Extreme heat and hospital admissions in older adults: A small-area analysis in the Greater Boston metropolitan area.

Extreme heat (EH) is a leading cause of weather-related fatalities in the United States. In Massachusetts, average temperatures have increased by 1.9 °C since the 20th century, higher than the global average increase of 1.1 °C. EH disproportionately impacts communities, exacerbating social inequities. This study examined the risks of heat-related hospitalizations in each small area of the Greater Boston Metropolitan Area using Medicare data (2000-2016). EH events included daily heat index (HI), days with an HI above the 90th percentile, and heat waves (≥2 consecutive EH days). We applied a case-crossover design to estimate area-specific associations between EH and hospitalizations and assessed effect modifications by an individual (age ≥85, sex, Medicaid dual eligibility) and ZIP-code characteristics (green space, poverty, educational attainment, and household income). Results were pooled using random effects meta-analysis. Area-specific analysis revealed higher hospitalization risks in Boston compared with surrounding areas. Pooled results indicated heat-related hospitalizations increased by 9.0% (95% CI = 5.7, 12.3) per 10 °C rise in HI, 14.4% (95% CI = 8.8, 20.3) on EH days, and 17.9% (95% CI = 11.1, 25.1) during heat waves. Risks were more pronounced in Boston, and some indications of elevated risk among males and residents in low-income, low-education areas. These findings underscore that heat-related health risks may be different across the level of geographic units and suggest the need for targeted public health strategies to mitigate the impacts of EH.