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How the classes work
Therapeutic springs are first divided by whether total dissolved solids are under 1 gram per kilogram. Under 1 g and 25 °C or more: a **simple hot spring**. Over 1 g: the dominant anion decides between **chloride**, **bicarbonate** and **sulfate** springs.
Separately, water containing enough of a specific substance is named a **carbon-dioxide**, **iron**, **acidic**, **iodine**, **sulfur** or **radioactive** spring. One water can meet several criteria, giving long official names such as "sulfur-bearing sodium chloride spring".
The 2014 revision added iodine springs, making ten classes. Older documents mention "copper-iron" and "aluminium" springs; these are no longer used.
Gentle waters: simple and bicarbonate
**Simple hot springs** are the most common in Japan. Being low in minerals, they are mild and are considered suitable for children and older bathers. Above pH 8.5 they are "alkaline simple" springs, which soften dead skin and feel slippery — Gero, Kakeyu, Dogo.
**Bicarbonate springs** contain sodium bicarbonate (baking soda) and similar salts that lift oils from the skin, leaving it fresh; most "beauty baths" are of this type — Ureshino, Ryujin, Shirahone. At Shirahone, calcium carbonate precipitates and turns the water white.
Asturio Cantabrio · CC BY-SA 4.0
Gero Onsen
下呂温泉
Gifu · 100 Onsen to Know Japan — No. 53
Alkaline water above pH 9 is piped at 55 °C to inns across town, and nine free foot baths within walking distance of the station let you feel the 'slippery' water without even undressing.
STA3816 · CC BY-SA 4.0
Ureshino Onsen
嬉野温泉
Saga · 100 Onsen to Know Japan — No. 93
One of Japan's three great 'skin-beautifying baths', famous for its slippery bicarbonate water, and for tofu that turns silky and clouds the broth when simmered in it.
Soica2001 (talk) · Public domain
Shirahone Onsen
白骨温泉
Nagano · 100 Onsen to Know Japan — No. 50
Clear as it emerges, white once it meets the air. Shirahone's milkiness comes not just from sulfur but from calcium carbonate that solidifies, even building the 'funto-kyu' spring mounds.
Warming waters: chloride and sulfate
**Chloride springs** are dominated by salt. A thin film of salt on the skin keeps heat in, so they are called "heat baths" and you stay warm afterwards. Common near the coast (Atami, Wakura, Shirahama) and where ancient seawater is heated underground (Matsunoyama). They taste salty.
**Sulfate springs** contain gypsum (calcium) or sodium sulfate. Long known as baths for wounds and circulation, they have a calm, settled feel — Hoshi, Shima, Yamanaka.
663highland · CC BY-SA 4.0
Atami Onsen
熱海温泉
Shizuoka · 100 Onsen to Know Japan — No. 56
More than 500 sources give about 20,000 litres a minute at an average of 63 °C; from the foot bath outside the station to the geyser up the hill, the whole town is built on hot water.
KQuhen · CC BY-SA 4.0
Matsunoyama Onsen
松之山温泉
Niigata · 100 Onsen to Know Japan — No. 37
Seawater trapped twelve million years ago comes back up heated to 88°C, salty enough to make salt, in a valley buried in snow.
C.K. Tse · CC BY-SA 2.0
Hōshi Onsen
法師温泉
Gunma · 100 Onsen to Know Japan — No. 27
In a bathhouse of 1895 with arched windows, the water rises between pebbles on the floor. One inn, one bath, and the old etiquette of mixed bathing still in use.
Strong characters: sulfur, acidic and iron
**Sulfur springs** smell of boiled eggs and often turn milky white — the archetypal onsen. The hydrogen-sulfide type smells stronger and clouds more readily: Manza, Takayu, Tsurunoyu in Nyuto, Myoban in Beppu. They blacken metal, so take off jewellery.
**Acidic springs** are rich in hydrogen ions and sting the skin slightly. Strongly antibacterial, they include Kusatsu (around pH 2), Tamagawa (pH 1.2, Japan's most acidic), Zao and Kawayu in Hokkaido. Avoid long soaks, and rinse afterwards.
**Iron springs** emerge clear but oxidise on contact with air into reddish-brown water: Arima's "gold spring", Ikaho's "golden water", Furofushi. They stain towels brown.
SpringField1967 · CC BY-SA 3.0
Manza Onsen
万座温泉
Gunma · 100 Onsen to Know Japan — No. 29
At 1,800 metres, 5.4 million litres of sulfur water a day. Thin air, deep snow, and a milky bath in the middle of it.
Aspere · CC0
Kusatsu Onsen
草津温泉
Gunma · 100 Onsen to Know Japan — No. 25
Thirty-two thousand litres a minute of pH 2.1 acid water rise in the middle of town and cool as they run down wooden channels.
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.
Rare waters: carbon-dioxide, iodine and radon
**Carbon-dioxide springs** hold dissolved CO₂ — "fizzy baths" where fine bubbles coat the skin and circulation improves even in lukewarm water. Because heat drives the gas off, they are usually cool: Nagayu in Oita, Arima's "silver spring", Ogon in Yamagata's Hijiori district. They are rare in Japan.
**Iodine springs**, added in 2014, are common in seawater-derived onsen and may tint the water yellow to brown — many are in Chiba and Tokyo.
**Radioactive (radon) springs** contain radon above a set level: Misasa, Masutomi, Arima, Murasugi. Since radon is a gas, much of it is inhaled from the bathroom air rather than absorbed in the tub; Misasa even offers "radon hot-air baths". See the article on radon springs.
NY066 · Public domain
Nagayu Onsen
長湯温泉
Oita · 100 Onsen to Know Japan — No. 89
Sit in the 32 °C pool and within minutes your whole body is sheathed in silver bubbles: 1,380 mg of free carbon dioxide per kilogram, one of Japan's richest carbonated springs.
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.
Spring types and "benefits"
Water analyses list "indications" for bathing and drinking by spring type. These are official statements under Ministry of the Environment guidelines, not guarantees of treatment. This atlas quotes indications as "posted on the analysis" and keeps them separate from facility descriptions and bathers' impressions; we do not write that a spring "cures" anything.
Spring type is still a useful guide to what suits you: start with simple or bicarbonate springs if you prefer mild water, or seek acidic and sulfur springs if you want a strong character.
Glossary
- Total dissolved solids (TDS)
- All dissolved matter (excluding gases) per kilogram of water. 1 g or more makes a "saline" spring.
- Anion
- Negatively charged dissolved components — chloride, bicarbonate, sulfate — that name a saline spring.
- Indications
- Conditions for which official guidelines say a spring type may be helpful; posted together with contraindications.
Related onsen
Calistemon · CC BY-SA 4.0
Noboribetsu Onsen
登別温泉
Hokkaido · 100 Onsen to Know Japan — No. 1
Ten thousand tonnes of water a day and nine kinds of spring, all boiling out of a single valley called Hell.
Zhaitengli · CC BY 4.0
Naruko Onsen
鳴子温泉
Miyagi · 100 Onsen to Know Japan — No. 18
Seven of Japan's ten official spring types in one valley. The communal Takinoyu by the station costs 300 yen, and the town lends you wooden geta to clatter from bath to bath.
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.
NY066 · Public domain
Nagayu Onsen
長湯温泉
Oita · 100 Onsen to Know Japan — No. 89
Sit in the 32 °C pool and within minutes your whole body is sheathed in silver bubbles: 1,380 mg of free carbon dioxide per kilogram, one of Japan's richest carbonated springs.