Medical experts have called for extra vigilance over heat-related illnesses this summer, amid growing fears that the wearing of face masks to prevent the spread of 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 Celsius, with several other prefectures registering temperatures over 30 Celsius.

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.