Q & A
Frequently Asked Questions
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What's the difference between an onsen and a regular bath?
An onsen uses natural spring water that meets Japan's legal temperature or mineral criteria. A regular public bath (sento) typically heats ordinary tap water — a legally distinct category.
Are there cold onsen?
Yes. Springs with source temperatures below 25 °C are still classified as onsen if they meet the mineral conditions. Cold carbonated springs and iron-rich springs are common examples.
Is it still an onsen if the water is heated or filtered?
Yes. Heating, diluting, circulating, or disinfecting the water doesn't change its onsen status as long as the source water meets the legal criteria. Facilities must disclose any such treatments.
Japan's 3 Legal Conditions for an Onsen
- Temperature: water temperature at the source must be 25 °C or above
- Dissolved solids: total dissolved solids must be 1,000 mg/L or more
- Specific minerals: must contain a regulated amount of at least one of 19 designated components (e.g. free carbon dioxide, radon)
Bathing Temperature vs. Legal Definition
Legally, any qualifying spring water is "onsen", regardless of bathing temperature. Most facilities heat the water to the comfortable range of 40–42 °C. When heating, diluting, or filtering is applied, facilities are required by law to display this information clearly.
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