Radon is a gas from rock
Radon is a colourless, odourless gas produced as uranium and radium in rocks such as granite decay. Groundwater passing through such rock dissolves it. That is why radon springs cluster not in volcanic zones but in granite country — Misasa in Tottori, Masutomi in Yamanashi, Murasugi in Niigata, Arima in Hyogo.
Many signs say "radium spring", but what the water carries is mostly radon; radium itself is present only in traces. Think of "radium" as a surviving old name.
How much makes a radioactive spring?
The therapeutic threshold is 30 × 10⁻¹⁰ curies of radon per kilogram of water — "8.25 Mache units" in the old unit, about 111 becquerels (Bq) per kg in today's.
- 1 Mache ≈ 13.5 Bq/kg
- 8.25 Mache ≈ 111 Bq/kg (the threshold)
Concentrations vary enormously, from just over the threshold to several tens of times it at some Misasa sources, so "contains radon" does not mean the same everywhere. Where we can, the atlas gives the value, unit and date of analysis.
Because radon is a gas, it escapes from the water into the air. The distance from source to tub, the water temperature and the bathroom's ventilation all lower the concentration you actually meet.
How to bathe: the air matters more than the tub
Most of the radon that enters the body comes not through the skin but by breathing the bathroom air. Therapeutic stays at radon springs therefore emphasise time spent in the bathroom rather than in the tub, and long, lukewarm soaks.
Misasa offers "hot-air baths", rooms filled with steam from the source where you sit and breathe, and public drinking fountains. Masutomi's sources are around 30 °C, and sitting still in the cool water for a long time is the traditional way. Arima's "silver spring" (carbonated, radon-bearing) can also be drunk.
Radon leaves the body in the breath within a few hours.
三塚 · CC BY-SA 4.0
Misasa Onsen
三朝温泉
Tottori · 100 Onsen to Know Japan — No. 74
Soak it, drink it, breathe it. A spa built around taking radon into the body three ways, with a free riverside bath and free drinking fountains open to the whole town.
Masutomi Onsen
増富温泉
Yamanashi · 100 Onsen to Know Japan — No. 44
Thirty quiet minutes in water around 30 °C. A therapeutic village where one of Japan's most radon-rich springs is bathed in unheated, exactly as it comes.
663highland · CC BY-SA 4.0
Arima Onsen
有馬温泉
Hyogo · 100 Onsen to Know Japan — No. 62
Salt and iron thicker than seawater in the 'gold' spring, carbon dioxide and radon in the 'silver': two utterly different waters rise five minutes' walk apart.
Effects on the body: what is and isn't known
There is a hypothesis that very small radiation doses stimulate the body (hormesis), and at Misasa a facility of Okayama University has studied the spring for nearly a century. Confirming effects at low doses is difficult, however, and the matter is not internationally settled.
What is certain: the radiation dose from bathing in a radon spring is very small compared with, say, medical imaging; these places have long attracted people seeking therapeutic stays; and no onsen can be said to "cure" anything.
This atlas does not assert medical effects in its own words. It quotes indications as posted on water analyses, notes where research exists, and keeps bathers' impressions separate.
Finding radon springs
Choose "contains radon" in the search to list areas whose official spring-type name includes "radioactive" or "radon-bearing". Where only descriptive text mentions radon and we could not confirm it in the official name, the entry is marked "from descriptive text". Some places, such as Arima, have two kinds of source — the iron-salt "gold spring" and the carbonated radon "silver spring".
Notable radon springs: Misasa (Tottori), Masutomi (Yamanashi), Arima (Hyogo), Murasugi (Niigata), Sekigane (Tottori), Nibukawa (Ehime), Yunohana (Kyoto), Tochiomata (Niigata), Futamata Radium (Hokkaido).
三塚 · CC BY-SA 4.0
Misasa Onsen
三朝温泉
Tottori · 100 Onsen to Know Japan — No. 74
Soak it, drink it, breathe it. A spa built around taking radon into the body three ways, with a free riverside bath and free drinking fountains open to the whole town.
Masutomi Onsen
増富温泉
Yamanashi · 100 Onsen to Know Japan — No. 44
Thirty quiet minutes in water around 30 °C. A therapeutic village where one of Japan's most radon-rich springs is bathed in unheated, exactly as it comes.
663highland · CC BY-SA 4.0
Arima Onsen
有馬温泉
Hyogo · 100 Onsen to Know Japan — No. 62
Salt and iron thicker than seawater in the 'gold' spring, carbon dioxide and radon in the 'silver': two utterly different waters rise five minutes' walk apart.
As6022014 · CC BY-SA 3.0
Nibukawa Onsen
鈍川温泉
Ehime · 100 Onsen to Know Japan — No. 85
In the hills behind Imabari, radon-bearing alkaline water rises from a granite gorge: one of the 'three baths of Iyo' alongside Dogo and Hondani.
Mti · CC BY-SA 3.0
Yunohana Onsen
湯の花温泉
Kyoto · 100 Onsen to Know Japan — No. 70
About an hour from Kyoto Station by train and bus: a quiet valley spa called 'Kyoto's inner parlour', with radon-bearing water where wounded samurai are said to have healed.
Glossary
- Becquerel (Bq)
- One nuclear decay per second. Radon in water is given in Bq/kg or Bq/L.
- Mache unit
- An old unit of radon concentration; 1 Mache ≈ 13.5 Bq/kg. Common on older analyses.
- Hot-air bath (nekkiyoku)
- Sitting in a room filled with steam from the source and breathing radon-bearing air. Practised at Misasa.
Related onsen
三塚 · CC BY-SA 4.0
Misasa Onsen
三朝温泉
Tottori · 100 Onsen to Know Japan — No. 74
Soak it, drink it, breathe it. A spa built around taking radon into the body three ways, with a free riverside bath and free drinking fountains open to the whole town.
Masutomi Onsen
増富温泉
Yamanashi · 100 Onsen to Know Japan — No. 44
Thirty quiet minutes in water around 30 °C. A therapeutic village where one of Japan's most radon-rich springs is bathed in unheated, exactly as it comes.
663highland · CC BY-SA 4.0
Arima Onsen
有馬温泉
Hyogo · 100 Onsen to Know Japan — No. 62
Salt and iron thicker than seawater in the 'gold' spring, carbon dioxide and radon in the 'silver': two utterly different waters rise five minutes' walk apart.
As6022014 · CC BY-SA 3.0
Nibukawa Onsen
鈍川温泉
Ehime · 100 Onsen to Know Japan — No. 85
In the hills behind Imabari, radon-bearing alkaline water rises from a granite gorge: one of the 'three baths of Iyo' alongside Dogo and Hondani.