What is onsen?
Definition of Onsen
In Japan, an Onsen is defined as “hot water, mineral water, water vapor and other gas (excluding natural gas whose major component is hydrocarbons) gushing out from the ground and having the temperatures or substances listed in Annex 1” by a law named the “Hot Springs Act”.
Therefore, when the temperature reaches 25 degree Celsius or more, it is regarded as an Onsen, and when it contains a specified amount of the substances (one or more of these 19 items) even when the temperature is less than 25 degree Celsius, it is regarded as an Onsen at the same time. And as long as such conditions are met, water vapor or gas can be an Onsen as well.
Annex 1
- Temperature (the temperature when collected from Onsen source)
25 degrees Celsius or more - Substances (one or more of the following)
| substance name | contained amount in 1kg |
|---|---|
| Dissolved substances | total amount over 1,000mg |
| Free carbon dioxide (CO2 ) | over 150mg |
| Lithium ion (Li+) | over 1mg |
| Strontium ion （Sr2+） | over 10mg |
| Barium ion (Ba+) | over 5mg |
| Ferro or ferri ion（Fe2+,Fe3+） | over 10mg |
| First manganese ion（Mn2+） | over 10mg |
| Hydrogen ion (H+) | over 1mg |
| Bromine ion (Br–) | over 5mg |
| Iodine ion (I–) | over 1mg |
| Fluoride ion (F-) | over 2mg |
| Hydro Arsenate ion (HAsO 4 2- ) | over 1.3mg |
| Meta Arsenous acid (HAsO 2 ) | over 1mg |
| Total sulfur (S) [corresponding to HS-+ S 2 O 3 2- + H 2 S] |
over 1mg |
| Metabolic acid (HBO 2 ) | over 5mg |
| Metasilicic acid (H 2 SiO 3 ) | over 50mg |
| Bicarbonate soda（NaHCO3） | over 340mg |
| Radon (Rn) | over 20 (unit: ten ppb curie) |
| Radium salt (as Ra) | over 100 million mg |
Definition of Sanitation spring
In the "The Guideline for analysis of Kosen" that the Ministry of the Environment established for component analysis of Onsen, the sanitation spring is defined as a sort of Onsen (excluding water vapor and other gases) having temperature or substance listed in Annex 2, and it "can be used particularly for the purpose of cure"
Annex 2
- Temperature (the temperature when collected from Onsen source)
a. 25 degrees Celsius or more - Substances (one or more of the following)
| substance name | content in 1kg |
|---|---|
| Dissolved substances | total amount over 1,000mg |
| Free carbon dioxide (CO2 ) | over 1,000mg |
| Ferro or ferri ion（Fe2+,Fe3+） | over 20mg |
| Hydrogen ion (H+) | over 1mg |
| Iodine ion (I–) | over 10mg |
| Total sulfur (S) [corresponding to HS-+ S 2 O 3 2- + H 2 S] |
over 2mg |
| Radon (Rn) | over 30 (unit: ten ppb curie) |
