Onsen water types: what makes water an onsen in Japanese law, the ten spring types, and how to read the sign by the bath

Every onsen offered to the public in Japan must post a sheet covered in chemical names and numbers somewhere easy to see. It is worth a minute, because it answers questions people usually guess at: whether this water is “really” an onsen, what kind it is, how hot it was when it came out of the ground, and what the operator does to it before it reaches the bath. This page explains the system behind that sheet, using the environment ministry’s own documents. It does not rank springs, and it does not say what any water will do for you.

What it is

An onsen is defined by law, and it need not be hot

Japan’s Hot Springs Act of 1948 defines an onsen (onsen) as hot water, mineral water, steam or other gas that wells up from underground — natural gas made up mainly of hydrocarbons is excluded — and that has either the temperature or the substances set out in a table attached to the law.

The environment ministry’s summary of that table: water is an onsen if it is 25°C or warmer at the source, or if one kilogram of it contains at least one of the following in the stated amount or more.

Substance	Per kg
Total dissolved substances (excluding gases)	1,000 mg
Free carbon dioxide	250 mg
Lithium ion	1 mg
Strontium ion	10 mg
Barium ion	5 mg
Total iron ion	10 mg
Manganese (II) ion	10 mg
Hydrogen ion	1 mg
Bromide ion	5 mg
Iodide ion	1 mg
Fluoride ion	2 mg
Hydrogen arsenate ion	1.3 mg
Metaarsenious acid	1 mg
Total sulphur	1 mg
Metaboric acid	5 mg
Metasilicic acid	50 mg
Sodium hydrogen carbonate	340 mg
Radon	20 × 10⁻¹⁰ curie (74 Bq, or 5.5 Mache units)
Radium salt (as Ra)	0.00000001 mg (1 × 10⁻⁸ mg, “one hundred-millionth of a milligram”)

These figures are as the ministry’s page gives them. For the radium line, the annexed table of the Act on e-Gov (“one hundred-millionth of a milligram”) and the ministry’s analysis guidelines (1 × 10⁻⁸ mg/kg) say the same. I have not checked every other line word for word against the Act.

The practical point is the word “or”. Water at 15°C with enough metasilicic acid is an onsen in law. Water at 40°C with nothing much dissolved in it is also an onsen. “Onsen” in Japan does not simply mean hot water.

A stricter tier, the therapeutic spring

Inside that legal definition sits a narrower category, the therapeutic spring (ryōyōsen): onsen water, not steam or gas, considered especially suitable for therapeutic use. It also qualifies at 25°C or above at the source, or by containing at least one of the following per kilogram.

• Total dissolved substances (excluding gases), 1,000 mg — the same as for an onsen
• Free carbon dioxide, 1,000 mg — four times the onsen figure
• Total iron ion (Fe²⁺ + Fe³⁺), 20 mg — double
• Hydrogen ion, 1 mg — the same
• Iodide ion, 10 mg — ten times
• Total sulphur, 2 mg — double
• Radon, 30 × 10⁻¹⁰ curie (111 Bq, or 8.25 Mache units) — one and a half times

The ten spring types below are types of therapeutic spring. The Japan Hot Spring Association, a private body, says that onsen water which does not meet the therapeutic standard gets no spring-type name, and the analysis states something like “an onsen under the Hot Springs Act” instead. I could not confirm that wording in the ministry’s own documents.

How many there are

The environment ministry’s survey for fiscal 2024 (as of the end of March 2025) counts 2,839 onsen areas, counting places that have lodging, and 27,899 sources. Of those, 4,106 flow naturally and 12,762 are pumped and in use; 3,697 natural and 7,335 pumped sources are not in use. Those four figures add up to 27,900, one more than the total; that is how the table gives them. The year before, the figures were 2,857 areas and 27,920 sources.

How it works

The rulebook

The method for analysing spring water is set by the ministry in the Guidelines for Mineral Spring Analysis, revised on 1 July 2014 to reflect newer analytical equipment. The same guidelines set out how a spring is classified and named. On the same day the ministry revised the standard for what an onsen must post about contraindications and bathing and drinking precautions, which says it reflects current medical knowledge.

The ten types

The guidelines sort therapeutic springs in three groups, which is where the number ten comes from.

1. Salt springs (enruisen) — 1,000 mg/kg or more of dissolved substances, named by the main negative ion. Three types.
2. The simple spring — under 1,000 mg/kg but 25°C or warmer. One type.
3. Springs with a special component — carbon dioxide, iron, hydrogen ion (acid), iodine, sulphur, radon. Six types.

The table gives each type’s threshold and the older name you may still see on signs or in older books. The Japan Hot Spring Association says the current names came in with ministry revisions from 1978 onwards.

Type (posted name)	What qualifies it	Older names
Simple spring (tanjun onsen)	Under 1,000 mg/kg dissolved, 25°C or warmer. “Alkaline simple spring” if pH 8.5 or higher on site	Same
Chloride spring (enkabutsu-sen)	Salt spring, chloride the main negative ion. “Strong salt spring” at 5,500 mg/kg sodium and 8,500 mg/kg chloride or more	Shokuen-sen (common-salt spring)
Hydrogen-carbonate spring (tansan-suisoen-sen)	Salt spring, hydrogen carbonate the main negative ion	Jūsō-sen, jūtansan-dorui-sen
Sulphate spring (ryūsan-en-sen)	Salt spring, sulphate the main negative ion	Bōshō-sen, sekkō-sen, seikumi-sen
Carbon dioxide spring (nisanka-tanso-sen)	Free CO₂ 1,000 mg/kg or more	Tanjun tansan-sen (simple carbonated spring)
Iron spring (gantetsu-sen)	Total iron 20 mg/kg or more	Tessen
Acidic spring (sansei-sen)	Hydrogen ion 1 mg/kg or more	Sansei-sen, tanjun sansei-sen
Iodine spring (gan’yōso-sen)	Iodide ion 10 mg/kg or more	Gan-yōso shokuen-sen
Sulphur spring (iō-sen)	Total sulphur 2 mg/kg or more. “Hydrogen sulphide type” where it is mainly free hydrogen sulphide	Iō-sen, ryūka-suiso-sen
Radioactive spring (hōshanō-sen)	Radon 8.25 Mache units or more. Under 50 units is “weakly radioactive”	Same

A few notes the table cannot hold.

• “Carbonated spring” is the older name. The official term is carbon dioxide spring; tansan-sen matches the old name for it.
• “Salt spring” is not an official type. It lines up with the old shokuen-sen and today’s chloride spring. The strong salt springs, the guidelines note, have become more common with deep drilling that reaches brine-like groundwater of the kind found in oil fields.
• Radioactive springs are decided by radon. The legal onsen definition lists radium salt and radon separately, but in the therapeutic classification only the radon content decides a radioactive spring. There is no type called “radium spring” in the guidelines, the old-and-new name table or the posting standard. The guidelines even work out the radium figure from the radon measured, treating the radon as having come from radium. “Radium spring” is a common name, used descriptively by, for example, Aomori prefecture.
• “Acidic” is decided by hydrogen ion, not by pH. An acidic spring is one with 1 mg/kg or more of hydrogen ion. “Acidic, under pH 3” is a separate scale, described below. The two usually point the same way, but the spring name follows the hydrogen-ion figure.

Temperature, pH and tonicity are separate scales

The type tells you the chemistry. Three more scales, all from the same guidelines, describe the water as it comes out of the ground.

Scale	Bands
Temperature at the source	Cold mineral spring under 25°C · low-temperature spring 25–34°C · onsen 34–42°C · high-temperature spring 42°C and over
pH at the source	Acidic under 3 · weakly acidic 3–6 · neutral 6–7.5 · weakly alkaline 7.5–8.5 · alkaline 8.5 and over
Tonicity (dissolved substances, excluding gases)	Hypotonic under 8,000 mg/kg · isotonic 8,000–10,000 · hypertonic 10,000 and over (or by freezing point)

Note that “onsen” appears twice: once as the legal term for any qualifying water, and again as the name of the 34–42°C band. I have not found a ministry explanation of what the tonicity bands mean for a bather, so this page does not offer one.

Reading the sign

What has to be on it

Anyone offering onsen water to the public for bathing or drinking must post, somewhere easy to see, the water’s components, contraindications, and precautions for bathing or drinking (Hot Springs Act, Article 18). The analysis must come from a registered analysis body and be repeated at set intervals — the ministry’s overview says every ten years — and the sign must be updated within 30 days of a new result. Changes are notified to the prefectural governor in advance, and the governor can order a change to protect health.

The enforcement regulations (Article 10) list what goes on it, as I read them in summary: the source name, spring type, temperature, components, date of analysis, the registered analysis body, contraindications, and how to use the water and what to watch for. On top of that, the operator must say whether, and why, it adds water, heats the water, recirculates or filters it, adds bath additives, or disinfects it. The ministry’s 2014 notice repeats that the fact and the reason must be posted. I have not checked the article’s wording word for word. What those five items mean in practice is a subject of its own, and is covered in the separate piece on kakenagashi.

Reading the name

A full spring name is built in a fixed order, and once you know it the long names stop being opaque.

• Salt springs read [special component]-[main positive ion]-[main negative ion] spring. Natorium-enkabutsu-sen, sodium-chloride spring, is a chloride spring where sodium is the main positive ion.
• Secondary components are listed too, in descending order, if they reach 20 mval% or more — for example sodium·calcium-chloride·sulphate spring.
• Special components above their threshold go at the front: sansei- (acidic), gan-nisanka-tanso- (containing carbon dioxide), gan’yōso- (containing iodine). If there are several, the order is hydrogen ion, sulphur, carbon dioxide, radon, iron, iodide. So an acidic sodium-sulphate spring is a sulphate spring that is also acidic.
• A salt spring below 25°C ends in “cold mineral spring” rather than “onsen”, as in sodium-chloride strong salt cold mineral spring.
• The three scales follow the name, by custom: the guidelines’ own example is “isotonic, neutral, high-temperature spring”.

The indications and contraindications

The posting standard lists, for each type, the conditions it officially counts as indications. They are reproduced here as the official list of indications, not as a promise. The same standard says the effect of spa therapy comes from the combined action of the water’s chemistry, heat and other physical factors, the landscape and climate of the spa town, and a change in daily rhythm; that its purpose is easing symptoms and restoring and improving health rather than curing a particular disease; that two to three weeks is usually suitable for full effect; that even an indicated condition can get worse depending on its stage; and that therapy is best done under a doctor’s instructions. It also leaves what is actually posted for indications to each prefectural governor.

Type	Bathing (official list)	Drinking (official list)
Simple	Autonomic instability, insomnia, depressive states (since the 2014 revision; the earlier standard set none for simple springs)	None listed
Chloride	Cuts, peripheral circulation disorders, sensitivity to cold, depressive states, dry skin	Atrophic gastritis, constipation
Hydrogen-carbonate	Cuts, peripheral circulation disorders, sensitivity to cold, dry skin	Gastric and duodenal ulcer, reflux oesophagitis, impaired glucose tolerance (diabetes), hyperuricaemia (gout)
Sulphate	As chloride	Biliary dysfunction, high cholesterol, constipation
Carbon dioxide	Cuts, peripheral circulation disorders, sensitivity to cold, autonomic instability	Weak digestive function
Iron	None listed	Iron-deficiency anaemia
Acidic	Atopic dermatitis, psoriasis vulgaris, impaired glucose tolerance (diabetes), superficial skin infections	None listed
Iodine	None listed	High cholesterol
Sulphur	Atopic dermatitis, psoriasis vulgaris, chronic eczema, superficial skin infections (hydrogen sulphide type adds peripheral circulation disorders)	Impaired glucose tolerance (diabetes), high cholesterol
Radioactive	Hyperuricaemia (gout), rheumatoid arthritis, ankylosing spondylitis and others	None listed

The standard also lists general contraindications for bathing in any onsen: the active phase of an illness, especially with fever; marked physical weakness such as active tuberculosis, advanced malignant tumours or severe anaemia; serious heart or lung disease that leaves you breathless after slight movement, or serious kidney disease with swelling; bleeding from the digestive tract or any visible bleeding; and the acute flare-up of a chronic illness. It defines a contraindication as a condition where even one bath or drink could cause harm, and adds that therapy under a doctor with specialist knowledge is not ruled out. For acidic and sulphur springs it adds two type-specific contraindications for bathing: people with sensitive skin or mucous membranes, and dry skin in older people.

Bathing and drinking, by the standard

The same ministry standard sets out precautions written for Japanese bathers. In summary:

• Before. Avoid bathing just before or after a meal or after drinking alcohol, and especially when drunk. Rest when very tired, and for about 30 minutes after exercise. Older people, children and people with disabilities should preferably not bathe alone. Pour water over your hands and feet first to get used to the temperature, and wash before getting in. Drink a glass of water.
• In the bath. Older people and those with high blood pressure, heart disease or a past stroke should avoid baths of 42°C or more. In the first few days, bathe once or twice a day, rising to two or three. Start with about 3 to 10 minutes a time, extending to 15 to 20 once used to it. Get out slowly. If you feel dizzy, call for help, keep your head low, get out slowly and lie down.
• After. Do not rinse off; towel dry, dress and rest about 30 minutes. People with sensitive skin, or after a strong spring such as an acidic or sulphur spring, or where chlorine is used, are better off rinsing. Drink a glass of water. Keep towels out of the bath water.
• Yuatari. Roughly three days to a week into a course of spa bathing, symptoms such as feeling unwell, poor sleep, stomach trouble or skin inflammation can appear. The standard says to stop or bathe less until they pass. I did not find an official explanation of why it happens.

JNTO’s English guide for visitors covers the same ground from the bather’s side: bathe naked, without a swimsuit; wash at the washing area first; pour water over yourself; keep the small towel out of the water, on your head or the edge; tie up long hair; do not swim or splash; talk quietly; no phones or photos in the bath; get out and drink cold water if you overheat; and avoid bathing after drinking alcohol. Washing first and keeping the towel out of the water appear in both the ministry standard and JNTO’s guide.

Some places offer onsen water to drink (inzen). The standard says drinking as therapy should be under a doctor with specialist knowledge, and anyone on medication should ask their own doctor. Children aged 15 and under should generally not drink it unless a doctor advises otherwise. Drink only at the designated spot, fresh from the source; a single serving is generally 100–150 mL and the daily total roughly 200–500 mL. Water below pH 3 is diluted with fresh water to pH 3 or above. Use your own or a disposable cup, ideally about 30 minutes before a meal, do not take the water home to drink, and do not drink it if you have trouble swallowing. Where water contains sodium, potassium, magnesium or iodide, the sign gives a daily upper limit worked out from the analysis.

Tattoos

There is no single rule. The tourism agency, in a Q&A for ryokan, says the response varies widely from inn to inn. Tattoos have been associated in Japan with organised crime, and inns have turned tattooed guests away to avoid unsettling other guests; in recent years, with wider understanding of tattoos as part of visitors’ culture, some inns set no restriction or judge small tattoos case by case. Where an inn has a private bath or an in-room bath, a tattooed guest can use that instead. The agency suggests asking the inn in advance.

JNTO says some traditional onsen do not accept tattooed guests but the number that do is growing, that you should check the policy beforehand, and that a private bath, kashikiri-buro, is an option. Some places sell or lend cover stickers, but those suit only very small tattoos. A JNTO blog post from December 2020 says about 30% of lodgings allowed tattooed guests to bathe, citing a 2015 tourism agency survey; I could not open the survey itself. I also could not confirm whether the Hot Springs Act says anything about tattoos.

What people get wrong

• “An onsen is hot water.” In law it is water that is 25°C or warmer at the source or carries enough of one listed substance. Cold water can qualify; warm water with little in it qualifies by temperature alone.
• “Every onsen has a spring type.” Only water that meets the stricter therapeutic standard gets one of the ten names. The Japan Hot Spring Association says the rest are labelled simply as onsen under the Act.
• “Carbonated spring” and “salt spring” are official types. They are older or informal names. The current posted names are carbon dioxide spring and chloride spring.
• “Radium springs” are a category. The classification decides radioactive springs by radon, and none of the ministry’s documents has a radium-spring type.
• “Simple springs only have the general indications.” That was true of the old standard. Since 2014 the simple spring has its own list.
• “The list of indications means it works.” The ministry calls its list indications for spa therapy, aimed at easing symptoms over weeks, best under a doctor, and leaves the posted wording to the prefectures.
• “Hotter is a higher grade.” Temperature, pH and tonicity are separate scales from the spring type. “High-temperature spring” is a band, not a ranking.
• “There is a national approval badge for natural onsen.” The Japan Hot Spring Association ran a 17-item display board for natural onsen, descended from a mark created in 1976, which graded six items on five steps. Every board it issued lapsed on 28 February 2019, and the scheme ended.