Arima Onsen — photo: 663highland · CC BY 2.5 · Wikimedia Commons
Onsen water types: reading the sign in the changing room
On the wall of almost every onsen changing room there is a framed sheet of dense Japanese text, a table of numbers, and a stamp. Most visitors walk straight past it. It is the only document in the building that tells you what is actually in the water, and why the bath down the road feels nothing like this one. You can read the parts that matter with about a dozen characters.
First: what makes water an "onsen" at all
Japanese law sets a low bar. Water that comes out of the ground at 25 °C or warmer counts as an onsen, and so does colder water if it carries enough of any one of a list of dissolved substances. That is why you will meet onsen towns whose water is heated in a boiler before anyone gets in: the water qualified on its minerals, not its temperature.
A second, stricter set of standards decides whether the water gets a type name. Only water that meets it is sorted into one of ten types, and it is the type name that appears on the sign. The rest of this page is about those ten names.
The sign has a name: 温泉分析書
The framed sheet is the 温泉分析書 (onsen bunsekisho), the hot spring analysis. It is produced by a registered laboratory, and the inn is required to display it. Three lines are worth finding:
泉質 (senshitsu), the water type. This is the long name such as ナトリウム－塩化物泉. Read it from the end: the last word is the type, and everything before it names the main dissolved ions, most important first.
泉温 (sen'on), the temperature at source, measured where the water leaves the ground, not in the tub.
pH値 (pH-chi), the pH. Below 3 is strongly acidic and will sting a cut. Above 8.5 is where water starts to feel slippery.
The sheet also lists conditions the water is said to suit and conditions under which you should not bathe. We do not repeat those lists here. If you have a medical condition, the second list is the one to ask the inn to read to you.
The ten types, and how common each one is
Every figure below is from our records: 1,538 onsen towns, of which 1,468 have at least one recorded water type. A town is counted under every type it has, so the numbers add up to more than 1,468.
- Chloride, 塩化物泉 (enkabutsu-sen): 689 towns. Salty to the taste, the most common type by far.
- Bicarbonate, 炭酸水素塩泉 (tansan suiso-en-sen): 355 towns. Often smooth and slightly slippery.
- Simple, 単純温泉 (tanjun onsen): 341 towns. Few minerals, usually clear and mild.
- Sulfate, 硫酸塩泉 (ryūsan-en-sen): 321 towns. Usually clear, sometimes with a faint bitter taste.
- Sulfur, 硫黄泉 (iō-sen): 311 towns. The egg smell.
- Radioactive, 放射能泉 (hōshanō-sen): 108 towns. A trace of radon; nothing you can see or smell.
- Iron, 含鉄泉 (gantetsu-sen): 81 towns. The rust-brown water.
- Acidic, 酸性泉 (sansei-sen): 47 towns.
- Carbon dioxide, 二酸化炭素泉 (nisanka tanso-sen): 21 towns. Bubbles on the skin.
- Iodine, 含よう素泉 (gan-yōso-sen): 13 towns, the rarest.
The one-line descriptions above are the typical case, not a promise. A type name is a chemical threshold. Sulfur water only needs 2 mg of sulfur compounds per kilogram to be called sulfur, and iron water 20 mg of iron; the chloride, bicarbonate and sulfate names need at least 1,000 mg of dissolved solids, and they are told apart by which ions dominate. Simple water is simple because it has less than 1,000 mg, not because it is plain. Two springs with the same name can look and feel very different, which is the subject of our page on sulfur onsen.
Most towns do not have one water
This is the part the lists of "ten types of onsen" leave out. Of the 1,468 towns with a recorded type, 911 have a single type. The other 557 have two or more, and 29 have five or more.
Arima Onsen near Kobe lists chloride, bicarbonate, iron and radioactive water, with sources from 40 to 98 °C. The famous brown 金泉 (kinsen), "gold water", and the clear 銀泉 (ginsen), "silver water", come from different sources in the same town. Kusatsu lists four types. Noboribetsu in Hokkaido lists six.
In a mixed town, the town's list tells you what exists, not what is in your bath. Each inn draws from one or a few sources, and the analysis board in its own changing room is the one that describes your water. If you chose the town for a particular water, check the inn, not the town.
Some types are rarely found alone. Of 311 sulfur towns, 142 are sulfur only. Of 689 chloride towns, 242 are chloride only. But of 81 iron towns, only 12 are iron only, and of 21 carbon dioxide towns, only 8.
The temperature line is the other half of the story
The analysis also classes the water by temperature at source. Of the 1,167 towns in our records with a measured source temperature, taking the hottest source in each town:
- 237 are below 25 °C, 冷鉱泉 (reikōsen), cold mineral springs
- 176 are 25 to 34 °C, 低温泉 (tei-onsen)
- 181 are 34 to 42 °C, 温泉 (onsen) in the narrow sense
- 573 are 42 °C or hotter, 高温泉 (kō-onsen)
Only the middle band comes out of the ground at a temperature you could sit in. Everything else is heated or cooled, and the types are not spread evenly across the bands. Of the sulfate towns with a temperature, 177 of 220 are 42 °C or hotter and only 7 are cold. Radioactive water is the opposite: 39 of 92 come out below 25 °C and only 14 are hot. Carbon dioxide water is cool too, 10 of 19 below 25 °C, because the gas escapes as the water heats up, so the bubbly baths are almost always lukewarm.
The second notice: what has been done to the water
Near the analysis there is usually a smaller notice, and it is the more useful one. Inns must say whether the water has been changed on its way to the tub. Four words cover it:
加温 (kaon), heated. 加水 (kasui), cold water added to bring the temperature down. 循環 (junkan), filtered and recirculated. 消毒 (shōdoku), disinfected, usually with chlorine.
Each is marked 有 (ari), yes, or 無 (nashi), no. None of them is a scandal. A 14 °C spring has to be heated; an 84 °C spring has to be cooled. But the combination tells you how close your bath is to what came out of the ground, and the phrase inns put on their front page when all four are 無, or nearly, is 源泉かけ流し (gensen kakenagashi): water from the source flows in and flows out, and is not reused.
The pH and the feel
Only about 480 towns in our records carry a pH class, and most are not acidic. 182 are listed as alkaline and 156 as weakly alkaline, 121 as neutral, and 20 as acidic or weakly acidic. Among simple springs with a recorded pH, 149 of 161 are 7.5 or above.
That is why so many clear, mild baths feel slippery: alkaline water reacts with the oils on your skin, which is the smooth feeling. Inns sometimes advertise this with the nickname 美人の湯 (bijin no yu), "beauty bath". It is a marketing phrase, not a water type, and you will not find it on the analysis.
Gero Onsen in Gifu is a simple spring at pH 9.0 and 84 °C at source. Dōgo Onsen in Ehime is also simple, 42 to 51 °C. Both are good examples that "simple" does not mean weak.
Some places to see a type at its clearest
For chloride, Kinosaki in Hyōgo is chloride only, with sources from 37 to 83 °C spread across seven public bathhouses. For carbon dioxide, Nagayu Onsen in Ōita, 40 to 45 °C, is one of the few places the bubbles are warm enough to sit in for long. For radioactive water, Misasa Onsen in Tottori, 34 °C at source, is the best-known name. For iodine, one of only 13, Izu Nagaoka in Shizuoka, pH 9.2 and 59 °C. For the extreme end, Tamagawa in Akita comes out at 98 °C and pH 1.05, around 9,000 litres a minute from a single source, and lists four types including acidic.
Flow matters as much as type. Of the 425 towns with a recorded flow, 135 produce less than 100 litres a minute for the whole town, and only 12 produce 10,000 or more. Yufuin, a simple spring, records about 38,600 litres a minute.
What not to do
Do not choose a town from a list of "the ten types" and expect one water. If the type is the reason for the trip, confirm it on the inn's own analysis or website.
Do not read the list of suitable conditions on the sheet as advice. It is a standard legal list, and it is not a reason to stay in longer. Hot water is hot water: ten to fifteen minutes at a time, then sit out.
Do not take a heated bath from a cold mineral spring as fake. It is still an onsen, and the notice saying so is honesty, not a warning.
New to onsen? Start with how an onsen stay actually works. Want the milky bath in the photos? See sulfur onsen: milky, clear, and where to find each. Deciding where to go? See where Japan's hot springs actually are.