Fig. 1: Mapping of County-Level Heat-related Deaths and the County Matching Process to the New Pathway.

a displays a distribution map of heat mortality at the county level in the United States, where varying colors represent the magnitude of death data. In the top-right corner are nine factors typically applied to traditional heat vulnerability assessments. b shows the county matching process, we identify counties with mortality data, labeled as “Vulnerable Counties” (e.g., County A), and those without mortality data, labeled as “ non-heat mortality county “ (e.g., Counties B to Z). We perform a county matching process based on mortality data for vulnerable counties. The matrix on the left represents factors related to heat vulnerability, while counties are listed at the top. The color intensity of each square reflects the magnitude of the factor for each county. Step (1) is population size filtering, where counties with similar population sizes are selected to mitigate population impacts. Step (2) calculates the Mahalanobis distance, identifying the top 50 non-heat mortality counties most similar to each vulnerable county. More detailed explanations are provided in the Method section and the Supplementary Information. Step (3) is outlier screening, sequentially evaluating the 50 counties in the matching pool for exceptionally high values until a suitable match is found. The identified non-heat mortality counties and their corresponding outlier are considered the matching county and the decisive factor for heat deaths in the vulnerable county, respectively.