Reusable cloth masks that are cooled and sold in vending machines are proving popular in northeastern Japan amid growing fears that wearing masks to prevent the spread of the novel coronavirus in the summer could cause breathing difficulties.

The masks are stored at 4 C, the lowest temperature setting in two vending machines installed in Yamagata Prefecture, and around 400 are sold daily for 690 yen ($6.50) a piece, even though they do not stay cool for long.

(Photo taken May 11, 2020, shows a vending machine in Yamagata, northeastern Japan, that sells reusable cloth masks. Produced by a local knitted fabric manufacturer, about 400 of the masks which are cooled to 4 C sell each day.)

Knitwear manufacturer Knit Waizu, which is based in the town of Yamanobe in the prefecture, produces the masks as it wanted to create something that would "make people smile during these dark times."

The company started producing them in mid-March following a serious shortage of masks amid the spread of the virus in Japan.

As a number of regions in Japan, such as Yamanashi and Gunma prefectures, recorded Monday temperatures above 30 C, the highest of the year so far, concerns are growing over the use of masks in hot weather.

Wearing face masks in hot weather makes it difficult for cool air to reach the lungs, activating the respiratory muscles and resulting in shortness of breath. It also allows heat to build up inside the body, according to medical experts.

"We're having a difficult time because we are barely making any profits from our main (knitwear) business," said Katsuyuki Goto, a 52-year-old executive of Knit Waizu. "(The masks) were also created to raise my spirits."