Warm Season Heat Exposure and Infant Mortality Across US Counties, 2017–2024

Warm season heat may be associated with infant mortality, but temperature definitions and county social context can influence estimates. We linked county mortality, daily maximum temperatures, and American Community Survey estimates in an exploratory ecological study. Complete samples comprised 1,483 overall county years during 2019–2024, 1,721 White county years and 963 Black county years during 2017–2024. Poisson mean models included county and year effects, live birth offsets, and county clustered uncertainty. Per 10 additional May–September days at or above 85°F, simultaneous adjustment for poverty, education, and Black population share yielded rate ratios of 1.0126 (95% CI, 1.0025–1.0227) for overall mortality, 1.0152 (95% CI, 1.0038–1.0267) for White mortality, and 1.0088 (95% CI, 0.9926–1.0252) for Black mortality. The overall 80°F association also persisted (rate ratio, 1.0135; 95% CI, 1.0018–1.0254). Formal Black–White comparisons were inconclusive. More warm days were associated with higher overall and White mortality after measured contextual adjustment. Exploratory cutoff selection and annual exposure aggregation limit interpretation of a minimum harmful temperature.