Q & A
Frequently Asked Questions
Tap any question to expand the answer
Where is the pH of an onsen written?
Why does alkaline onsen water feel slippery?
The five pH bands
pH 7 is neutral; lower numbers are more acidic and higher numbers more alkaline. Onsen use the bands defined in Japan's mineral spring analysis guidelines, and the band also appears in brackets in the spring type name (e.g. "Simple spring (hypotonic, weakly alkaline, hot)").
| Band | pH | How the water tends to feel |
|---|---|---|
| Acidic | below 3 | Often tingles; sensitive skin should keep soaks short |
| Weakly acidic | 3 to below 6 | Milder than acidic; feels crisp |
| Neutral | 6 to below 7.5 | Low irritation; gentle on the skin |
| Weakly alkaline | 7.5 to below 8.5 | Often feels smooth |
| Alkaline | 8.5 and above | Often feels slippery |
Reading pH on the analysis sheet
The pH on the sheet is measured from water taken at the source. Water in the bath can change with added water or recirculation, so different baths at the same facility may feel different.
- An acidic spring is defined by hydrogen ion content (1 mg/kg or more), not by pH alone. Most are strongly acidic, below pH 3
- Alkaline water feels slippery because it is said to mix with the skin's natural oils
- pH does not measure how "effective" a spring is. Read it together with the spring type and minerals
Find onsen by pH
On Explore, use "Sort by component" to order onsen by the pH measured on their analysis sheets.
Onsen towns are often rural with patchy Wi-Fi and limited public transport. Sort out data, tours and a car before you go.
- Japan eSIMAiralo
- Japan eSIMYesim
- Tours, passes & transfersGetYourGuide
- Tours, passes & transfersKKday
- Car rentalDiscover Cars
Includes affiliate links
References
EXPLORE RELATED TOPICS
Related Articles
