The interplay of heatwaves, air pollution, and green space on all-cause mortality in older adults with diabetes mellitus: a national cohort study

BACKGROUND: While extreme heat represents an established risk factor for mortality in patients with diabetes mellitus (DM), its synergistic effects with air pollution and green space remain incompletely elucidated. This study aimed to investigate the joint effects of heatwaves, air pollution, and green space on all-cause mortality in older adults aged ≥ 45 years with diabetes mellitus. METHODS: We conducted a longitudinal analysis of data from 1,400 adults aged ≥ 45 years with DM, drawn from the China Health and Retirement Longitudinal Study (CHARLS, 2011-2020). Heatwaves were defined according to six intensity-duration thresholds, based on two temperature thresholds (95th/97.5th percentiles of daily maximum temperature) and three duration criteria (≥ 2/≥3/≥4 consecutive days). Time-varying Cox proportional hazards models were used to estimate hazard ratios (HRs), with adjustments for individual PM₂.₅, NO₂ and greenness exposures as time-varying covariates. Multiplicative and additive interactions were evaluated using product terms and the relative excess risk due to interaction (RERI), complemented by the attributable proportion due to interaction (AP) and the synergy index (S). RESULTS: Heatwave exposure was significantly associated with increased mortality risk in a graded exposure-response relationship, with HRs increasing from 1.027 (95% CI: 1.013-1.041) to 1.070 (95% CI: 1.048-1.093) as heatwave intensity and duration increased. Additive interaction analyses demonstrated significant synergistic effects between heatwaves and both air pollutants and reduced greenness. These combined effects were more pronounced among older individuals and urban residents. CONCLUSION: Heatwaves interact synergistically with air pollution and diminished greenness to elevate mortality risk in diabetic patients. These findings highlight the imperative for integrated public health strategies that concurrently address these coexisting environmental exposures. CLINICAL TRIAL REGISTRATION: Not applicable.