Urban-suburban-rural gradient variations in mortality associated with compound exposure to heat wave and ozone...

Ever-increasing heat wave and frequent ambient ozone (O₃) pollution in China has been exacerbating premature mortality. However, the urban-suburban-rural compound exposure inequity has seldom been explored. Here, a space-time-stratified case-crossover study of 1,040,125 all-cause deaths from 1390 townships in Anhui province, China, during 2013-2021 was conducted to comprehensively investigate the spatiotemporal variations in excess mortality attributed to compound exposure to both hazards and its driving factors along the UR (urban area)-MU (urban-dominated mixed region)-MR (rural-dominated mixed region)-RU (rural area) fourfold structure. Under various definitions, the odds ratios (ORs) of mortality associated with compound exposure to heat wave and O₃ pollution ranged from 1.09 (95% CI, 1.06-1.12) to 1.28 (1.19-1.37) in UR; 1.13 (1.09-1.18) to 1.44 (1.32-1.56) in MU; 1.15 (1.13-1.16) to 1.50 (1.43-1.56) in MR; and 1.15 (1.12-1.18) to 1.69 (1.43-1.94) in RU. Correspondingly, the estimated excess mortality rates per 100,000 people ranged from 0.06 (95% CI, 0.04-0.07) to 0.38 (0.25-0.50) in UR; 0.05 (0.04-0.07) to 0.56 (0.37-0.76) in MU; 0.05 (0.04-0.06) to 0.87 (0.78-0.96) in MR; and 0.04 (0.02-0.05) to 0.89 (0.69-1.08) in RU. Notably, the excess mortality attributed to exposure to concurrent heat wave and O₃ pollution was found to exhibit significant gradient variations across the UR-MU-MR-RU spatial structure. Overall, increased exposure to compound days (83.03%), rising baseline mortality (10.25%), and population aging (6.65%) were the primary drivers for the increase of attributable deaths. However, along the UR-MU-MR-RU gradient, the contribution of exposure to excess mortality decreased from 86.72% to 79.9%, while baseline mortality contributions increased from 5.4% to 13.42%, highlighting the roots of structural vulnerability in marginalized communities facing compound climate hazards. These findings support the spatially targeted measures for urban, suburban, and rural areas to better protect populations from increasingly prevalent compound climate hazards.