Thirty-five years of accelerating heat stress in Kenya: subnational patterns of hazard, exposure burden, and vulnerability in priority populations

Background: Extreme heat is an increasing public health threat in sub-Saharan Africa,yet evidence on subnational disparities and group-specific vulnerability remains limited.We characterized heat hazard, exposure, and vulnerability across all 290 Kenyansubcounties, focusing on pregnant women, children under five years, and adults aged60 years and older.Methods: We conducted a nationwide ecological panel study using annual subcountyobservations from 2015 to 2025, with extended climatological context from 1991 to2025. Heat hazard was defined using Universal Thermal Climate Index (UTCI)-derivedmetrics from the Copernicus ERA5 HEAT product, including annual counts of days withUTCI > 38°C (H38) and UTCI > 46°C (H46). Group-specific population denominatorswere derived from WorldPop, and socioeconomic vulnerability was represented usingthe Relative Wealth Index. We quantified long-term trends, anomalies relative to the1991 to 2020 baseline, hotspot persistence, person-day exposure burden, and acomposite Heat Vulnerability Index for each population group.Results: National area-averaged mean annual H38, defined as days per year withUTCI > 38°C, increased significantly over 1991 to 2025 (Sen slope 0.97 days per year;p < 0.001), reaching record levels in 2024 and 2025. In 2025, 136 subcountiesrecorded zero H38 exposure, whereas 61 experienced sustained heat, defined as H38≥ 30 days per year. These 61 sustained-heat subcounties accounted for 96.2% ofannual person-day heat burden for pregnant women, 96.8% for children under five,and 95.0% for older adults. Heat exposure was inversely associated with subcountylevel relative wealth (Spearman rho = -0.561; p < 0.001). In a separate hotspot-stabilityanalysis, 62 subcounties entered the annual top-quintile H38 tier at least once during2015 to 2025, and 50 of these remained in that tier in all 11 years. Heat VulnerabilityIndex rankings were highly concordant between pregnant women and children underfive (Spearman rho = 0.977), but less so for older adults, whose priority geographyextended further into coastal transitional subcounties. Rankings based on 2025conditions were highly consistent with rankings based on mean conditions over 2015 to2025.Conclusions: Heat stress in Kenya has increased markedly over the past 35 years andis concentrated in a structurally stable set of socioeconomically disadvantagedsubcounties. These findings support tiered, geographically targeted heat-healthplanning, with shared priority areas for maternal and child health and additional agespecific targeting for older adults.