# Effects of high summer temperatures on heatstroke-related ambulance dispatches in Japan: A nationwide time-stratified case-crossover analysis

## TL;DR

High summer temperatures in Japan are linked to increased ambulance calls for heatstroke, with effects lasting up to two days and varying by region.

## Contribution

This study quantifies the temperature-morbidity relationship for heatstroke ambulance dispatches in Japan using nationwide data and a novel statistical approach.

## Key findings

- Higher temperatures significantly increased heatstroke-related ambulance dispatches in Japan.
- The risk of ambulance dispatches persisted for up to two days after high temperature exposure.
- Geographic variation in heatstroke cases was significant across Japanese prefectures.

## Abstract

While the short-term effects of ambient temperature on heatstroke mortality have been studied across various countries, the impact on morbidity and its spatial distribution remains insufficiently examined. We quantified the association between maximum temperature and heatstroke-related ambulance dispatch (HSAD) cases in Japan using daily national data.

Daily HSAD counts and meteorological data (daily maximum temperature (°C), relative humidity (%), wind speed (m/s), and sunshine duration (h)) were analyzed for June–September (summer), 2015–2019, across 47 Japanese prefectures. A time-stratified case-crossover study with conditional quasi-Poisson regression combined with a distributed lag non-linear model was used to estimate maximum temperature-HSAD associations in Japan between 2015 and 2019. A random-effects meta-analysis pooled country-level association.

A total of 300,528 HSAD cases were analyzed. Higher temperatures were associated with an increased risk of HSAD cases. The overall lag-cumulative relative risk at the 95th percentile of maximum temperature (35.3 °C) compared to the minimum morbidity temperature (18.2 °C) was 19.23, with a 95 % confidence interval of 13.95 to 26.51. Lagged effects of higher temperatures persisted for up to 2 days. Significant geographical variation in HSAD cases was observed (Cochran Q test, p < 0.001; I2 = 84.0 %). The association between maximum temperature and HSAD cases was significantly modified by the use of air conditioning (likelihood ratio test, p = 0.01).

Our study demonstrated the influence of heat on HSAD cases in Japan, emphasizing the necessity of preventive strategies to reduce the impact of temperature on heatstroke morbidity.

•Temperature was associated with heatstroke-related ambulance dispatches in Japan.•Risk persisted for approximately 2 days post-exposure.•Geographical heterogeneity among prefectures was observed.•Air conditioning use modified the exposure–response association.

Temperature was associated with heatstroke-related ambulance dispatches in Japan.

Risk persisted for approximately 2 days post-exposure.

Geographical heterogeneity among prefectures was observed.

Air conditioning use modified the exposure–response association.