Concerns are growing about possible heatstroke or heat exhaustion at the 2020 Olympic marathon due to the sweltering and humid weather expected in Tokyo.

Experts are calling on the organizers to change the route or set up rest stations as the Tokyo Olympic event is likely to be extremely tough for runners and spectators compared with previous games.

Akio Hoshi, a professor of health science at Toin University of Yokohama, said the risk of summer heatstroke in Tokyo has escalated in recent years, while noting the Olympics are expected to take place in conditions when sports activities should normally be halted.

Heatstroke risk levels are determined by temperature, humidity and other factors. Based on Hoshi's analysis of data for the last 50 years, the risk level in Tokyo has surged since 2004, mainly due to rising temperatures.

Between July 24 and Aug. 9, when the Olympics will be held, the average highs in Tokyo were 34.6 C in 2015, 31.6 C in 2016 and 31.3 C in 2017.

"The number of people transported by ambulance due to heatstroke or heat exhaustion has peaked in early August in recent years. So the Tokyo Olympics fall in the period with the highest risk," Hoshi said.

Makoto Yokohari, a professor of city planning at the University of Tokyo, measured the temperature, humidity and sunlight in August last year on the expected route of the Tokyo Olympic marathon and divided the course into four sections labeled safe, caution, danger and extreme danger, in line with a U.S. heat risk assessment method.

The temperature was at 30 C when he left the starting point at the New National Stadium at 7:30 a.m. but gradually rose to 35 C in Asakusa around the halfway point. Most of the return route, especially around the Imperial Palace where shade is lacking, was considered extremely dangerous, with even severer heat from the road surface and stronger sunlight.

Although the 1984 Los Angeles Games and the 2004 Athens Games took place in hot weather, Yokohari said "the conditions in Tokyo would be worse compared with past games" as high humidity levels increase the risk of heat-related illnesses.

In addition to the athletes, it is feared that elderly people, children and foreigners among the spectators could also suffer from heatstroke or heat exhaustion, Yokohari said.

The Japanese government and the Tokyo metropolitan government are planning to lay pavements that emit less surface heat and plant taller roadside trees.

But Hoshi recommends that the government also take into account other measures such as seeking cooperation from shops along the marathon route to set up temporary shelters.

While Yokohari is proposing a different route passing through buildings in the Marunouchi business district instead of the area near the Imperial Palace, its feasibility remains uncertain due to security concerns.

"With less than three years to the Olympics, I want the government to take measures swiftly," Yokohari said.