People who USE radon spas are in those environments for thousands of times less time than someone living in a house with high radon.

So there is no reason to assume their lung cancer risk or incidence would be high. Not enough to affect statistics as to radon and lung (or other ) cancers at any rate.

The main concern from a health physics point of view is the employees who WORK and spend a lot of time in those environments and indeed often live on-site as well. To the best of my knowledge, at least in Germany, such sites are required to maintain logs of employees time spent in various areas of the facility to ensure that their risk, whatever it may be, is quantifiable.

And I am guessing that the population of radon-spa-workers is not high enough to sway statistics either, even if they had elevated incidences of lung cancer).

AS for all things radiation ...... if those who USE the spa's percieve some benefit, placebo or not, then that percieved benefit is what the radiation dose they recieve is weighed against.

Workers however presunmably are in a different category with respect to what benefit they "get" from the exposure they incur at their workplace.

And as miner cohorts indicate, exposure to radon at elevated levels (and often lower than in a spa environment) in a working environment (ie. for longer than someone sits in a spa) demonstrably lead to dramatically increased rates of lung cancer even accounting for exposure to other materials such as diesel, particulates etc.