Exposure and inequality of compound day-night heat among the elderly population in Asia

Persistent extreme heat during both day and night exacerbates health risks, particularly for the elderly. Based on Global Climate Model (GCM) data from the Inter-Sectoral Impact Model Intercomparison Project phase 3b (ISIMIP3b), this study investigates variations in the occurrence of Independent Hot Day (IHD), Independent Hot Night (IHN), and Compound Day-Night Heat (CDNH) and the resulting exposure risk among Asia's elderly population under Shared Socioeconomic Pathways (SSPs). Across all scenarios, CDNH prevalence increases considerably, driving a gradual shift from IHD and IHN toward CDNH and further elevating elderly exposure. During the reference period (1995–2014), CDNH exposure of Asia's elderly population was 6.88 million persons. Under all future scenarios, exposure surges significantly; inter-scenario differences are modest in the near term (2021–2040) but widen sharply thereafter, reaching 211.57 million persons in the middle term (2041–2060) and 830.38 million persons in the long term (2081–2100) under SSP585. High-exposure regions are concentrated in the Ganges Plain, eastern China, and Southeast Asia. Two inequality dimensions are examined: extreme inequality (low-income minus high-income countries) and moderate inequality (low- plus lower-middle-income minus upper-middle- plus high-income countries). Extreme inequality remains persistently negative and intensifies with emissions, indicating greater CDNH exposure burden in high-income countries. Moderate inequality shifts from negative to positive during the mid-to-late century, signaling a reversal whereby the exposure burden increasingly falls on lower-income country groups, which generally possess fewer economic and institutional resources for adaptation.