What is ONSEN? – Part 1

“温泉 (ONSEN)” is the Japanese general term meaning “hot spring”. From ancient times, ONSEN has been an ultimate relaxation and indispensable for Japanese people’s lives. People have been enjoying this blessings of nature by healing their fatigue, curing their diseases or injuries, and using ONSEN for their cooking and washing as well.


Although there are a lot of hot springs not only in Japan but also all over the world, I as an ONSEN Sommelier, will introduce what the ONSEN in Japan is all about and what kind of roles or functions ONSEN have been playing in Japanese history and culture.


ONSEN used to be the medicines in Japan. Depending on the spring quality of ONSEN, it has a certain medical effect to cure your diseases or injuries. By knowing the spring quality, pH, and osmotic pressure more in detail, you can become to enjoy ONSEN much much more to maintain your physical and mental health, not just making your body warm and relaxed.


I hope this information could be the tips to help you to enjoy your ONSEN trip more.


Definition of “温泉 (ONSEN)” under “温泉法 (Hot Spring Law)” in Japan


“温泉 (ONSEN)” in Japan is defined by “温泉法 (Hot Spring Law)” of Ministry of Environment as below.


  1. Water temperature at the spring source has to be over 25°C.
  2. More than one among 19 specific components (such as lithium ion, hydrogen ion, iodine ion, fluorine ion, metasilicate element, sodium bicarbonate, etc.) reaches the predetermined value.


If either one of these is met, it is officially defined as ONSEN. It means, the spring which has the source with temperature of 25°C or more, or even if the temperature is less, if only one of 19 elements meets the predetermined value, it can be defined as ONSEN in Japan.


Since there is the pyromagma inside the earth, the temperature goes up 2°C with digging every 2 meters depth. That means, if you dig the source at 1300 meters depth and draw up the groundwater from this source, the temperature of this spring source would theoretically be more than 25°C, thus it can be also defined as ONSEN. Like this, though ONSEN is only perceived as the blessings of nature, at the present day with the advanced drilling technology, many ONSEN have been excavated one after another even in Tokyo metropolitan.


In this new era of ONSEN, it is interesting to care about the difference between the ONSEN artificially excavated and the ONSEN naturally springing out.


What is “療養泉 (RYO YO SEN / Medical ONSEN)”?


Among officially defined ONSEN, some ONSEN which meets either of the below standards are further defined as “療養泉 (RYO YO SEN / Medical ONSEN)”.


  1. Water temperature at the spring source has to be over 25°C.
  2. More than one among 7 specific components reaches the predetermined value.


Although I omit the detailed explanation here since it is a bit complicated, No. 2 above is the clear difference between “ONSEN” and “Medical ONSEN”. In short, the ONSEN which is defined as Medical ONSEN has better spec which can be utilized as medical treatment. Once registered as Medical ONSEN, it is required by law to submit its “温泉成分分析表 (Analysis table of hot spring components)”. When you visit the Medical ONSEN, you can refer to it to know what the benefits and risks are.


Reference: Analysis table of hot spring components at JAKOTSU YU in Tokyo.


Classification by “pH”


ONSEN is classified by pH as below.


  • 強酸性泉 (Strong acidic spring): Less than pH2
  • 酸性泉 (Acidic spring): pH2 ~ less than pH3
  • 弱酸性泉 (Weak acidic spring): pH3 ~ less than pH6
  • 中性泉 (Neutral spring): pH6 ~ less than pH7.5
  • 弱アルカリ性泉 (Weak alkaline spring): pH7.5 ~ less than pH8.5
  • アルカリ性泉 (Alkaline spring): pH8.5 and above


Higher acidic spring has bactericidal effect like lemon, and it peels the dirt of your skin surface. Neutral spring has similar pH with the skin, thus very gentle on the skin. Higher alkaline spring has the effect to make your skin beautiful just like soup by dissolving sebum and removing keratin.


Just for your information, lemon is pH2.5, tap water is pH6.5, sea water is pH8, and soup is pH7 ~ pH10. It is interesting to choose your best fit ONSEN with thinking about pH score.


If you want to bathe in strong acidic spring, 玉川温泉 (TAMAGAWA ONSEN) in AKITA Prefecture (pH1.2), 酸ヶ湯温泉 (SUKAYU ONSEN) in AOMORI Prefecture (pH1.9), and 草津温泉 (KUSATSU ONSEN) in GUNMA Prefecture (pH2) could be the best choices. On the contrary, if you want to bathe in strong alkaline spring, 白馬八方温泉 (HAKUBA HAPPO ONSEN) in NAGANO Prefecture (pH11.5) and 飯山温泉 (IIYAMA ONSEN) in KANAGAWA Prefecture (pH11.3) could be the best choices.


I will share the posts of these ONSEN separately.


Classification by “浸透圧 (Osmotic pressure)”


ONSEN is liquid with a lot of kinds of salts dissolved, and “浸透圧 (osmotic pressure)” is also one of the parameters to classify ONSEN. The liquid called “等張液 (Isotonic solution)” is the liquid which has same osmotic pressure with the liquid in human body, and it is more or less similar with salt solution which has 8.8g of salt in 1 liter of water. ONSEN is classified into 3 groups by having this isotonic solution as the standard, and it is mentioned in the Analysis table of hot sprig components.


  • 低張泉 (Hypotonic spring): Lower osmotic pressure (less than 8g per kg), thus the skin absorbs more water and becomes sodden with water. You can enjoy the effect of ONSEN slowly by bathing long.
  • 等張泉 (Isotonic spring): The same osmotic pressure with isotonic solution (8g ~ less than 10g per kg)
  • 高張泉 (Hypertonic spring): Higher osmotic pressure (more than 10g per kg), thus the skin absorbs more components in shorter time. It is advised to bathe in ONSEN for comparatively shorter time.


This story is to be continued in Part 2. In Part 2, I will introduce the classification by the spring quality. The ONSEN in Japan is classified into 10 groups by the spring quality, and each of them has special features. Depending on the diseases or injuries you want to cure, or depending on your skin condition, you can choose your best fit spring quality and which ONSEN place you visit accordingly.


To be continued in Part 2: What is ONSEN? – Part 2

Related articles about ONSEN: https://onsen-wakipedia.com/category/onsen/

Related article: Your first experience of TOUJI – Enjoy different way of ONSEN trip

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