I have read about some onsen being rich in radium, a radioactive and carcinogenic element that has been blamed for Marie Curie's premature death by cancer.

Misasa Onsen in Tottori prefecture advertises itself as a place to cure cancer thanks to its high levels of radium. I have also read that radium is good for lowering blood pressure, and preventing hardening of the arteries.

The very famous and popular Arima Onsen, near Kobe, also has springs rich in radium, and clearly indicates on its website that it has radioactive hot springs.

Quote Originally Posted by Arima Onsen
Ginsen (Radioactive spring (Radon spring)
Spring gas goes through the whole body by inhalation and improves your natural healing power.
~Ailments relieved by this spring water~
Myotonic spine disease, degenerative joint disease, chronic multiple joint disease, degenerative spine disease, chronic gout, joint and muscle rheumatism, mild peripheral arterial circulation disorder, menopausal discomfort, bronchial asthma, etc.
I wonder how much research has been made on the subject. Onsen don't seem to me to have very scientific claims in their advertisements, and the Japanese are well-known for being gullible when it comes to "miracle cures" and cosmetic treatments. Is it certain that people aren't getting cancer from bathing in radium ? That seems to fly in the face of scientific evidence that radium is radioactive and carcinogenic (causing notably bone cancer, one of the worst type of cancer).