The ministry selected the 14 venues and set observation equipment outdoors at the venues from July 11 through Sept. 15 last year to measure the WBGT index.
WBGT is a measure of heat stress based on temperature, humidity and direct sunlight, which was invented in the 1950s in the United States. The Japan Sports Association divides guidelines for exercise into several categories based on the index. According to the association, it is "generally safe" to do exercise when the index is below 21; "caution" is needed at 21 or above but below 25; "warning" is given at 25 or above but below 28; "severe warning" is given at 28 or above but below 31; and the association suggests not engaging in exercise in principle if the index rises to 31 or above.
Of the 14 venues, an index reaching 31 or above was recorded at Kasumigaseki Country Club in Kawagoe, Saitama Prefecture, for 27 days; new National Stadium in Tokyo and Tokyo Stadium in Chofu, Tokyo, for 26 days; and Saitama Stadium for 25 days. The index reached at least 31 once every three days or so.
Index 31 or above is classified into the most dangerous level of the five-level scale set by the Japan Sports Association. The association suggests that all exercise - except for special cases - should be stopped.
For example, the index reaches 31 if the temperature rises to 35 C and the humidity is 55 percent or above. The index is lower in the shade, in the early morning and after sunset.
The ministry plans to create guidelines to take steps against the heat based on the measured figures and other sources to advertise the guidelines to related organizations.
In July last year, the Tokyo Organizing Committee of the Olympic and Paralympic Games launched a study committee comprising experts to work out countermeasures to protect against the summer heat. They have started discussing how to deal with the heat for players, spectators and volunteers.
The committee plans to work out a policy by the end of this fiscal year to protect spectators from the sun while they wait in line for games around a stadium, securing drinking water for them and setting up a first-aid system, among other measures.
There are concerns over possible heatstroke for runners at the Tokyo Olympic marathon. Authorities have already started working on constructing pavements with a surface that curbs temperature increases and planting roadside trees to provide shade.
They are also considering making the starting time earlier than the initially scheduled 7:30 a.m.
The new National Stadium is designed to take in natural air flows so heat and humidity is not retained inside the stadium.
When the 2014 FIFA World Cup was held in Brazil, official water breaks were called due to excessive heat and humidity.
"We recognize that taking measures against the heat will become an important issue. We're considering measures for athletes, spectators and Games staff," said an official of the organizing committee. "It will also be important to provide information regarding the heat to spectators, including foreigners."
Osaka International University Prof. Yoshimitsu Inoue, who specializes in thermal physiology, said special consideration for the elderly, children and foreigners who are not used to Japan's summer heat is needed. "Even if all possible measures are taken, there may be no perfect protective measure. But we should make the best efforts as a host country," he said.
