Impact of the urban heat island on heat-related mortality in the Tokyo Metropolitan Area, Japan

Abstract With global temperatures rising, urban heat islands (UHIs) intensify heat exposure in cities, potentially elevating heat-related mortality. However, the specific burden of UHI-attributable mortality remains inadequately quantified. This study investigates the impact of UHIs on heat-related mortality in the Tokyo Metropolitan Area (TMA), Japan, from 2010 to 2019. Daily time series data on maximum and mean ambient temperatures and all-cause mortality were obtained for 149 municipalities. Urban municipalities were identified by the presence of densely inhabited districts, with adjacent non-urban municipalities at similar elevations serving as a baseline (Zone 1). UHI intensity was quantified by calculating municipality-specific UHI anomalies (UHIa) as the deviation of daily maximum temperatures in each urban municipality from the average mean temperatures of Zone 1. Based on UHIa values, urban municipalities were grouped into nine zones (Zones 2–10). Heat-mortality relationships across zones were estimated using distributed lag nonlinear models, with relative risks (RRs) calculated at 30 °C using 26 °C as the reference. Heat-related mortality was computed for observed and counterfactual scenarios, the latter assuming urban temperatures matched non-urban levels. Differences in estimated mortality represented UHI-attributable impacts in the TMA. A mean UHIa difference of 0.72 °C was observed between Zones 2 and 10, with UHIa values ranging from 4.77 °C to 5.49 °C. No significant heterogeneity in RRs was observed across zones. UHI-attributable mortality was estimated at 2207 deaths (95% empirical confidence interval (eCI): 1200, 3119), accounting for 30.0% of urban heat-related deaths from 2010 to 2019. Zone-specific UHI-attributable mortality ranged from 9 deaths (95% eCI: 5, 13) to 593 deaths (95% eCI: 323, 839). These findings highlight the potential health impact of UHIs in the TMA and underscore the importance of integrating UHI effects into future temperature-mortality assessments to better understand urban heat-related health risks.