France's Aloise Retornaz (left) and Camille Lecointre compete in the women's two-person dinghy 470 class competition in a sailing test event for the Tokyo Games, off the coast of Enoshima in Kanagawa Prefecture on Thursday. Photo: AFP

Sailing authorities have loosened the rules on life jackets at their Tokyo 2020 Olympic test event to help athletes cool down, the latest sport to take measures to beat sweltering heat in the Japanese capital.

Fears over intense heat and humidity in the Tokyo summer have crystallized at several test events in recent weeks, with some competitors and spectators taken ill and other events curtailed.

Competitors in the windsurfing ­final on Wednesday were permitted to remove their life jackets, revealed Rio Olympics gold medalist Charline Picon.

"The doctors realized that [the heat] was too fierce, that [the jackets were increasing our heat levels and that it could be dangerous. We took them off. Everyone is feeling their way," said Picon.

French team doctor Fanny Mevellec said the heat "requires additional physical effort to achieve equal performance. It also creates significantly more ­fatigue."

"This year, we are able to wear cooling jackets [with ice packs], and the international federation is changing the lycra we wear, which is too tight and not ventilated and which can provoke heatstroke," said Mevellec.

Despite these changes, not everything is "in perfect working order," said the medical expert, who added however that the heat and humidity had not come as a surprise. "We knew exactly where we were going."

The World Sailing federation said it was closely monitoring the situation and could further tweak the rules depending on conditions.

"We have to be very careful with the heat, clearly. It's much hotter than the last two Olympics venues and many of the venues that we are used to," acknowledged Alastair Fox, director of events at World Sailing.

"We're looking at potential rules to put in place next year to keep the athletes safe and not get too hot and unable to compete." 

Some of the regulations under consideration are a compulsory break for athletes to cool down or an upper limit for water temperature.

Marathon swimming - whose water-temperature limit of 31 C was nearly breached at a test event earlier this month - has been brought forward to earlier in the morning to avoid the heat.

But sailing needs to take other conditions into account, said Fox.

"Of course, starting racing earlier in the day is a nice idea but if there is no wind we can't race."

Many athletes said they simply had to come to terms with the heat.

"The first thing that you need to do is to accept it," said double Olympic champion Dorian van Rijsselberghe from the Netherlands.

"They had a lot of trouble with this year's Tour de France with the heat and they said, 'We shouldn't be cycling.' You can't stop the Tour de France! 

"The Tour goes on and it's the same with the Olympics. The Olympics doesn't stop for one day of heat, it goes on. So you have to be better prepared for it."

Japanese-Finn sailor Hajime Kokumai said there was no need to wear cooling jackets and that he kept well ­hydrated and protected from the direct sun.

"Two or three years ago, it was 45 degrees. This year, it's not that hot," he said.