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Become an Expert in Classifying Onsen!

Let's look deeper into what makes an “onsen”
Hot Springs
If you are in Japan for any length of time, you will hear the topic of “quality” come up in discussions about hot springs, called Senshitsu in Japanese. Simply put, it is a matter of clearly indicating the mineral composition of the waters and how concentrated those substances are. As you can see from the timeline below, the definition has quite a history.

In Japan, the official classification for each onsen is usually displayed in the changing rooms of the onsen. But here is where it gets a bit complicated. Any one of the three classifications indicated above—or a combination—may be used. Let’s look at the three types of classifications a single onsen might display.

1. Firstly let’s compare the Hot Springs Quality Classification Display Standard name with the New Hot Springs Quality Classification name


In the table above, pay attention to the Hot Springs Quality Classifications Display Standard name and New Hot Springs Quality Classifications circled in red. We can break down the New Hot Springs Quality Classification name as follows:

Now, you are probably thinking, Why does the Hot Springs Quality Classifications Display Standard name “Hydrogen Carbonate Spring” give priority to the composition of negative ions? Well, there is a reason for that, too. If you look at the alternative names for classifications in the New Hot Springs Quality Classifications section of the table, you can see that there are also springs in this category with mineral compositions rich in sodium, calcium magnesium, and so on.

From a scientific perspective, this means that the biggest shared composition of this kind of onsen as a group is actually that of the negative ions—and this is true for many onsen classifications. Therefore, the composition of the negative ions is the characteristic that broadly defines the group of onsen with the proportion of the mineral composition contained in the positive ions being thought of as indicating the distinctive character and qualities of the spring.

2. Next, let’s compare the Hot Springs Quality Classifications Display Standard name with the Old Hot Springs Quality Classifications name

Going back to the previous graph, let’s compare the display standard name with the older classification. If we now compare the three classifications circled in red, we can understand the connection as follows:

In this way, we can see that the older classifications are actually indicating exactly the same classification as the new ones. They present the same information in a simple, less scientific way, but help people to quickly get an idea of what the hot spring is going to be like.

Below you can see an example of some of the older classifications and the effects of their respective mineral compositions.

Salt Water (not sea water) Helps heat retention by promoting circulation. Bathers can prevent themselves from catching a chill, and the salt also acts as a disinfectant.
Sulfurous Helps capillaries expand and is, therefore, good for those suffering from hardening of the arteries and high blood pressure. Also softens hardened skin cells.
Alum With anti-bacterial qualities and high heat retention, alum-rich waters are good for those with anemia, athlete’s foot, and other skin conditions, as well as women who are menstruating.
Earthy Indicates onsen rich in calcium and alkaline earth metals. Effective against inflammation and for pain relief.
Bitter A high concentration of magnesium lowers the blood sugar level and is beneficial to those who have suffered a stroke or hardening of the arteries.
As you can see, these classifications help you understand the benefits of the hot spring waters quickly and simply.

With all that understood, now you can pick an onsen that best suits your body and health. Next time you are at an onsen, be sure to check how its waters are classified and experience its therapeutic effects for yourself!

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