Heat-related impacts on all-cause emergency hospitalisation differ by area deprivation and urbanicity: a time-stratified case-crossover study in Japan

Background: Climate-related health impacts have been a global public health concern. Identifying vulnerable populations is critical in implementing adaptation strategies. This study aimed to examine how heat-related impacts on all-cause emergency hospitalisations differ by area deprivation and urbanicity.

Methods: All-cause emergency hospitalisations were identified in the Japanese nationwide administrative database during the warm season between 2011 and 2019. A time-stratified case-crossover study was conducted to examine short-term associations between daily mean temperature and hospitalisation. Days of heat exposure were defined as days when the daily mean temperature exceeded the minimum morbidity temperature (ie, temperature with the lowest relative risk between the 25th and the 75th percentiles of the daily mean temperature distribution). Analyses were stratified by area deprivation index and urbanicity. Heat-related excess hospitalisations were quantified using the population attributable fraction (PAF), derived as a fraction of heat-attributable emergency hospitalisations to the total number of emergency hospitalisations for all study subjects or within specific subgroups.

Results: We identified 5 914 084 hospitalisations. Among all study subjects, PAF for heat-related excess hospitalisations was 1.69% (95% CI 1.54% to 1.87%). PAF for heat-related excess hospitalisations was more pronounced in people living in the most deprived areas (1.87%, 95% CI 1.68% to 2.06%) than those in the least deprived (1.19%, 95% CI 0.98% to 1.41%) and in urban populations (2.03%, 95% CI 1.78% to 2.30%) than rural ones (1.42% (95%CI 1.24% to 1.60%)). When further stratified by deprivation and urbanicity simultaneously, PAF for heat-related excess hospitalisations was most significant among urban populations living in the most deprived areas (2.62%, 95% CI 2.26% to 3.03%).

Conclusion: These findings revealed that individuals living in the most deprived areas in urban settings were particularly vulnerable to heat exposure. Adaptation strategies tailored to socioeconomic and geographical inequalities can potentially reduce future heat-related health impacts.