When the coronavirus pandemic hit last year, Japan's booming sauna industry had a problem: Packed hot rooms seemed to violate all the new rules of social distancing and ventilation.

But instead of spelling the end of the good times, the challenge has led to a burst of innovation, with the emergence of saunas for individuals and even outdoor versions at such scenic spots as riverbanks.

"It has become an opportunity for novelty to become pervasive," said Yasutaka Kato, 37, representative director of the Japan Association of Sauna and a physician.