Variation in estimates of heat-related mortality reduction due to tree cover in U.S. cities

Association of Extreme Heat and Cardiovascular Mortality in the United States: A County-Level Longitudinal Analysis From 2008 to 2017

BACKGROUND

Extreme-heat events are increasing as a result of climate change. Prior studies, typically limited to urban settings, suggest an association between extreme heat and cardiovascular mortality. However, the extent of the burden of cardiovascular deaths associated with extreme heat across the United States and in different age, sex, or race and ethnicity subgroups is unclear.

METHODS

County-level daily maximum heat index levels for all counties in the contiguous United States in summer months (May-September) and monthly cardiovascular mortality rates for adults ≥20 years of age were obtained. For each county, an extreme-heat day was identified if the maximum heat index was ≥90 °F (32.2 °C) and in the 99th percentile of the maximum heat index in the baseline period (1979-2007) for that day. Spatial empirical Bayes smoothed monthly cardiovascular mortality rates from 2008 to 2017 were the primary outcome. A Poisson fixed-effects regression model was estimated with the monthly number of extreme-heat days as the independent variable of interest. The model included time-fixed effects and time-varying environmental, economic, demographic, and health care-related variables.

RESULTS

Across 3108 counties, from 2008 to 2017, each additional extreme-heat day was associated with a 0.12% (95% CI, 0.04%-0.21%; P=0.004) higher monthly cardiovascular mortality rate. Extreme heat was associated with an estimated 5958 (95% CI, 1847-10 069) additional deaths resulting from cardiovascular disease over the study period. In subgroup analyses, extreme heat was associated with a greater relative increase in mortality rates among men compared with women (0.20% [95% CI, 0.07%-0.33%]) and non-Hispanic Black compared with non-Hispanic White adults (0.19% [95% CI, 0.01%-0.37%]). There was a greater absolute increase among elderly adults compared with nonelderly adults (16.6 [95% CI, 14.6-31.8] additional deaths per 10 million individuals per month).

CONCLUSIONS

Extreme-heat days were associated with higher adult cardiovascular mortality rates in the contiguous United States between 2008 and 2017. This association was heterogeneous among age, sex, race, and ethnicity subgroups. As extreme-heat events increase, the burden of cardiovascular mortality may continue to increase, and the disparities between demographic subgroups may widen.

Association of Extreme Heat With All-Cause Mortality in the Contiguous US, 2008-2017

IMPORTANCE

The number of extreme heat events is increasing because of climate change. Previous studies showing an association between extreme heat and higher mortality rates generally have been limited to urban areas, and whether there is heterogeneity across different populations is not well studied; understanding whether this association varies across different communities, particularly minoritized racial and ethnic groups, may allow for more targeted mitigation efforts.

OBJECTIVE

To the assess the association between extreme heat and all-cause mortality rates in the US.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional study involved a longitudinal analysis of the association between the number of extreme heat days in summer months from 2008 to 2017 (obtained from the Centers for Disease Control and Prevention's Environmental Public Health Tracking Program) and county-level all-cause mortality rates (obtained from the National Center for Health Statistics), using a linear fixed-effects model across all counties in the contiguous US among adults aged 20 years and older. Data analysis was performed from September 2021 to March 2022.

EXPOSURES

The number of extreme heat days per month. Extreme heat was identified if the maximum heat index was greater than or equal to 90 °F (32.2 °C) and in the 99th percentile of the maximum heat index in the baseline period (1979 to 2007).

MAIN OUTCOMES AND MEASURES

County-level, age-adjusted, all-cause mortality rates.

RESULTS

There were 219 495 240 adults aged 20 years and older residing in the contiguous US in 2008, of whom 113 294 043 (51.6%) were female and 38 542 838 (17.6%) were older than 65 years. From 2008 to 2017, the median (IQR) number of extreme heat days during summer months in all 3108 counties in the contiguous US was 89 (61-122) days. After accounting for time-invariant confounding, secular time trends, and time-varying environmental and economic measures, each additional extreme heat day in a month was associated with 0.07 additional death per 100 000 adults (95% CI, 0.03-0.10 death per 100 000 adults; P = .001). In subgroup analyses, greater increases in mortality rates were found for older vs younger adults (0.19 death per 100 000 individuals; 95% CI, 0.04-0.34 death per 100 000 individuals), male vs female adults (0.12 death per 100 000 individuals; 95% CI, 0.05-0.18 death per 100 000 individuals), and non-Hispanic Black vs non-Hispanic White adults (0.11 death per 100 000 individuals; 95% CI, 0.02-0.20 death per 100 000 individuals).

CONCLUSIONS AND RELEVANCE

These findings suggest that from 2008 to 2017, extreme heat was associated with higher all-cause mortality in the contiguous US, with a greater increase noted among older adults, men, and non-Hispanic Black individuals. Without mitigation, the projected increase in extreme heat due to climate change may widen health disparities between groups.