JAPAN'S FINEST ONSEN & SAUNA
The Finest Onsen & Sauna in Japan
57 SELECTIONS
Most of what we surveyed never even made it onto this site. This book is a further cut from what did. Our editor bathed in every onsen and sauna in it, and wrote only what they confirmed on the ground.
A simple guide to Japan's hot spring water types, with an easy-to-scan chart of 10 therapeutic spring categories. Learn the definitions, features, how to read pH and mineral data, and how beginners can choose.
Published: Oct 22, 2025
A simple guide to Japan's hot spring water types, with an easy-to-scan chart of 10 therapeutic spring categories. Learn the definitions, features, how to read pH and mineral data, and how beginners can choose.
Published: Oct 22, 2025
Hot spring water type refers to a classification of the minerals dissolved in the water. Labels such as "sulfur spring," "bicarbonate spring," and "chloride spring" seen at Japanese hot spring resorts express the character of the water in a single phrase.
In short, beginners do not need to memorize everything. First, grasp the broad differences among the 10 therapeutic spring types and the key points to check first in the mineral analysis sheet, and you will be able to understand what kind of water a hot spring has. This article organizes those essentials based on Ministry of the Environment standards.
First, let's separate two confusing terms.
One is the legal definition of a hot spring. Under the Hot Springs Act, water is defined as a "hot spring" if the source temperature is 25°C or higher, or if it contains at least one of the 19 specified components at or above the regulated amount, even if the temperature is lower.
The other is a therapeutic spring. This is a hot spring that meets the stricter standards set by the Ministry of the Environment's "Guidelines for Mineral Spring Analysis," and only therapeutic springs are given one of the 10 water type names. If you see a sign that says "○○ spring," you can think of it as meeting the conditions for a therapeutic spring. The requirements are that the total dissolved solids are 1,000mg/kg or more, or that specific components are present at or above the regulated levels.
Therapeutic springs are classified into the following 10 types according to the components they contain. Start with this table to get the full picture, then move on to the detailed explanation of any type that catches your interest.
| Water type | Rough definition (therapeutic spring) | Typical feel | Representative hot spring areas (examples) |
|---|---|---|---|
| Simple spring | Dissolved solids under 1,000mg/kg, source temperature 25°C or higher | Mild and soft, with little character | Gero, Kinugawa |
| Chloride spring | Dissolved solids 1,000mg/kg or more, with chloride ions as the main anion | Said to retain warmth well because of its salt content | Atami, Kinosaki |
| Bicarbonate spring | Same as above, with bicarbonate ions as the main component | Often feels smooth on the skin and is also called a beauty spring | Ureshino, Ryujin |
| Sulfate spring | Same as above, with sulfate ions as the main component | A clean bathing feel, also called a wound spring | Yamanaka, Hoshi |
| Carbon dioxide spring | Free carbon dioxide 1,000mg/kg or more | Bubbles cling to the skin even in lukewarm water | Nagayu (Oita) |
| Iron spring | Total iron ions 20mg/kg or more | Turns brownish when exposed to air | Arima (Kinsen), Nagayu |
| Acidic spring | Hydrogen ions 1mg/kg or more (roughly pH under 3) | Strongly stimulating and antibacterial | Kusatsu, Tamagawa |
| Iodine spring | Iodide ions 10mg/kg or more | Changes from yellow to brown over time | Plains of Chiba and Niigata |
| Sulfur spring | Total sulfur 2mg/kg or more | Smells like eggs and may turn cloudy white | Manza, Takayu |
| Radioactive spring | Radon 8.25 Mache units (about 111Bq/kg) or more | Often clear and colorless | Misasa, Masutomi |
The figures are based on the Ministry of the Environment's Guidelines for Mineral Spring Analysis (revised in 2014). The representative hot spring areas are only examples, and even within the same area, the water type differs by source.
You do not need to read every detail of the "hot spring analysis report" posted in the changing room or bath area. Beginners can get a rough sense of the water by checking these five points.
For a more detailed explanation, see how to read a hot spring analysis report.
Along with the water type name, pH (acidity/alkalinity) is another major factor that affects the bathing experience. The Guidelines for Mineral Spring Analysis classify it roughly as follows.
| pH | Acidity/alkalinity | Features and examples |
|---|---|---|
| up to 3 | Strongly acidic | Strongly stimulating and antibacterial (Tamagawa, Kusatsu) |
| 3 to 6 | Weakly acidic | Gentle acidity |
| 6 to 7.5 | Neutral | Standard |
| 7.5 to 8.5 | Weakly alkaline | Smooth, slightly slippery feel |
| 8.5 and above | Alkaline | Silky feel (Hakuba Happo, etc.) |
Acidic water tends to feel more irritating on the skin and is considered to have antibacterial properties, while alkaline water is said to soften dead skin cells and sebum, so it is often described as "slippery" or "silky." However, this is only a description of the feel and does not guarantee any cosmetic effect. For details, see how to read pH values.
It is best to avoid oversimplifications such as "sulfur spring is the best" or "simple spring is boring." The actual bathing experience is determined not only by the water type name, but also by the temperature, whether water is added or heated, whether it is circulated, and even the size and crowding of the bath area.
For example, even a simple spring with a low mineral content can feel much more pleasant than a stronger spring if it is free-flowing from the source and at a comfortable temperature. A water type is a clue for understanding the water; it is not a ranking of better or worse.
If you are unsure, choose in this order:
Rather than aiming for a famous water type, it is more satisfying to find water that feels good to you.
Even if the water type changes, the basic way to bathe stays the same. Rinse your body before entering, do not stay in too long at first, get out before you feel overheated, and hydrate before and after.
However, for strongly stimulating waters such as acidic springs or highly mineralized waters such as sulfur springs, it is best to start with a shorter soak. Older adults and people with chronic conditions should also check the precautions before entering a hot spring.
No. It is enough to know the 10 names and the general tendencies of smell, color, and skin feel. You can check the rest on the analysis sheet on the spot.
These are not legal water type names, but customary labels. Alkaline or bicarbonate springs may feel smoother on the skin, but they do not guarantee cosmetic effects.
Water type is only one factor. The bathing experience changes greatly depending on the water temperature, usage conditions such as dilution, heating, and circulation, and the facility environment.
Warmth and skin feel can often be noticed right away, but the indicated benefits of therapeutic springs are generally based on repeated use over time. Individual differences are large, so do not expect too much.
A water type is a classification that shows the character of the minerals dissolved in a hot spring, and only hot springs that meet the standards are given a water type name as a therapeutic spring. If you understand the broad differences among the 10 types and the five points to check on the analysis sheet—water type name, source temperature, pH, osmotic pressure, and usage status—you can read the character of a hot spring even on your first visit. Do not overthink it; start with the one bath that feels good to you.
