There are thermal baths with quite high concentration of radon.

https://www.bbc.com/future/article/20180306-a-spa-where-pati...

Most exposed are not the visitors, but the thermal bath workers.

"Workers and visitors in thermal spas are exposed to radiation from radon and short-lived daughters. Radon concentrations in the premises of a thermal bath in Budapest were investigated; high radon concentrations (up to 7.15 kBq.m-3) were found in some rooms. Bronchial dose and effective dose equivalent values were estimated for the staff and the visitors. In some cases the effective dose was about or in excess of 20 mSv annual limit for the workers. Doses for the visitors were about 4 mSv per year, but exposure of the public from radon is still outside the scope of the system of dose limitation."

https://academic.oup.com/rpd/article-abstract/56/1-4/319/310...

There are few places on Earth with high natural background radiation:

Ramsar, Iran (the maximal estimated dose-rate received by inhabitants is 80 mSv/year

Kerala (India), Orissa (India), Guarapari (Brazil), Yangjiang (China)

https://pmc.ncbi.nlm.nih.gov/articles/PMC12402632/

https://academic.oup.com/jrr/article/65/Supplement_1/i32/792...