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Kusatsu Onsen — photo: おにく · CC0 · Wikimedia Commons

Is onsen water clean? How Japanese baths are kept clean, chlorine, and what cloudy water really is

Usually, yes, and not by luck. A shared onsen bath in Japan is a public bath under the law, and the people who run it have rules on disinfecting the water, how often it is changed, how the pipes are cleaned and how often the water is tested. Some baths are cleaner by design than others, and the notice on the wall tells you which kind you are in. This guide covers what the rules ask for, how to read that notice, why white, green or brown water is not dirt, and the part that depends on the people in the bath, which is you.

The short answer

• Is there chlorine in onsen water? Often, yes. Most prefectures' rules ask for about 0.4 mg of free chlorine per litre in the tub. Not every bath uses it, and the notice says whether yours does.
• How often is the water changed? It depends on the tub. A tub that is filled fresh each day is drained every day. A tub that filters and reuses its water must be drained completely at least once a week.
• Is cloudy water dirty? No. White, milky, green or brown water is the colour of the minerals in it. Dirty water looks different: a greasy film on the surface, hair, or a smell of anything other than the spring.
• What keeps it clean most of all? Everyone washing before they get in, and nothing but a clean body going into the water.

What the rules ask for

The national guidelines for public baths are written by the health ministry, and each prefecture or city turns them into its own ordinance, so the exact wording differs from place to place. The core is the same almost everywhere:

• Chlorine. Free chlorine in the tub kept at about 0.4 mg/L, with an upper limit of 1.0 mg/L, and measured every day. For comparison, Japanese tap water must have at least 0.1 mg/L at the tap, so a bath is a few times stronger than tap water, not a swimming pool.
• Changing the water. Tubs that are refilled daily are drained and cleaned daily. Tubs that run through a filter and are not changed for more than 24 hours must be drained completely, cleaned and disinfected at least once a week. Filters are backwashed and pipes disinfected on a schedule too.
• Testing. The water is tested for Legionella bacteria, the main thing these rules are written against, at least once or twice a year depending on the tub and the local ordinance. Bacteria that grow on the inside of pipes and filters, not the people in the bath, are the usual concern.

Rules are only as good as the operator who follows them. In 2023 an inn in Fukuoka Prefecture admitted that it had changed the water in its main bath only twice a year and skipped the chlorine, and it made international news precisely because it is rare. The local health office found it through an inspection, which is how the system is meant to work.

Where chlorine is hard to use

Chlorine does not suit every spring. It works less well in strongly alkaline water, it reacts with iron and sulfur, and in some water it barely lasts. In our records 215 of the 596 onsen towns with a pH reach 8.5 or above, and there are 311 sulfur towns and 81 iron towns. For water like this the guidelines allow another disinfectant, monochloramine, kept at about 3 mg/L, and some baths rely on high flow and frequent draining instead. That is a decision for the health office and the operator, not something you can see, which is why the notice matters.

Reading the notice

Near the spring analysis on the wall there is usually a small notice saying what has been done to the water. Two of its four words are about cleanliness:

• 循環 (junkan): the water is filtered and reused. Marked 有 (ari), yes, or 無 (nashi), no. A recirculated bath is not a dirty bath; it is the kind the weekly-change rule is for.
• 消毒 (shōdoku): disinfected. If it says 有, the notice often adds which method, such as 塩素系薬剤 (enso-kei yakuzai), a chlorine-based agent.

The other two, 加温 (heated) and 加水 (water added), and the phrase 源泉かけ流し (gensen kakenagashi), water that flows in from the source and out over the edge without being reused, are explained in onsen water types. A flow-through bath is constantly replacing itself, which is the main reason people who care about this look for one. It can still be chlorinated, and the notice will say so.

Two more signs you will see at the door: 清掃中 (seisōchū), "being cleaned", usually for an hour or so in the late morning at inns, and 入れ替え (irekae), when the men's and women's baths swap over, which often happens at the same time. When inn baths are open and closed is in when to soak and how long.

Cloudy, coloured and floating bits: not dirt

Many of the most famous baths in Japan look, to a first-time visitor, like something has gone wrong. Nothing has.

• White or milky. Typical of sulfur water, which turns cloudy as it meets the air. The smell of eggs is the spring, not the drains. See the sulfur onsen guide.
• White or grey flakes. Called 湯の花 (yu-no-hana), "hot-water flowers": mineral deposits that form in the water and settle on the floor of the tub. Inns in sulfur towns are proud of them.
• Brown, orange or reddish. Iron water is clear when it comes up and turns brown as the iron meets the air, which is why the famous gold bath at Arima is the colour of strong tea. Ikaho's brown bath is coloured by iron in the same way. The brown stains on the stone are the same thing. Iodine water, 13 towns in our records, can turn brown too.
• Green. Some sulfur and acidic water looks green in a pale tub or in sunlight.
• Black. Some baths in and around Tokyo use dark water coloured by ancient plant matter; see onsen in Tokyo.

Strongly acidic water is its own case. 26 of the 596 towns with a pH record go below pH 3, and Kusatsu reaches 1.0. Water that sharp can sting a cut and damage metal, so keep it away from your eyes and take off jewellery; see onsen etiquette.

The part that is up to you

A bath shared by dozens of people a day stays pleasant mainly because each of them arrives clean. That is what the rituals are for:

• Wash your whole body at the shower seat, with soap, and rinse it off before you go near the tub.
• Pour a few bowls of bath water over yourself at the edge, かけ湯 (kakeyu).
• Keep the small towel out of the water, and tie long hair up so it stays out too.
• Do not wash, shave or rinse anything in the tub itself.
• Many baths ask people who are unwell, have an open wound, or have been drinking heavily not to bathe; the notice usually says ご遠慮ください (go-enryo kudasai), "please refrain".

The full routine, with the Japanese words, is in onsen etiquette. If you have a period, there is a separate guide: onsen on your period.

If you want to be choosy

• Read the notice before you undress. 循環 無 and 源泉かけ流し mean the water is not being reused.
• Go after the cleaning. At an inn, the first hour after the baths reopen in the late morning or the afternoon is when the water is freshest and the floor has just been washed.
• Ask. お湯は毎日入れ替えていますか (oyu wa mainichi irekaete imasu ka), "is the water changed every day?" Inns that do it are glad to say so.
• Book a bath to yourself. A private bath or a room with its own bath means you are the only one in the water; see baths you can have to yourself.

How we wrote this

The rules summarised here are from prefectural and city guidance on public baths that follows the health ministry's guidelines (Wakayama, Matsuyama, Kyoto, Tokyo), and the wording and test frequency differ by area. We did not name inns or rank baths for hygiene, because there is no public data that would let us. The counts of water types and pH come from our own records of 1,538 onsen towns, 1,468 of them with a water type and 596 with a pH. Nothing here is medical advice.

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