Q & A
Frequently Asked Questions
Tap any question to expand the answer
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 aboveThis is one of the three criteria under Japan's Onsen Act. If the water's natural temperature at the source is 25°C (77°F) or higher, it legally qualifies as an onsen. Springs with cooler water, known as kōsen (鉱泉), can still be classified as onsen if they meet separate mineral content conditions.
- Dissolved solids: total dissolved solids must be 1,000 mg/L or moreThis is an alternative condition for water to be legally classified as an onsen. If the water is cooler than 25°C, it must contain at least 1,000 milligrams of total dissolved solids per liter. This measurement includes all non-gaseous mineral components dissolved in the water.
- Specific minerals: must contain a regulated amount of at least one of 19 designated components (e.g. free carbon dioxide, radon)This is the final alternative condition defined by the Onsen Act. Even if the water is cool and has less than 1,000 mg/L of total solids, it can still be an onsen. It must contain a specified minimum amount of at least one of 19 designated chemical components, such as free carbon dioxide or 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.
References
EXPLORE RELATED TOPICS
Related Articles
