Proposition of heat-stroke risk assessment based on potential effective sweating possible to measure with portable equipment and its application to the Tokyo 2020 Summer Olympic marathon event

Abstract

We developed a method to assess heat-stroke risk during exercise in the outdoor environment based on the potential effective sweating (PES). The PES indicates the minimum effective sweating required to maintain a constant body temperature when undertaking an activity in the outdoor environment. It can be calculated easily using observed meteorological data, body size, clothing, and exercise intensity. Additionally, we propose the concept of globe PES (PESg) and PESWBGT, which can be measured using portable equipment, such as a wet-bulb globe temperature (WBGT) meter. Especially, PESWBGT can be estimated by only WBGT observations. We performed a thermal-load exposure experiment with human subjects in an outdoor urban environment, and compared the three kinds of effective sweating and actual potential sweating. The calculated PES values had deviations of 33.3-42.7% from actual potential sweating. We assessed the risk of heat stroke based on the decrease in a runner‘s weight using routine meteorological data observed in recent decades. Calculations were made for start times set at 30-min intervals from 05:00 to 19:00 on August 2 and 9, the dates of the marathon event in the 2020 Summer Olympics in Tokyo. When the start time was set from 9:30 to 11:00, a runner‘s weight was estimated to decrease by an average of 7.37-7.40% and a maximum of 8.09-8.20% by the end of the race. A postural movement or convulsion is likely to occur if the runner did not drink water. When the start time was set to 06:00, the weight decrease was predicted to be 6.18-7.15%; the thermal load at this time was equivalent to that of a 15:00 or 15:30 start time. Thermal loads may be smaller in the early evening than in the early morning. The average wind speed is weaker in the morning than in the afternoon, thereby decreasing the effect of wind on cooling the surfaces of the human body and the road in the morning. Hence, a change in the start time to early morning may not significantly decrease the thermal load.