Diurnal Temperature Range and Its Association with Mortality and Morbidity in Japan, 2015–2019

While diurnal temperature range (DTR) is a risk factor for mortality, its association with morbidity remains unclear. This study aimed to examine the associations of DTR with mortality and morbidity in Japan and identify vulnerable populations. We collected mortality and emergency ambulance dispatch (EAD) data for all causes, circulatory and respiratory diseases from 47 prefectures in Japan from 2015 to 2019. We employed two-stage time series analyses using Quasi-Poisson regression with distributed lag nonlinear models and multivariate meta-analysis. Increased DTR was a risk factor for mortality and EAD from all causes and respiratory diseases. For circulatory diseases, a positive association was observed only with mortality. The risk for mortality was delayed by 1 day and persisted for 1–3 days, whereas the relative risk for EAD was highest at lag 0. Males showed significantly higher sensitivity to increased DTR for all-cause EAD, while individuals aged 65 years and older were more vulnerable to circulatory and respiratory EAD. For all-cause EAD, the Relative Risk in southern regions was significantly higher than in other regions. Our findings suggest that DTR is an important climate-related risk factor for both mortality and EAD, with variations observed in lag duration, disease, and subgroups.