Medical experts have called for extra vigilance regarding heat-related illnesses this summer, amid growing fears that the wearing of face masks to prevent the spread of the novel coronavirus could cause breathing difficulties and dehydration.

Cities in Yamanashi and Gunma prefectures on Monday recorded the country's highest temperature this year of 33.4 C, with several other prefectures registering temperatures of over 30 C the same day.

Wearing masks during the hot and humid summer months "will be a new experience for many people," said Yasufumi Miyake, head of the advanced emergency medical service center at Teikyo University Hospital.

As wearing masks in hot weather makes it difficult for cool air to reach the lungs, the respiratory muscles are activated resulting in shortness of breath, which in turn makes it easier for heat to build up inside the body.

Miyake, who is well-versed in measures to prevent heat-related illnesses, stressed it is important to cool the body down in order to prevent heat exhaustion and heatstroke. He suggested turning on air conditioners, drinking water at regular intervals and increasing rest time.

(People walk in Tokyo's Ginza district in unseasonably hot weather on May 11, 2020.)

"People should not overdo it on days they go to the office or school," as they have not had time to acclimatize to the heat after remaining at home for telework and when schools were closed, he said.

Noriyuki Koibuchi, a professor at Gunma University's Graduate School of Medicine specializing in environmental physiology, said that as there are many nerve fibers in the face, it is more sensitive to heat and cold than other parts of the body.

He recommended alleviating discomfort by cooling the forehead and neck even while keeping a mask on.

"As you take more frequent breaths when wearing a mask, you should definitely avoid vigorous exercise (when wearing one)," Koibuchi said.

In China, at least two junior high school students collapsed and died last month while wearing face masks during physical exercise examinations.

The deaths have prompted experts to warn of the dangers of wearing high-grade masks during intense exercise, which could lead to oxygen deficiency.

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