Figure 1 illustrates the variation in the number of heatwave-occurring days, mortality rate of non-accidental deaths, and occurrence of heat-related illness in different regions. Supplementary Fig. S1 and Supplementary Tables S2-S6 provide additional information on the average maximum temperatures and annual trends. In 2018, Korea experienced its most severe heatwave, with a total of 31 heatwave-occurring days and the highest occurrence of heat-related illness (108.5 patients per 100,000 people) during the summer. However, in 2020, only 7.7 heatwave-occurring days were reported, similar to the 6.7 days recorded in 2014. The number of heat-related illness patients per 100,000 people also decreased in 2020 compared to 2014. On the other hand, non-accidental deaths were highest in 2020, with 209.9 deaths per 100,000 people.
Table 1 and Table 2 present the regional differences in excess non-accidental deaths and heat-related illness percentages in Korea during the summer of 2020. Notably, certain regions such as Busan, Gwangju, Daejeon, Chungnam, Gyeongbuk, and Gyeongnam experienced significant excess non-accidental deaths. Seoul, the epicenter of the second wave of COVID-19 in Korea, also had some excess non-accidental deaths, although not significant. Daegu, the epicenter of the first wave, had both deficits and excesses in heat-related illness patients. Gyeongnam, with relatively fewer COVID-19 cases, experienced steady excess non-accidental deaths throughout the summer.
Throughout the summer, the number of patients with heat-related illnesses continuously decreased, with Seoul having the highest deficit of patients, especially during August, which saw a surge in COVID-19 cases and the highest maximum temperature. In Daegu, deficits in heat-related illness patients were observed from May to July, but August saw a significant excess. Conversely, the Gyeongnam area experienced a constant decrease in the number of heat-related illness patients, with the largest decrease in May and the smallest in August.
The relative risk (RR) and absolute cumulative density function (ACF) provide insights into the impact of temperature on non-accidental deaths and heat-related illnesses. In 2020, the threshold temperature for excess non-accidental deaths was lower compared to previous years, while the threshold temperature for heat-related illnesses was higher in previous years than in 2020. Additionally, the ACF showed a steep increase in non-accidental deaths in 2020 at temperatures above 25°C, while the ACF for heat-related illnesses in 2020 was smaller compared to previous years.
Figure 4 presents the ACF results for Seoul, Gyeongnam, and Daegu. In Seoul, the ACF for non-accidental deaths in 2020 was higher compared to 2018 at a similar temperature. However, the ACF for heat-related illnesses decreased by 47% in 2020 compared to 2018. Similar patterns were observed in Gyeongnam and Daegu, with higher ACF for non-accidental deaths in 2020 and lower ACF for heat-related illnesses at similar temperatures compared to 2018.