Aspere · CC0
What is an onsen?
Not every warm pool that rises from the ground is an onsen. Japan has a legal definition — and under it, even cold water can be an onsen.
Japan Onsen Atlas
Learn about onsen
What a hot spring is, the ten spring types, radon, kakenagashi, how to read a water analysis, and the culture of onsen towns and toji stays — written for readers without specialist knowledge.
Aspere · CC0
663highland · CC BY 2.5
How many onsen does Japan have?
"2,839 onsen areas." "27,899 spring sources." Familiar numbers — but what they count changes everything.
Asturio Cantabrio · CC BY-SA 4.0
Onsen area, source and facility — what's the difference?
"Going to Kusatsu Onsen" involves a town (the area), a spring called Yubatake (the source), the inn you stay in (the facility) and its open-air bath (the bath). They are four different things.
Calistemon · CC BY-SA 4.0
The ten therapeutic spring types
A spring type is the name of what is dissolved in the water. There are ten, from "simple" to "radioactive". Knowing how each tends to look, smell and feel changes how you choose.
三塚 · CC BY-SA 4.0
What is a radon spring?
"Radioactive spring" sounds alarming, yet these waters have been used for therapeutic stays in Japan for centuries. What is radon, how much is there, and how do you bathe in it?
Purplepumpkins · CC BY-SA 3.0
What is kakenagashi (free-flowing) bathing?
Fresh spring water flows into the tub, and whatever overflows is simply let go. Simple — but only possible when flow and temperature allow. We also sort out how it differs from "100% source water".
Asturio Cantabrio · CC BY-SA 4.0
Added water, heating, recirculation and disinfection
Four words on the changing-room notice. Each has its reasons, and together they tell you not whether a bath is good or bad, but how far the water has travelled from the source.
Soica2001 (talk) · Public domain
Milky springs and springs that change colour
Colour is not a spring type. A sulfur spring can be clear, and milky water has more than one cause. Here we treat colour as a separate, visual classification.
Belle Equipe · CC BY-SA 3.0
What is yunohana?
The white threads floating in the tub, the stony crust on its rim, the crystals grown in thatched huts — all of them are "yunohana", flowers of the hot spring.
Aspere · CC0
Source temperature and bath temperature
You can bathe at a "98 °C" spring because the water is cooled on its way to the tub. Keeping the two temperatures apart shows how each onsen solves the problem.
三塚 · CC BY-SA 4.0
How to read a water analysis
That sheet of numbers on the changing-room wall tells you exactly what the water is. There are only five places to look.
小石川人晃 · CC BY-SA 4.0
Onsen towns, communal baths and toji
In Japan an onsen is more than a bath. It is a town walked in yukata, bathhouses kept by villagers, and stays of weeks for one's health — ways of living that grew up around hot water.
Takuya Matsuyama · CC BY 3.0
Your first onsen: how to bathe and the rules
Bathe naked, wash first, keep your towel out of the water. Simple once you know, confusing if you don't — here are the rules and the reasons, plus mixed bathing, bathing wear and tattoos.
Bsx · Public domain
Going to an onsen alone
Inns turn solo guests away, prices are high, there's nowhere to eat. The old wisdom is half true. How to find onsen that welcome one, and what to check.
Spring type (therapeutic classes)
Simple hot spring
Under 1 g/kg of dissolved minerals and 25 °C or warmer. Gentle on the skin; pH 8.5 or higher is called "alkaline simple".
451Chloride spring
Salt is the main component. A film of salt keeps you warm after bathing — the "heat spring". Common near the coast.
706Bicarbonate spring
Baking-soda-like water that lifts oils from the skin, leaving it smooth — often called a "beauty bath".
311Sulfate spring
Contains gypsum or sodium sulfate. Traditionally called a bath for wounds and circulation.
239Carbon-dioxide spring
Dissolved CO₂ gas forms bubbles on the skin — the "fizzy bath". Warms the body even at low temperature; rare in Japan.
40Iron spring
Contains iron, which oxidises on contact with air and turns the water reddish-brown.
46Acidic spring
High in hydrogen ions and sharp on the skin, with strong antibacterial action. Kusatsu and Tamagawa are examples.
24Iodine spring
Added to the therapeutic classes in 2014. Common in seawater-derived springs; the water can be tinted yellow to brown.
9Sulfur spring
The boiled-egg smell. Often milky white — the most "onsen-like" of all. Comes in hydrogen-sulfide and sulfur types.
269Radioactive (radon) spring
Contains radon above a set threshold; bathers receive a very small dose of radiation. Misasa, Masutomi and Arima are examples. Concentrations vary widely.
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