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From Gifu! Hot Spring Museum Episode 18: Are the Radioactive Springs in Gifu Prefecture Safe?

Due to the tragic accident at the Fukushima Daiichi Nuclear Power Plant, concerns about radioactivity are spreading. Amidst this, voices are being heard asking, "Are radioactive springs safe?" For those who have continued to bathe in radioactive springs expecting their benefits, it is natural for concerns about radioactive springs to rise when there is so much alarm about radioactivity.
The Tono region of Gifu Prefecture is one of the leading areas in the country for radioactive springs, with many such springs scattered throughout.
To prevent these hot springs from being wrongly stigmatized, I would like to take another look at radioactivity and radioactive springs.

"Radioactivity" refers to the property (ability) of "radioactive substances" to emit "radiation." Radioactive substances are a general term for materials that emit radiation, such as radioactive isotopes. Radioactive isotopes emit radiation and eventually transform into stable elements. For example, in the case of uranium-238 found in nature, it undergoes stages of transformation into thorium, protactinium, radium, radon, polonium, bismuth, and thallium, eventually becoming stable as lead over a long period. Radioactive substances, such as radioactive isotopes, release particles or energy outside the atom as they attempt to transform into more stable elements, which is called radiation.
Radiation includes alpha rays (emitting atomic nucleus particles), beta rays (emitting electrons), and gamma rays (emitting electromagnetic waves). Radiation has high energy, and the energy of radiation ultimately transforms into thermal energy. The mechanism of artificially converting this thermal energy into electricity is nuclear power generation.
The magnitude of radiation dose is expressed in units called sieverts (Sv). The unit of radioactivity is becquerel (Bq). Radioactivity is measured by "how many times radiation can be emitted per second."
There are natural radioactive nuclides and artificial radioactive nuclides produced by nuclear power generation and atomic bombs. Artificial radioactive nuclides are incomparably more radioactive than natural radioactive nuclides. For example, the radioactivity of one gram of naturally occurring uranium-238 is 12,000 becquerels, while cesium-137, an artificial radioactive nuclide produced by nuclear power generation, is 32 billion becquerels, and iodine-131, another artificial radioactive nuclide, is as much as 4.6 trillion becquerels.

In the Tono region of Gifu Prefecture, rocks such as granite and gaps underground contain radium, which is in the process of decaying from naturally occurring uranium-238, and radon gas, which is produced by the decay of radium, as well as thoron gas, which is produced by the decay of thorium. All of these are easily soluble in water, allowing them to easily dissolve into groundwater. This is how the radioactive springs of the Tono region are formed. In other words, the radioactive substances contained in the hot springs are radium, radon gas, and thoron gas. However, radon is overwhelmingly more present than the equilibrium amount of its parent nuclide, radium, before decay, so it can be said that radon is the main radioactive component contained in radioactive springs. Not only in the Tono region but also in various parts of the country, there are hot springs named "○○ Radium Onsen," but in most cases, the actual component contained is not solid radium but gaseous radon.
According to Japan's Hot Springs Law, in the case of radioactive substances, a hot spring is defined as one that contains 74 becquerels or more of radon per kilogram of hot spring water or 100 billionths of a milligram or more of radium salt. Furthermore, the Ministry of the Environment's "Guidelines for Hot Spring Analysis" stipulates that among the hot springs defined by the Hot Springs Law, those containing 111 becquerels or more of radon as a special component are defined as radioactive springs or springs containing radioactivity. Thoron is not included in the radioactive substances that define radioactive springs because its amount is small and it disappears in a short period, making it not very common as a main component of hot springs.

The radioactive springs represented by the Tono region are hot springs containing radon and have radioactivity. However, the regulated amount of radon contained in the hot springs is very small, and even if the gaseous radon enters the body through breathing, it is considered to be expelled from the body in about three hours due to radon's biological half-life of about 40 minutes. Therefore, both in terms of quantity and the characteristics of radon's half-life, it is unlikely that bathing correctly in "radioactive springs" will have adverse effects on the body. You can bathe with peace of mind while expecting excellent benefits.