Manza Onsen — photo: 663highland · CC BY 2.5 · Wikimedia Commons
Sulfur onsen in Japan: milky, clear, and where to find each
You booked the inn because the listing said sulfur spring. You pictured the photograph — steam over a pale blue-white pool, the water so cloudy your hands disappear — and what you got was a clear bath with a faint smell and nothing to photograph. Nobody lied to you. The label was accurate. It just does not mean what the pictures suggest.
What "sulfur spring" actually means on the sign
Japan sorts hot spring water into ten types by what is dissolved in it. The sulfur type is 硫黄泉 (iō-sen), and it is a chemical threshold, not a description of colour: the water has to carry at least 2 mg of sulfur compounds per kilogram. Cross that line and the inn can write 硫黄泉 on the analysis board in the changing room, whether the water is milky, clear, grey, or faintly green.
The smell is the one thing they all share, and it is not sulfur itself. It is hydrogen sulfide, 硫化水素 (ryūka suiso), the gas that gives off the rotten-egg note. Even a weak sulfur spring usually has some of it, which is why you can often smell a sulfur town from the station before you see a single bath.
Most sulfur water is clear
Here is the part the lists of famous sulfur onsen skip. Our records hold 1,468 onsen towns with a recorded water classification, and 311 of them — about one in five — include sulfur water. Of those 311, 105 have a measured pH, and the spread is not what the photographs imply:
- 15 are strongly acidic, below pH 3
- 5 are mildly acidic, pH 3 to 6
- 22 are near neutral, pH 6 to 7.5
- 63 are alkaline, pH 7.5 or above
The milky white bath people travel for belongs mostly to that small acidic group. In acidic water, the sulfur comes out of solution as fine particles, and the particles are what turn the water opaque. Alkaline sulfur water tends to keep its sulfur dissolved, so it stays clear or only slightly hazy, feels smooth and slippery on the skin, and smells much milder. Both are sulfur springs. They are very different baths.
So if the milky bath is what you want, the word to look for is not just 硫黄泉 but 白濁 (hakudaku), cloudy white, and ideally a pH figure below 3 on the inn's analysis. If you would rather have something gentle with just a hint of the smell, an alkaline sulfur town is the easier bath.
The strong end: acidic sulfur towns
Only 32 towns in our records are classified as both acidic and sulfur. They are almost all volcanic, high up, and far from the big cities. A few worth knowing by name:
Kusatsu Onsen in Gunma runs from pH 1.0 to 2.2, with sources from 44.9 to 94.5 °C and around 32,300 litres a minute across the town. It is 148 km from Tokyo and the single most famous strong-water town in Japan. The water in the central 湯畑 (yubatake), the open wooden channels in the middle of town, is cooled before it reaches any bath.
Manza Onsen, on the other side of the same mountain from Kusatsu, sits at around 80 °C at source with nine inns, and is one of the places the milky blue-white bath in the pictures comes from. Kawayu Onsen in eastern Hokkaido records pH 0.8, the lowest figure of any sulfur town in our data. Goshougake Onsen in Akita is pH 1.8 and 85 °C, in an area of steaming ground that you walk through on a marked path.
Water this acidic stings any cut, scratch, or freshly shaved skin, and it is the reason the inns post a suggested time in the water. Short soaks, out onto the edge, in again. Rinse with plain water before you dress if the inn provides a tap for it; many people find the acidic film drying.
The gentle end: clear, slippery, alkaline
At the other extreme, Hirugami Onsen in Nagano is a sulfur spring at pH 9.0, Onneyu Onsen in Hokkaido pH 9.1, and Tsunagi Onsen outside Morioka between pH 8.7 and 9.5. These are the baths where you notice the smell when you walk in and the smoothness when you sit down, and not much else. They are a good first sulfur bath.
Tsukioka Onsen in Niigata, pH 8.0 and 50 °C at source, is known for water that turns a pale green rather than white. That is the same rule at work: the colour comes from what the sulfur is doing in that particular water, and "sulfur" alone does not predict it.
Hot at source, and often cold
Of the sulfur towns with a recorded source temperature, 72 come out of the ground at 60 °C or above — Kurokawa in Kumamoto is 80 to 98 °C, Yunomine in Wakayama 92 °C. But 45 come out below 25 °C, cold enough that the inn has to heat them. Kashio Onsen in Nagano is 14 °C at source. A heated bath is marked 加温 (kaon), and there is nothing wrong with it; it just means the water you sit in has been through a boiler, and some of the gas has gone with the heat.
This matters for the smell more than for anything else. Hydrogen sulfide leaves the water quickly once it is exposed to air and heat. A bath fed straight from the source, 源泉かけ流し (gensen kakenagashi), will usually smell stronger than one that is heated and recirculated, even from the same spring.
The white flakes are supposed to be there
In a real sulfur bath you will see soft white or grey flakes floating in the water or settled on the bottom of the tub. They are 湯の花 (yu-no-hana), "hot water flowers", mineral deposits that form as the water cools. They are not dirt and nobody expects you to fish them out. In towns like Kusatsu and Beppu the dried deposits are collected and sold in packets as a souvenir.
What sulfur does to your things
Silver goes black. Take off silver rings, chains and earrings before you go in, and do not leave them in an open basket in the changing room either — the air in a strong sulfur bath is enough to start the tarnish. Some pieces clean up afterwards and some do not. Gold and platinum are fine. Watches with metal bands are worth leaving in the room.
The smell stays in your hair, your towel and your swimsuit if you brought one for a foot bath. It fades in a day or two. Use the inn's towels, not your own good ones, and put the ones you carry in a separate bag on the train home.
Signs you have to obey
Hydrogen sulfide is heavier than air and collects in hollows, and in the strongest volcanic areas it is dangerous in concentration. This is why some sulfur towns rope off parts of the steaming ground and why paths and outdoor areas are sometimes closed when the gas readings rise. A town in Hokkaido is literally named Yūdoku Onsen, "poison onsen". The sign to recognise is 立入禁止 (tachiiri kinshi), no entry. Stay on the marked paths, do not climb over a rope to get a better photograph, and if an inn tells you a bath is closed because of the gas, that is the whole reason.
Indoors, sulfur baths are built with high ceilings and open windows for the same reason. Do not close the window in a private sulfur bath to keep the heat in.
Where to find sulfur water near your base
Sulfur towns are spread across the country but cluster where the volcanoes are. By prefecture, the most in our records are Hokkaido with 35, Nagano with 32, then Fukushima and Niigata with 16 each, and Kagoshima, Akita, Tochigi and Ōita with 15 each.
From Tokyo, Nikkō Yumoto is a sulfur town at 49.3 °C at source with 23 inns at the far end of the Nikkō valley, and Hakone has sulfur water among at least five types, mostly in the upper, more volcanic part of the valley. In Kyushu, Beppu lists five water types including sulfur and acidic, and Unzen in Nagasaki is a sulfur town built around steaming vents. From Sapporo, Noboribetsu has sulfur alongside five other types, so you can compare them in one stay.
Of the 311 sulfur towns, 142 list sulfur as their only water type. The rest mix it with something else, and in a mixed town the bath you get depends on which source your inn draws from. If the milky bath is the point of the trip, check the inn's own description, not just the town's.
Three things not to do
Do not book a town because it is on a list of sulfur onsen and assume the water will be white. Look for 白濁, or a pH under 3, or a photograph of the inn's own bath.
Do not wear silver into the bath, or leave it in the changing room basket.
Do not step off the path in a steaming volcanic area or go into a bath that has been closed. The gas is the reason for both.
New to onsen? Start with how an onsen stay actually works. Choosing between water types for a first stay? See planning a first onsen trip.