Difference in heat-stroke ambulance visit risk by month in Japan

Abstract

Aim: Comparison of the all-cause mortality risk between early summer and late summer has been done and risk reduction was observed in late summer. This was considered as an adaptation to heat. Here, we evaluate the seasonal adaptation effect on heatstroke morbidity by comparing the risk from early summer to late summer. Methods: We obtained the data of heatstroke ambulance visit from Fire and Disaster Management Agency, Japan and temperature data from Japan Meteorological Agency for 47 prefectures in Japan. Using smoothing spline with the degree of freedom being 6, we evaluated the relation between daily maximum temperature and number of heatstroke ambulance visits for July, August, and September. Then, we calculated the risk ratio (August vs July and September vs July) at 30 degC and 35 degC. Results and Discussion: At 30 degC, the August risk ranged from 0.46 to 1.08, and the mean was 0.73; the September risk ranged from 0.21 to 0.59, and the mean was 0.38. At 35 degC, the August risk ranged from 0.55 to 1.02, and the mean was 0.72; the September risk ranged from 0.22 to 0.58, and the mean was 0.38. At both temperature levels, the risk in August was more than 25% lower than July. It is worthy of note that the daily maximum temperature was higher in August on average (30.9 degC vs 31.9 degC). The risk in September was even lower; 62 % lower than July. We speculate that this adaptation includes both physiological adaptation and behavioral adaptation (proper air-con use, or avoiding unnecessary outside activities).