t="_self">Figure 1. Estimation of the relation between mean air temperature and ambulance transports due to acute disease by quadratic curve in Sakata area, Yamagata prefecture, Japan.
Table 3. Approximation by quadratic curve between ambulance transports per day and climate parameters.
temperature and daylight hours. The correlation coefficient between mean air temperature and ambulance transports per day was the highest among variables. Finally, from the regression line, a 14.4˚C in mean air temperature was corresponded to a change point of ambulance transports per day.
The main finding of this study was to explore the link between ambulance transports due to acute disease and low air temperature in Sakata area, Yamagata prefecture, Japan by ecological study.
Although direct effects of low air temperature are only in part of the result of increased stress on the circulatory system , low air temperature was associated with higher mortality rates form myocardial infarction by retrospective analysis in Northern Ireland . According to the relation between air temperature and total mortality, Braga et al. showed that, in cold cities in US, both high and low air temperature were associated with increased death. The effect of cold air temperatures persisted for days and no clear humidity effect was observed . In this study, we firstly evaluated the link between ambulance transports due to acute disease and air temperature in Sakata area, Yamagata prefecture, Japan. Lower air temperature was closely linked to higher ambulance transports, as well as higher air temperature was linked to higher ambulance transports by using monthly data [1,2]. In addition, the correlation coefficient between mean air temperature and ambulance transports due to acute disease was the highest among variables by estimation of quadratic curve, and a 14.4˚C in mean air temperature was corresponded to a change point of ambulance transports by quadratic curve. These findings suggest that we need to pay attention to dealing with the relation between low air temperature and its related health conditions as well as high air temperature.
In turn, we have previously proved that positive changes in air temperature were noted and these were accelerated in all 47 prefectures of Japan [1,13]. Wanitschek et al. reported that the average air temperature increase of 7.5˚C from the cold to the warm winter was associated with a decrease in acute coronary angiographies, in particular due to a lower incidence of ST elevation myocardial infarctions . Martens reported that global climate change was likely to read to a reduction in mortality rates due to decreasing winter mortality and this effect is the most pronounced for cardiovascular mortality in elderly people in cities who experience temperate or cold climates at present . Doyon et al. reported that the summer increase and the annual mortality range from about 2% and 0.5% for the 2020 period, to 10% and 3% for the years around 2080 in Quebec, Canada . Taken together, analysis of any climatic change also should take into account low air temperature-related health problems in future.
Potential limitations still remain in this study. First, we used an ecological study. The link between ambulance transports due to acute disease and air temperatures, which was noted in this study, may not apply for the link among individuals. Second, detailed daily and individual (i.e. age and sex) data of ambulance transports due to acute disease could not be obtained and analyzed in this study. Therefore, we could not evaluate the link between ambulance transports and air temperatures as accurately as we wished. Further ongoing studies are urgently required to prove such a link.