Types of Hot Springs

Mineral springs can be classified into many categories according to Japanese law. Just like wine in France, onsen in Japan have a detailed classification system. Almost every onsen will have a sign that, along with basic information like the address of the well, when it was drilled, the well temperature and so on there are four important factors in describing each spring.

Classifications are done using 4 criteria. 

1 - well temperature
2 - pH
3 - osmotic pressure
4 - type of spring

WELL TEMPERATURE

  • cold spring - 冷鉱泉 - wells that have a temperature up to 20 C.
  • warm spring - 微温線 - wells that have a temperature from 25 C to 34 C.
  • hot spring - 温泉 - wells that have a temperature from 34 C to 42 C.
  • extra hot spring - 高温泉 - wells that have a temperature above 42 C.

The common temperature for a bath is 41 C. Many places with a spring colder than 41 C will heat up their spring water. This is called kaon 加温 in Japanese. 

Conversely, springs that are too hot will cool the spring by adding cool water to the baths. This will dilute the amount of minerals in the bath. It is called kasui 加水 in Japanese. 

If the water is used straight out of the well without any heating or cooling it is called kakenagashi 掛け流し or gensen 源泉.

pH

    • Acidic - 酸性 - Wells that have a pH of 3 or lower.
    • Weak acidic - 弱酸性 - Wells that have a pH of 3 to 6.
    • Neutral - 中性 - Wells that have a pH of 6 to 7.5.
    • Weak alkaline - 弱アリカリ性 - Wells that have a pH of 7.5 to 8.5.
    • Alkaline - アリカリ性 - Wells that have a pH of 8.5 or greater.

    Acidic wells are rare in Japan. Most common you’ll find neutral to weak alkaline wells. The pH of the water has a large effect on how the water feels in the bath. The more alkaline the baths are the softer and soapier the water feels. Alkaline baths are thought to be better for the skin and also called "beauty springs" or bijin-no-yu 美人の湯 in Japanese. Acidic baths will, conversely, feel much harder or firmer on the body. Both sensations are enjoyable and are a matter of opinion. Some bathers dislike the strongly alkaline baths as they dislike the slippery feeling  while others dislike acidic baths because of the dry skin feel.

    OSMOTIC PRESSURE

    • Low - 底張性 - 8g/kg of dissolved solids or lower and a freezing point of -0.55 C or higher.
    • Isotonic - 等張性 - 8g/kg to 10/kg of dissolved solids with a freezing point of -0.55 C to -0.58 C
    • High - 高張性 - 10g/kg or higher and a freezing point of at least -0.58 C

    This is the total weight of the dissolved solids in a given amount of the spring water and the water’s ability to hold those minerals in suspension. You can tell the difference between the onsen in this category by just looking at the water. Those that contain a high percentage of dissolved solids will typically be opaque. Baths that are cloudy or entirely opaque are called nigori-yu. The word nigori (濁り) is also used to describe cloudy sake.

    HOT SPRING TYPE

    This category is defined by the chemical and mineral content of the spring water. There does not seem to be a standardized nomenclature for these names and the actual name given seems to be at the discretion of the well chemist who verifies the contents of the well. I’ve broken the types of onsen down into larger categories which then contain the variations on those categories. Some types listed in the reviews might not be listed here or written in a different format.

    Simple type 単純温泉 These are hot springs that do not contain enough minerals to be classified in detail but are still considered onsen.

    • Simple hot spring / 単純温泉
    • Simple alkaline hot spring / アルカリ性単純温泉

    Chloride springs 塩化物泉 The most common type of hot spring in Japan. The salt content helps clean the body of sweat while helping the body retain heat.

    • Sodium chloride spring / ナトリウム−塩化物泉
    • Strong sodium chloride spring / ナトリウム−塩化物強塩泉
    • Sodium chloride carbonated spring / ナトリウム−塩化物・炭酸水素塩泉
    • Sodium chloride sulfate spring / ナトリウム−塩化物・硫酸塩泉
    • Sodium and calcium chloride carbonated spring / ナトリウム・カルシウム−塩化物・炭酸水素塩泉

    Carbonated springs 炭酸水素塩泉 These have a softening effect on your skin. They also leave the body feeling refreshed and cool after leaving the bath.

    • Sodium and calcium bicarbonate spring / ナトリウム・カルシウム−炭酸水素塩泉
    • Calcium and sodium bicarbonate chloride spring / カルシウム・ナトリウム−炭酸水素塩・塩化物泉
    • Calcium and sodium bicarbonate sulfate spring / カルシウム・ナトリウム−炭酸水素塩・硫酸塩泉

    Sulfuric acid springs 硫酸塩泉 These springs help reduce pain, combat liver disease and constipation. They also promote the process that oxygenates the blood. The calcium type also has a calming effect. The magnesium helps lower blood pressure.

    • Calcium sulfate spring / カルシウム−硫酸塩泉
    • Sodium sulfate chloride spring / ナトリウム−硫酸塩・塩化物泉
    • Magnesium sulfate spring / マグネシウム-硫酸塩泉

    Carbon dioxide springs 二酸化炭素泉 There are only a few of these naturally occurring in Japan. When you get in your entire body will be covered in bubbles. They leave the skin feeling warm even if the temperature of the bath is low. Bathing in them helps expands the capillaries and is good for lowering ones blood pressure. Many supersento add carbon dioxide to their baths for a similar effect.

    • Simple carbon dioxide spring / 単純二酸化炭素泉

    Acidic springs 酸性泉 These springs are very powerful. If you have weak skin you should be careful bathing in them. They have a strong antibiotic power and are good at fighting types of eczema.

    • Simple acidic spring / 単純酸性泉

    Sulfuric springs 硫黄泉 Helpful with various skin diseases and diabetes.

    • Simple sulfur spring / 単純硫黄泉
    • Sulfur and sodium chloride and calcium carbonated spring / 含硫黄−ナトリウム−塩化物・炭酸水素塩泉
    • Simple sulfur spring (hydrogen sulfide type) / 単純硫黄泉(硫化水素型)
    • Acidic sulfur (sodium) sulfate spring (hydrogen sulfide type) / 酸性−含硫黄(−ナトリウム)− 硫酸塩泉(硫化水素型)

    Iron springs 含鉄泉 The iron in the spring is absorbed by the water and helps in blood production. Although these springs are clear when they emerge from the earth, the iron in them will oxidize and turn a brownish color.

    • Iron carbonated spring / 鉄−炭酸水素塩泉
    • Acidic iron sulfuric acid spring / 酸性−鉄−硫酸塩泉

    Aluminum springs 含アルミニウム泉 These springs have a disinfectant affect and are good against skin diseases.

    • Aluminum and iron sulfate spring / アルミニウム−鉄・硫酸塩泉

    Radioactive springs 放射能泉 The radon is absorbed into the body and breathed in as the gas escapes from the water. The use of radioactivity for positive medical effects is greatly debated, but has been done for many years. Radon caves where people go to breath in the gas for therapeutic reasons are still in operation in parts of North America. One should use their own judgment when deciding to expose themselves to radioactivity like this.

    • Simple radioactive spring / 単純放射能泉
    • Simple weak radioactive spring / 単純弱放射能泉

    The data for this page comes from personal research and references such as Mappuru Higaeri Onsen Kansai 2009