Japan has a written government standard for how to use a hot spring, and it says something that travel advice in English rarely repeats: after the bath, do not rinse the onsen water off. The same paragraph immediately names the people who should rinse. Both halves come from one notice, issued by the Ministry of the Environment on 1 July 2014, and the ministry has published its own English translation of it.
Three separate permissions stand between the water underground and the bath you pay to enter
The governing law is the Hot Springs Act, Act No. 125 of 1948, last amended by Act No. 121 of 30 November 2007. The ministry’s outline of the act sets out three distinct permissions, and an operator needs all three in sequence.
| Stage | Who grants the permission |
|---|---|
| Drilling, deepening, or installing pumping equipment | Prefectural governor |
| Extracting the water | Prefectural governor, unless the governor has confirmed that the flammable natural gas concentration needs no safety measures |
| Supplying the water for public bathing or drinking | Prefectural governor, or the mayor of a city or ward that runs its own public health centre |
The outline also states the condition that keeps the notices in changing rooms current. A facility with a utilisation permission has to post the water’s components and its contraindications, and that posting has to rest on an analysis carried out by a registered analysis body, with a periodic re-analysis once every ten years. The board on the wall is not decoration; it is the legal output of a laboratory test with an expiry horizon.
The posting standard is a 2014 notice issued as technical advice, not a penalty rule
The document that fixes what goes on that board is titled, in the ministry’s own numbering, notice Kanjisohatsu 1407012, dated 1 July 2014 and signed by the Director-General of the Nature Conservation Bureau. It was issued to prefectural governors, to mayors of cities with public health centres and to special ward mayors as technical advice under Article 245-4, paragraph 1 of the Local Autonomy Act, Act No. 67 of 1947. The same notice abolished two earlier instruments, a notice of 25 May 1982 and an administrative communication of 28 March 2002.
Its scope is stated at the end of each section: the criteria apply to use at accommodation facilities and public bathhouses that supply onsen for public bathing or drinking. Where a medical institution uses onsen as part of treatment, the notice says not all of the criteria apply.
The standard tells you not to rinse the water off, and then names who should
The Ministry of the Environment’s English leaflet, The ABCs for the Safe and Secure Use of Onsen, dated March 2019 and compiled under the supervision of the Japanese Society of Balneology, Climatology and Physical Medicine, prints the instruction as two lines on the same page. The main line reads: “It is preferable not to rinse the onsen constituents off the body. Just wipe moisture off with a towel.” The footnote directly beneath it reads: “However, it is better to rinse off if you have sensitive skin, or if onsen contains stimulant constituents (e.g. acidic springs, sulphur springs).”
The Japanese original adds one more case to that footnote that the English leaflet leaves out: where chlorine disinfection or similar treatment is in use. The Japanese text also attaches a duration to the step that follows — after wiping down, dress and keep warm and rest for about 30 minutes, and drink about a glass of water. The same page carries the rule about towels: do not put towels in the bathtub, stated in the leaflet as being in order to keep the water clean.
Acidic and sulphur springs are the only two qualities on the contraindication list, and the only two indicated for atopic dermatitis
This is the part of the standard that is hardest to summarise in one direction, because the notice puts the same two spring types in both of its lists.
On the contraindication side, the table of contraindications by spring water quality has exactly two rows, acidic springs and sulphur springs, and the sulphur row simply says “same as acidic springs”. Both rows name the same two conditions for bathing: hypersensitive skin or mucous membrane, and dry skin in the elderly. No other spring quality appears in that table at all.
On the indications side, the table of indications by spring water quality lists ten spring qualities, and acidic springs and sulphur springs are the only two whose bathing indications include “atopic dermatitis”. The acidic row reads: atopic dermatitis, plaque psoriasis, epidermoid suppuration and impaired glucose tolerance (diabetes). The sulphur row reads: atopic dermatitis, plaque psoriasis, chronic eczema and epidermoid suppuration, with peripheral circulatory failure added for the hydrogen sulphide type.
So the two spring types the notice singles out as capable of inflaming sensitive skin are the same two it lists against atopic dermatitis. The notice does not reconcile the two tables anywhere in the text read here. What it does say, in its own framing note, is that a contraindication means “diseases or states of illness with a risk of adverse events by even one occasion of bathing in an onsen or drinking the mineral water”, and that even where a contraindication applies, taking an onsen cure under the guidance of a doctor with relevant expertise is not ruled out.
The general contraindication list, which applies regardless of spring quality, has five entries for bathing: the active stage of a disease, especially with a fever; active tuberculosis, an advanced malignant tumour, or significant debility such as severe anaemia; severe cardiac or lung disease involving shortness of breath, or severe kidney disease with oedema; visible bleeding, including gastrointestinal bleeding; and the acute exacerbation stage of a chronic disease.
The bathing numbers are all in the same notice
The notice puts numbers where general advice puts impressions. These are the figures it prints for bathing.
| What the notice sets out | The figure it prints |
|---|---|
| Temperature to avoid for the elderly, people with hypertension or cardiac disease, and people who have had a cerebral haemorrhage | 42 degrees Celsius or more |
| Baths per day in the first days of a stay | 1 to 2 |
| Baths per day once accustomed | up to 2 to 3 |
| Length of a single bath at first | 3 to 10 minutes |
| Length once accustomed | up to 15 to 20 minutes |
| Rest after exercise before bathing | 30 minutes |
| Rest after bathing | about 30 minutes |
| Length of an onsen cure for adequate effect | 2 to 3 weeks |
The notice also says that people with cardiopulmonary insufficiency should bathe halfway or partially rather than submerging the whole body, and that before entering the tub you pour water over yourself starting from the extremities, both to get used to the temperature and to rinse the body.
Feeling unwell after a few days has a name and a date range in the standard
The notice treats feeling unwell partway through a course of bathing as an expected event, and gives it a name and a timetable. The English leaflet puts it this way: “Around 3 to 7 days or so after starting onsen cures, some persons may experience bath reaction, feel unwell, or suffer from insomnia, digestive symptoms or dermatitis.” The Japanese text gives the window as roughly three days to about one week. The instruction attached is to stop bathing or reduce the frequency until the symptoms resolve.
That is a statement about a multi-day course of bathing, not about a single soak on a day trip. The notice separates the two throughout: the bathing-method figures above are written for a stay, and the cure section is written for two to three weeks.
Drinking the water has its own limits, and they are calculated spring by spring
Drinking fountains at hot springs are governed by the same notice, with tighter numbers. The general amounts are 100 to 150 mL at one time and roughly 200 to 500 mL per day, taken about half an hour before a meal, from a clean or disposable cup, at the place where drinking is permitted and not carried away. The Japanese text says that in principle people aged 15 or under should not drink, except under the guidance of a doctor with relevant expertise.
The per-spring limits are formulas rather than fixed millilitres. For a spring containing A milligrams of a constituent per kilogram of water, the notice sets the daily ceiling at (1,200 divided by A) times 1,000 mL for sodium ions, (900 divided by A) times 1,000 mL for potassium ions, (300 divided by A) times 1,000 mL for magnesium ions, and (0.1 divided by A) times 1,000 mL for iodide ions. For arsenic, copper, fluorine, lead, mercury and free carbon dioxide the notice prints no figure of its own; it says only that where those are present the amount is to be shown by a separately prescribed method. Those figures appear instead in the English leaflet, which gives total intakes of 0.1 mg for arsenic, 0.2 mg for lead, 1.6 mg for fluorine, 0.002 mg for mercury and 2.0 mg for copper, each set out as that figure divided by A and multiplied by 1,000 mL, with free carbon dioxide given as (1,000 divided by A) times 1,000 mL for one time. The notice adds that if the calculated limit comes out at 500 mL or more, the contraindication does not have to be posted at all, because the general daily ceiling has already cut in below it.
Two milligrams of sulphur per kilogram changes what the building has to have
A separate instrument, Ministry of the Environment Notification No. 66 of 1 September 2017, sets structural criteria for facilities supplying onsen for public bathing. It applies to any onsen containing total sulphur of 2 milligrams or more per kilogram, and it exists to prevent hydrogen sulphide accidents.
The criteria are specific. Hydrogen sulphide in the air must not exceed 20 ppm measured 10 cm above the surface of the bath water, or 10 ppm measured 70 cm above the bathroom floor. The room must have ventilation openings, or aeration equipment between the source and the bathroom, or both; where ventilation openings are used, there must be at least two of them in the bathroom and at least one of those must be set at the same height as the floor. The bath water surface has to sit higher than the bathroom floor, and the inlet feeding the tub has to sit above the water surface. Nothing in the room may be built or installed in a way that lets hydrogen sulphide pool locally.
That 2 milligram figure is the same threshold the leaflet uses to define a sulphur spring. Sulphur springs are therefore the one quality that appears in all three documents read here: the contraindication table, the indication table, and a notification about how the room itself has to be built.
One thing the English leaflet promises and does not deliver
The leaflet’s contents page lists a section called “Q&A on onsen use” at page 18. In the file downloaded from the ministry in September 2026, page 18 is the contact page for the Office for Conservation and Promotion of Hot Springs. The Q&A is not in the file. Anyone relying on the English leaflet alone should expect the tables and the cautions, and should not expect that section.
For hot-spring water that visitors watch rather than enter, the timing question at the snow monkey park has a published answer: when the monkeys are actually in the hot spring.
Sources Checked
Ministry of the Environment — Outline of the Hot Springs Act
Confirmed that the act is Act No. 125 of 1948, last amended by Act No. 121 of 30 November 2007; that drilling and deepening, extraction, and supply for public bathing or drinking each require a separate permission; that the supply permission comes from the prefectural governor or the mayor of a city or ward with a public health centre; and that posted components and contraindications must rest on an analysis by a registered analysis body re-done once every ten years.
https://www.env.go.jp/nature/onsen/outline/index.html
Ministry of the Environment — criteria for posting contraindications and cautions for bathing or drinking under Article 18(1) of the Hot Springs Act, 1 July 2014
Confirmed the notice number and date and its issue as technical advice under Article 245-4(1) of the Local Autonomy Act, Act No. 67 of 1947; the abolition of the two earlier instruments; the five general contraindications for bathing; the two-row table of contraindications by spring quality naming only acidic and sulphur springs; the ten-row table of indications by spring quality, with atopic dermatitis appearing only in the acidic and sulphur rows; the bathing figures of 42 degrees, 1 to 2 then 2 to 3 baths a day, 3 to 10 then 15 to 20 minutes, 30 minutes of rest; the bath-reaction window; the drinking amounts of 100 to 150 mL and 200 to 500 mL, the half hour before a meal, the clean or disposable cup and the ban on carrying the water away; the formulas for sodium, potassium, magnesium and iodide ions; the statement that arsenic, copper, fluorine, lead, mercury and free carbon dioxide are to be shown by a separately prescribed method; the instruction in principle against drinking for people aged 15 or under; the two-to-three-week cure period; and the scope covering accommodation and public bathhouses.
https://www.env.go.jp/nature/onsen/pdf/2-5_p_11.pdf
Ministry of the Environment — The ABCs for the Safe and Secure Use of Onsen, March 2019
Confirmed the English wording of the rinsing instruction and its footnote, the towel instruction, the definition of a contraindication, the bath-reaction sentence, the acidic and sulphur indication rows, the 2 milligram per kilogram definition of a sulphur spring, the drinking limits for arsenic, lead, fluorine, mercury, copper and free carbon dioxide, and that the contents page lists a Q&A at page 18 while page 18 of the file carries the bureau’s contact details.
https://www.env.go.jp/nature/onsen/pdf/pamph_full_en.pdf
Ministry of the Environment — criteria on the structure and equipment of onsen facilities supplied for public bathing, Notification No. 66 of 1 September 2017
Confirmed that the criteria apply to onsen containing total sulphur of 2 milligrams or more per kilogram; the hydrogen sulphide limits of 20 ppm at 10 cm above the water surface and 10 ppm at 70 cm above the floor; the requirement for ventilation openings or aeration equipment; at least two openings with one at floor height; and that the water surface must sit above the floor with the inlet above the water surface.
https://www.env.go.jp/nature/onsen/pdf/2-5_p_9.pdf
Last updated: September 2026
