# A short-term association between hospitalizations for mental disorders and ambient temperature in Japan: an ecological study using the LIFE Study data

## TL;DR

This study found that higher temperatures in Japan are linked to increased hospitalizations for mental disorders, with younger people being more affected.

## Contribution

The study provides new evidence on the short-term association between ambient temperature and mental disorder hospitalizations in Japan.

## Key findings

- Higher temperatures were associated with increased risk ratios for hospitalizations across all mental disorders.
- Younger people (<65 years) showed stronger associations between temperature and hospitalization risk compared to older people (≥65 years).
- Temperature-related attributable fractions were higher in younger age groups regardless of disorder type.

## Abstract

Few studies have investigated the association between ambient temperature and the risk of mental disorders in Japan. In this study, we investigated a short-term association between the risk of hospitalizations for mental disorders and ambient temperature using municipal health insurance data.

We used the data of the Longevity Improvement & Fair Evidence Study in Japan, and the data of 17 municipalities were employed in the analysis. The daily number of hospitalizations for schizophrenia, depressive disorders, and anxiety disorders was used as the outcome variable. The time-stratified case-crossover design was employed in this ecological time-series study, and a distributed-lag non-linear model using a conditional quasi-Poisson regression model was employed to investigate an association between ambient temperature and hospitalizations for the abovementioned mental disorders. The model was applied to each municipality, and a multivariate meta-analysis was conducted to pool the results of municipalities. In addition, subgroup analyses by sex and age groups were conducted, and temperature-related attributable fractions of the mental disorders were also calculated.

The results of the overall cumulative effect of ambient temperature on hospitalizations for mental disorders indicated that the risk ratio (RR) tended to increase with an increase in temperature regardless of the type of mental disorder. An analysis by sex indicated that the RR tended to increase with an increase in temperature regardless of sex. In addition, an analysis by age group indicated that an increase in RR with increasing temperature was more evident in persons aged <65 years compared to those aged ≥65 years regardless of mental disorders, and that the temperature-related attributable fractions were also higher in persons aged <65 years.

Higher temperatures were associated with a higher risk of hospitalization for mental disorders in Japan, while the degree of the association differed by age group.

The online version contains supplementary material available at https://doi.org/10.1265/ehpm.25-00377.