Radon is a naturally-occurring radioactive gas that we are warned about in our basements. We know the dangers of radiation, which was driven home forcefully after Hiroshima and during the arms race. There was a time before that in which people sought out the miracle of radiation in cosmetics and even medicine. That belief hasn't quite died out.
There are places in the world that people go to and pay good money to be exposed to radon. Radon therapy seems to help some people, but is it a miracle cure, a placebo, or a matter of faith? Tom Scott visits the radonstollen (radon tunnel) in Bad Kreuznach, Germany. It's the site of a thriving pain clinic. The unique feature of this clinic is that the radon can be turned on or off! It sounds like the perfect place to propose a double-blind study on the effects of radon, but that probably won't happen. Even if you could find enough volunteers, good luck getting that idea past a university's ethics board.