Figure 1 shows the number of heatwave-occurring days, the mortality rate of non-accidental deaths, and the occurrence of heat-related illness and the regional characteristics of these values, including average maximum temperatures are shown in Supplementary Fig. S1 and the annual trend is shown in Supplementary Tables S2–S6. From 2013 to 2020, the patterns of the number of heatwave-occurring days varied by year; Korea experienced the most severe heatwave in 2018, recording a total of 31 heatwave-occurring days, with the highest occurrence (108.5 heat-related illness patients per 100,000 people) during the summer. In 2020, 7.7 heatwave-occurring days were reported, which was similar to the 2014 record of 6.7 heatwave-occurring days. In total, 48.4 heat-related illness patients per 100,000 people occurred in 2020, and the number of patients decreased compared to that in 2014 (56 heat-related illness patients per 100,000 people). Moreover, non-accidental deaths were the highest in 2020, with 209.9 deaths per 100,000 people.
In 2020, throughout the study period of May–September, 110,377 non-accidental deaths were reported in Korea, exceeding the 892 predicted deaths (Fig. 2). Excess non-accidental deaths were the highest in August, with an additional 658 deaths; 4% of excess deaths were from COVID-19 (n = 26), while 96% of excess deaths were due to other causes of death. In 2020, 46% of deficit in the patients with heat-related illnesses were observed throughout the study period, with the highest 63% of deficit in September. Moreover, a positive maximum temperature anomaly was observed in June and August, and Korea experienced a second wave of COVID-19 with an epicenter in the capital metropolitan area in mid-August.
The magnitude of excess deaths differed by region in Korea. Tables 1 and 2 show the results of the excess changes in non-accidental deaths and heat-related illness percentages. Supplementary Table S7 shows the characteristics of maximum temperature and COVID-19 confirmed cases by region in 2020. In Korea, 3% of non-significant non-accidental deaths were observed in August, with significant excess non-accidental deaths in Busan (5.1%), Gwangju (7.7%), Daejeon (5.5%), Chungnam (6.8%), Gyeongbuk (7.7%), and Gyeongnam (7.1%). Seoul, the epicenter of the second wave in Korea, experienced 0.8% of non-significant excess non-accidental deaths. Moreover, in Daegu (the epicenter of the first wave), the highest cumulative COVID-19 confirmed cases occurred until the summer of 2020, and exposure to the highest maximum temperature resulted in 3.7% of non-significant excess non-accidental deaths in August. Gyeongnam, which experienced the lowest COVID-19 confirmed cases in Korea during the summer, experienced non-significant but constant excess non-accidental deaths throughout the summer, with 7.1% of the highest significant excesses in August.
In Korea, a constant decrease in the number of patients with heat-related illnesses was observed throughout the summer (May (− 44.5%), June (− 48.7%), July (− 48.6%), August (− 40.7%), and September (− 51%)). Furthermore, in August, the second wave of COVID-19 surged in Korea, and the highest maximum temperature was observed. Moreover, the number of patients with heat-related illnesses differed by region and month. Seoul, which had the highest incidence of COVID-19 confirmed cases, constantly experienced a deficit of heat-related illness patients during the summer (May (− 56%), June (− 58.3%), July (− 58.6%), August (− 59.2%), and September (− 63.3%)). However, in Daegu, constant deficits in heat-related illness patients were observed from May to July, whereas 35.9% of the significant excesses in heat-related illness patients were observed in August. In August, Daegu experienced the highest maximum temperature of 36.1 °C and the average maximum temperature in August was 32.1 °C. In the Gyeongnam area, which was relatively less affected by COVID-19 in Korea during the summer, there was a constant decrease in the number of heat-related illness patients, with the largest decrease in May (− 69.4%) and the smallest decrease in August (− 28.1%).
The RR of non-accidental deaths and heat-related illnesses in 2020 was compared to previous years (2013–2019) (Fig. 3). The RR of non-accidental deaths at 33.1 °C was 1.00 (CI 0.99–1.01) and 1.04 (CI 1.02–1.07) in 2013–2019 and 2020, respectively. The threshold temperature (RR > 1) was lower in 2020 (29.1 °C) when compared to the value of historical years, 2013–2019 (32.8 °C). The RR of heat-related illness at 33.1 °C was 1.44 (CI 1.42–1.45) and 1.59 (CI 1.54–1.64) in 2013–2019 and 2020, respectively. In terms of the RR of heat-related illness, the threshold temperature (RR > 1) was higher in 2013–2019 (28.9 °C) than 2020 (27.4 °C). The absolute cumulative functions (ACF) of non-accidental deaths and heat-related illnesses in 2020 were compared between 2013 and 2019 (Fig. 3). The ACF of non-accidental deaths in 2020 had increased steeply compared to other historical years, and the ACF in 2020 showed the biggest value at temperatures above 25 °C. However, the ACF of heat-related illnesses was the smallest in 2020 compared to previous years. For example, the ACF of heat-related illnesses was approximately 92.14 per 100,000 people in 2018, the hottest summer, compared to 48.33 per 100,000 people in 2020.
Figure 4 shows the ACF results for non-accidental deaths and heat-related illnesses in Seoul, Gyeongnam, and Daegu. In Seoul, the ACF of non-accidental deaths in 2020 was 1.43 per 100,000 people (30.06 °C), which was higher than 2018 at a similar temperature (1.17 per 100,000 people at 30.35 °C). However, the ACF for heat-related illness in 2020 was 0.52 per 100,000 people (35.05 °C), which was a 47% decrease from the ACF of 2018 at a similar temperature (0.98 per 100,000 people at 35.28 °C). In Gyeongnam, the ACF of non-accidental deaths in 2020 was 6.35 per 100,000 people (30.05 °C), which was higher than 2018 (4.86 per 100,000 people at 30.04 °C). However, the ACF of heat-related illness in 2020 was 1.82 per 100,000 people at the highest maximum temperature (34.23 °C), which was 50% less than the ACF of 2018 at a similar temperature (3.58 per 100,000 people at 34.24 °C). In Daegu, the ACF of non-accidental deaths in 2020 was 7 per 100,000 people (30.05 °C), which was higher than 2018 (5.49 per 100,000 people at 30.05 °C). The ACF of heat-related illness in 2020 was 1.14 per 100,000 people at 35.05 °C, which was a 15% decrease from the ACF of 2018 at a similar temperature (1.33 per 100,000 people at 35.05 °C).