10 Types of Japanese Onsen and Their Benefits, with Recommended Towns to Visit

Why Do Onsen Vary in Color and Scent?

Have you ever visited a Japanese hot spring (onsen) where the water was a cloudy emerald green, stained a rust red, or carried a distinctive scent like rotten eggs, and thought to yourself, “I had no idea onsen could be so different”?

Those differences come from the minerals dissolved in the water. Hot springs that meet certain official criteria are classified into 10 types of therapeutic springs (ryōyōsen) based on their components, and each type has its own characteristic color, scent, feel on the skin, and traditional benefits.

This article introduces all 10 types. Think of it as a field guide to onsen waters, and let it help you decide which kind of bath you would like to try on your next hot spring trip.

For background on the definition and history of onsen, see the article below.

”Spring Quality”: A Natural Recipe Dissolved in the Water

“Spring quality” (sen-shitsu) refers to how a hot spring is classified based on the mineral content dissolved in its water. Just as a soup stock changes flavor depending on whether it is made with kelp, bonito, or dried sardines, an onsen takes on its own character based on the minerals it picks up while traveling underground.

In Japan, the Ministry of the Environment’s Mineral Spring Analysis Guidelines (Kōsen Bunseki - hō Shishin) sort hot springs into 10 categories according to the types and amounts of components they contain.

Simple Hot Spring: Gentle and Welcoming Waters

Modest in color and scent, with a soft touch on the skin: this is the simple hot spring (tanjun onsen), the most common spring type in Japan.

“Simple” means that the total dissolved mineral content is relatively low (less than 1,000 mg per kg of spring water). Because the stimulation is mild, these waters are gentle enough for children and the elderly to enjoy with peace of mind.

The water is colorless and transparent, with little to no scent, and typically feels smooth on the skin. When the pH is 8.5 or higher, it is called an alkaline simple hot spring (arukarisei tanjun onsen) and takes on a distinctive slippery, silky feel. The alkalinity softens the outer layer of skin, and many such springs are fondly known as “beauty baths” (bihada no yu).

Chloride Spring: The “Warming Bath” That Keeps the Chill Away

A chloride spring (enkabutsu-sen) is a hot spring in which salt takes the leading role. The mineral makeup resembles seawater, and the water often tastes salty when tasted. It is one of the more common spring types found throughout Japan.

Salt that clings to the skin during bathing continues to slow the evaporation of sweat after you step out of the bath. Because it warms the body to the core and helps you stay warm long after your bath, this water has long been called the “hot bath” (netsu no yu) or the “warming bath” (atatamari no yu). The effect is especially noticeable in cold winters or in snowy hot spring towns.

The color ranges from colorless to a pale yellow. In springs that originate from ancient seawater trapped underground, the intense saltiness can be surprising.

Bicarbonate Spring: The “Beauty Bath” for Silky Skin

The moment your skin touches the water, you feel a mysterious slippery sensation. That is the bicarbonate spring (tansansuiso-en-sen).

The main component, bicarbonate ions, helps wash away excess oils and old surface skin. Because skin feels remarkably smooth after a bath, sodium-type bicarbonate springs have long been popular as “beauty baths” (bihada no yu) and the affectionately named “baking soda springs” (jūsōsen).

The water is colorless and transparent. There is almost no scent, leaving only that silky feel on the skin. In contrast, types rich in calcium and magnesium can leave white, lime-like deposits around the spout and the rim of the bath; these mineral traces are part of the charm of a bicarbonate spring.

Sulfate Spring: The Quiet “Wound-Healing Bath”

The main component is sulfate ions. Known for its calm feel on the skin, this water has long been called the “wound-healing bath” (kizu no yu) and the “stroke-care bath” (chūbu no yu). Chūbu is an old term for stroke, and these springs were valued at tōji (long-stay therapeutic bathing) retreats as waters that improved circulation.

Sulfate springs come in sodium type (formerly called bōshōsen), calcium type (formerly sekkōsen), and magnesium type (formerly shōkumisen). The feel differs depending on the cation involved, but all types are highly moisturizing and leave the skin feeling soft and supple after the bath.

The color ranges from colorless to a pale brown (when iron is also present). The scent is restrained, and the water sometimes has a slightly bitter taste.

Carbon Dioxide Spring: The Tingling “Bubble Bath”

Sink into the water and tiny bubbles cling all over your skin. That fizzy, ticklish sensation is the signature of a carbon dioxide spring (nisanka-tanso-sen).

Those bubbles are carbon dioxide gas dissolved in the water at high concentrations. Absorbed through the skin, the gas dilates blood vessels and promotes circulation. That is why even relatively cool water can warm the body thoroughly, making these springs perfectly suited to slow, lingering soaks in lukewarm baths.

However, carbon dioxide escapes easily as temperature rises, which makes this a rare spring type in Japan. In a volcanic archipelago dominated by high-temperature sources, few hot springs surface with their carbonation intact.

The water is colorless and transparent, and may have a slightly metallic taste when sipped.

Iron Spring: The “Golden Spring” That Changes Color in the Air

Colorless and transparent as it emerges from the ground, then turning reddish-brown or golden the moment it touches the air: an iron spring (gantetsu-sen) is like watching a chemistry experiment unfold before your eyes.

The iron ions in the water react with oxygen and oxidize, changing color. Scoop some water in your palm at the spout and compare it with the darker bath that has been sitting for a while, and you can see the transformation clearly.

The color ranges from reddish brown to gold. The water has a distinctive iron scent and a metallic taste in the mouth.

Acidic Spring: A Water of Sharp Stimulation and Antiseptic Power

Sour to the taste, tingling on the skin, and able to corrode an iron nail dipped into it: an acidic spring (sansei-sen) is a bold, powerful kind of water.

Rich in hydrogen ions, it shows strong acidity, often with a pH below 3. Its antiseptic action is exceptional, and these waters have long drawn tōji-style visitors seeking relief from skin conditions. On the other hand, the stimulation can be too strong for those with sensitive skin or for children, so the standard practice is to rinse off with fresh water after bathing.

Globally, it is rare to find this many strongly acidic hot springs concentrated in one place, making them one of Japan’s most distinctive types of onsen water.

The color ranges from colorless to slightly cloudy. The water has a sharp, pungent scent and, if tasted, is more sour than a lemon.

Iodine Spring: A Golden Water That Holds Memories of the Sea

This is the newest addition to the 10 types. It was recognized as a therapeutic spring (ryōyōsen) in the 2014 revision of the Mineral Spring Analysis Guidelines.

Its defining feature is that it contains a high level of iodine, the same element abundant in seaweed. Rather than being tied to volcanic activity, these springs often surface from ancient marine sedimentary layers, with notable examples found in the plains of Chiba and Niigata Prefectures and along the coast of Hokkaidō.

The water is pale when it first emerges, but as the iodine oxidizes in the air it shifts to yellow or yellow-brown. You may also notice a faint oily scent.

Sulfur Spring: A Cloudy White Bath with a Distinctive Scent

As you approach the town, a distinctive scent like rotten eggs drifts faintly through the air. That is the sign of a sulfur spring (iō-sen).

Hot spring water containing hydrogen sulfide shifts in appearance from milky white to emerald green with the play of air and light. Within the bath, white mineral particles called yu-no-hana (“flowers of the hot spring”) drift through the water, making for a visually dramatic bathing experience.

Sulfur springs dilate blood vessels and break down skin keratin, and have traditionally been used for caring for skin conditions and arteriosclerosis. The stimulation, however, is on the stronger side. Those with sensitive skin should keep baths short and gently rinse off with fresh water afterward.

Some hot spring towns are even said to have the strongest sulfur scent in Japan, and many enthusiasts travel from one onsen to another in pursuit of that aroma.

Radioactive Spring: Entrusting Yourself to Trace Natural Radiation

The word “radioactive” may sound alarming, but there is no cause for concern. What a radioactive spring (hōshanō-sen) contains is only a trace amount of radon, a gas that exists naturally in our environment. The levels are far below anything that would harm health.

Because radon is a gas, it enters the body not only through the skin during bathing but also through breathing. The theory of radiation hormesis suggests that very small amounts of radiation may have beneficial effects on the body, and these springs have long been favored by visitors seeking tōji-style relief for conditions such as gout and rheumatoid arthritis.

The water is colorless, transparent, and odorless. Many hot springs of this type are also known for a culture of long, slow soaking in lukewarm water.

Beauty? Recovery? Wellbeing? Choosing an Onsen by Purpose

Each of the 10 spring types has its own areas of strength. Here is a summary by purpose of bathing.

Note: These are traditionally recognized indications based on Japan’s Ministry of the Environment guidelines, not medical claims. The effects of hot springs vary by individual, and some waters may not suit certain people. If you are bathing for therapeutic purposes, please consult a doctor before going in.

Insen: What to Know About Drinking Hot Spring Water

In addition to bathing, there is also a Japanese practice known as insen, the drinking of hot spring water.

Insen is a traditional method of taking in the minerals of an onsen by drinking it. Similar in spirit to the Kurhaus tradition in Germany, it has long been part of Japanese tōji (long-stay hot spring therapy) culture.

• Records of drinking hot spring water in everyday Japanese life go back to the era of the Nihon Shoki (Chronicles of Japan, compiled in 720), and medical texts describing how to drink it appeared during the Edo period (1603–1868).
• During the Meiji era (1868–1912), the German physician Erwin von Bälz (1849–1913), who taught at the University of Tokyo’s medical school, helped modernize Japanese insen practice, using Ikaho Onsen as a model.
• In the world of tōji, drinking the water alongside bathing has long been seen as a way to take in the minerals of the spring from both inside and outside the body.

In some hot spring towns, you will find designated drinking spring fountains (insenjo), where you can scoop water from a tap or spout using a provided cup.

On the other hand, scooping water from the bath itself to drink is not insen, and should be avoided for hygiene reasons. Insen means drinking freshly emerging water from a tap that has been specifically prepared for drinking. Because you are taking the water into your body rather than just bathing in it, it is important to keep a few basic points in mind:

• Drinkable hot springs are limited: only at towns with a designated insenjo.
• Amount and timing: 100–150 ml (about 3.4–5 oz) per session, up to 200–500 ml (about 6.8–17 oz) per day. Around 30 minutes before meals is a common guideline.
• Drinking is generally not recommended for those aged 15 or younger (unless under a doctor’s guidance).
• If you are on medication, please consult your doctor first.

Ten Stories of Water, Painted by the Earth

We use a single word, “onsen”, but as we have seen, different minerals dissolved in the water can yield wildly different colors, scents, textures, and traditional benefits.

The water may be groundwater warmed by volcanic heat, or the remnants of an ancient sea.

Each onsen carries layered stories of the long journey it took before reaching the surface. A hot spring is, in a sense, the memory of the geological layers that gathered its minerals over vast spans of time.

When you sink into the bath thinking about the millions of years of underground travel hidden beyond the steam, the water somehow shows you a different face.

We hope this article will be a helpful companion as you choose your next hot spring destination.

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References

• Hot Spring Act (Onsen - hō) | e - Gov Law Search
• Ministry of the Environment: Protection and Use of Hot Springs
• Ministry of the Environment: Mineral Spring Analysis Guidelines (2014 revision)
• Ministry of the Environment: Classification of Therapeutic Spring Qualities
• Ministry of the Environment: Basics of Safe and Secure Onsen Use
• Japan Spa Association: Varieties of Hot Spring Qualities
• Japan Spa Association: How to Read a Hot Spring Analysis Report

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