Impact of Maximum Air Temperature on Ambulance Transports Owing to Heat Stroke During Spring and Summer in Tottori Prefecture, Japan: A Time-stratified Case-crossover Analysis

[Background] The frequency and intensity of hot weather have increased. In Japan, there have been many studies of the relationship between ambulance transports owing to heat stroke (ATHS) and high air temperature in the summer season. However, there have been very few reports focusing on ATHS in spring. Therefore, we investigated the effect of the maximum air temperature on ATHS not only in summer but also in spring, to help with development of effective measures to prevent heat stroke. [Methods] We obtained daily ATHS and meteorological data from April to September in 2017 in Tottori Prefecture. We used a time-stratified case-crossover method for data analysis. [Results] A total 382 cases of ATHS were identified from April to September in 2017 in Tottori. The number of cases was highest in July, followed by August and May. Maximum air temperature was significantly linked to ATHS. The risk of ATHS was increased 1.13 times when maximum air temperature rose by 1°C. In summer, the risk on extremely hot days (maximum air temperature ? 35°C) increased by 5.55 times or more compared with that on days below 30 °C (< 30°C). The risk was approximately four times greater on hot days (? 30°C and < 35°C) than that on relatively cooler days (< 30°C) during the spring months of April through May. [Conclusion] Maximum air temperature was significantly linked to ATHS. It is necessary to pay particular attention to heat stroke prevention not only on extremely hot days in summer but also on hot days in the spring.