Five events (men's and women's 20 km race walk, men's 50 km race walk, women's marathon, and men's marathon) were held at Sapporo from August 5th to August 8th, 2021 (Table 1). Men's 50 km race walk, men's marathon, and women's marathon were scheduled in the early morning to minimize the influence of solar radiation and ambient temperature. The race walk used a loop course starting at Odori Park, using 20-laps of 1 km circuit course for the 20 km race walk, and 25-laps of 2 km circuit course for the 50 km race walk. The start and the end of the Marathon course were also set at Odori Park. The marathon course consisted of a large loop, which covered the first 20 km of the race, followed by a smaller loop, ~10 km in the distance, which the athletes ran twice, totaling three laps. All athletes who required medical attention were transferred to the athlete medical station, which was located adjacent to the finish line.

The athlete medical station was equipped with rectal thermometers and ice baths to provide cold-water immersion when the collapsed athlete was diagnosed with exertional heat stroke (Hosokawa et al., 2021). Once the collapsed athlete was admitted to the athlete medical station, medical volunteers who were trained in the exertional heat stroke prehospital care determined the need to assess the athlete's internal body temperature using a rectal thermometer (DataTherm II Continuous Temperature Monitor, Geratherm Medical AG). Athletes with confirmed hyperthermia (rectal temperature ≥40.5°C) and central nervous system dysfunction were cooled using whole-body cold water immersion. Water temperature in the ice bath was kept between 10 and 15°C. Rectal temperature was monitored throughout the duration of cooling, and it was ceased when the rectal temperature reached <39°C. EHS diagnosis and treatment were carried out by a physician, nurses, and physical therapists.

Medical volunteers stationed at the athlete medical station took medical records (e.g., physician, nurse, physical therapist). The data were first recorded on the paper chart and later entered into an electronic system for statistical analysis.

Environmental conditions observed at the start line during men's 50 km race walk, women's marathon, and men's marathon are summarized in Figures 1, 2. The start time T_a, RH, and wet bulb globe temperature (WBGT) suggest that the corresponding flag system for thermal stress as defined by World Athletics was “Caution (WBGT, 21 to 25°C)” (men's 50 km race walk, T_a = 25°C, RH = 86%, WBGT = 24°C; women's marathon, T_a = 25°C, RH = 84%, WBGT = 23°C; men's marathon, T_a = 28°C, RH = 72%, WBGT = 24°C) (Adami et al., 2020).

The percentage of DNF athletes in each event was 8.8, 8.6, 20.3, 17.0, and 28.3% in men's and women's 20 km race walk, men's 50 km race walk, women's and men's marathon, respectively (Table 2).

A total of fifty athletes were transferred to the athlete medical station: 28 athletes completed the race (i.e., collapsed after finish line), while 22 were DNF athletes transported from the field of play.

Forty-eight (96%) of athletes who were admitted to the athlete medical station exhibited signs and symptoms of EHI. HE was most common (n = 22), followed by EAMC (n = 12), HS (n = 12), and EHS (n = 2) (Tables 3A,B).

EHI incidence rate per 1,000 finishers was highest in 50 km men's race walk (170.2 per 1,000 finishers), followed by men's marathon (118.4 per 1,000 finishers), women's marathon (109.6 per 1,000 finishers), men's 20 km race walk (38.5 per 1,000 finishers) and women's 20 km race walk (18.9 per 1,000 finishers) (Table 4). Two athletes diagnosed with EHS and three athletes with severe HE (rectal body temperature >39.5°C with or without central nervous system disturbance) were cooled using whole-body cold water immersion at the heat deck located within the athlete medical station. All athletes were cooled successfully and recovered without any complications. The average recorded cooling rate to reach 39°C was 0.11 ± 0.03°C (EHS, 0.13 ± 0.02°C; HE, 0.10 ± 0.02°C). These athletes required an average of 14 ± 9.4 min (range, 6–30 min) to cool their rectal temperature below 39°C (Figure 3).

Epidemiological data from race walk and marathon events suggest that a high incidence rate of EHI is associated with high T_a and RH (Armstrong et al., 2007; Hosokawa et al., 2018). The 2020 Summer Olympics in Tokyo was no exception, which led the Tokyo Organizing Committee of the Olympic and Paralympic Games to relocate the race walk and marathon competitions to Sapporo, which was predicted to experience much milder heat. A study conducted after the announcement of the relocation revealed that Sapporo was indeed modeled to experience less thermal stress than Tokyo when the Physiologically Equivalent Temperature and the modified Physiologically Equivalent Temperature were calculated (Wu et al., 2020). Another study conducted at a similar time examined the predicted WBGT and universal thermal climate index (UTCI) between Tokyo and Sapporo during the Games (Seo and Honjo, 2021). Their study also suggested a milder heat strain in Sapporo than in Tokyo, where the predicted average WGBT and UTCI were lower by 4.4 and 6.3°C, respectively. Nonetheless, Sapporo experienced extreme heat during the Games (Figures 1, 2), which suggests that a data-driven preparation could not sufficiently detect the risk of extreme heat. In fact, the %DNF and EHI incidence rates in women's marathon could have been much worse if the competition had started at the scheduled time since the WBGT was already at 27.5°C by 7:00 am. Therefore, future races should consider a much earlier start, such as before sunrise (i.e., 4:00 am), which would allow the race to conclude around 6–7 am. In addition, future organizers of elite endurance events should prepare a contingency plan for unexpected heat events to make sure that the event operation has the flexibility to accommodate sudden changes in the event schedule. The contingency plan should, at minimum, address arrangements of event and medical volunteers, traffic and road closure controls, audience control and security, ambulance services, and broadcasting and media relations.

The percentage of DNF athletes documented at Sapporo was less in all events except for the men's marathon when compared to the World Athletics Championships in Doha, 2019. They experienced one of the most extreme heat events for elite endurance competitions in recent years (event start T_a range, 29–32°C; event start RH range, 51–77%) (Bermon and Adami, 2019), resulting in %DNF of 23.1, 13.3, 39.1, 24.7, and 41.2% in men's and women's 20 km race walk, men's 50 km race walk, women's and men's marathon, respectively. The reduction in %DNF under comparable environments may have been due to increased preparedness among athletes since the Doha World Championships.

Almost all (96%) of the athletes who were transferred to athlete medical station were EHI. Five athletes required cold water immersion due to EHS and severe HE. EHI incidence rates at the Olympic race walks and marathons in this study were high compared to other mass-participation endurance events. For example, the combined incidence rates of EHS and HE at a shorter summer road race (11.3 km) reported an incidence rate of 3.10 runners per 1,000 finishers (Hosokawa et al., 2018). EHI incidence rates in mass-participation marathon events could range from 11.7 per 1,000 entrants in cold to mild environments (Ta = −4~16°C) (Roberts, 2000) to 148.9 per 1,000 entrants in hot environments (T_a = 32–34°C) (Ogwumike and Adeniyi, 2013). In elite-level international athletics championships, the incidence rate of EHI in marathon and race walking increases to 71.8 and 68.6 runners per 1,000 registered athletes, respectively (Hollander et al., 2021). However, the data observed in Sapporo exceeded further, where the average EHI incident rate in men's and women's marathons combined was 114.1 per 1,000 finishers, and the average EHI incidence rate of the three-race walks in Sapporo was 72.4 per 1,000 finishers. When examined by event, long-duration events that were held in the morning (men's 50 km race walk, men's marathon, and women's marathon) reported extremely high EHI incidence rates per 1,000 finishers, compared to shorter-distance events that were held in the late afternoon (men's and women's 20 km race walk). This suggests that the plan to avoid mid-day heat for long-duration events did not sufficiently work to reduce the risk of EHI in Sapporo, whose historical environmental condition is much milder than in Tokyo. While it may not be a conventional time schedule, race medicine in future elite endurance events that are being scheduled during the warm season may need to consider much earlier start times in the morning.

Five athletes (two EHS and three HE) were cooled using whole-body cold water immersion at the heat deck. The initial temperature of these athletes ranged from 39.55 to 42.6°C (Figure 3) with an average cooling rate of 0.11 ± 0.03°C to reach 39°C. This cooling rate is lower than the previously reported average cooling rate from 274 EHS runners who were treated at the Falmouth Road Race (average cooling rate, 0.22 ± 0.11°C) (Demartini et al., 2015). The reason for the slower cooling rate observed in our dataset compared to the Falmouth Road Race is unknown (for example, the heat deck in Sapporo was air-conditioned while the heat deck at the Falmouth Road Race is not); however, the difference in the total number of patients (Sapporo, n = 5; Falmouth Road Race, n = 274) and inter-variability of the body temperature response to whole-body cold water immersion may have influenced this results (Poirier et al., 2018; Hosokawa et al., 2020). Nevertheless, the cooling rate documented during marathons and race walks during the Games was sufficient to reduce the runner with the highest initial rectal temperature below 39°C within 30 min (Runner 11, Figure 3A), which is within the best medical practice of EHS prehospital management (Hosokawa et al., 2021). Therefore, our dataset suggests that the standardized EHS prehospital management implemented by the trained medical volunteers during the Tokyo Summer Olympic Games sufficiently addressed the medical emergency.