant of heat alerts and could potentially affect individual behaviors independent of heat alerts.