Urban Heat Island Impacts on Heat-Related Cardiovascular Morbidity: A Time Series Analysis of Older Adults in US Metropolitan Areas
The United States (US) population largely resides in metropolitan areas experiencing urban heat islands (UHIs) and climate change-driven temperature increases. Extreme heat has been linked to increased cardiovascular disease (CVD) risk, yet little is known about how this association varies between cities or with UHI intensity (UHII). We aimed to identify the US urban populations most at-risk of and burdened by heat-related CVD morbidity while considering the role of UHII. ZIP code-level daily counts of CVD hospitalizations among Medicare enrollees, aged 65-114, were obtained for 120 US metropolitan statistical areas (MSAs) between 2000-2017. Local average temperatures were estimated by interpolating daily weather station observations. ZIP codes were stratified into low and high UHII areas using the first and fourth UHII quartiles with an equal number of hospitalizations. MSA-specific associations between temperature and hospitalization were estimated using quasi-Poisson regression with distributed lag non-linear models and pooled via multivariate meta-analyses. Stratified analyses were performed by age, sex, race, and chronic condition status in low and high UHII areas. We also calculated the CVD hospitalizations attributable to heat. Extreme heat (MSA-specific 99th percentile, ~28.6°C) increased CVD hospitalization risk by 1.5% (95% CI: 0.4%, 2.6%), with considerable variation among MSAs. Between 2000-2017, there were an estimated 37,028 (95% CI: 35,741, 37,988) heat-attributable admissions, with most due to extreme temperatures. Risk in high UHII areas (2.4% [95% CI: 0.4%, 4.3%]) exceeded that in low UHII areas (1.0% [95% CI: -0.8%, 2.8%]). High UHII areas accounted for 35% of the total heat-related burden and disproportionately impacted already heat-vulnerable populations. The most at-risk and burdened populations were females, individuals aged 75-114, those with chronic kidney disease or diabetes living in high UHII areas. Overall, extreme heat increased cardiovascular morbidity risk and burden among older urban populations. UHIs exacerbated these heat-related impacts, especially among those with existing vulnerabilities.