Q & A
Frequently Asked Questions
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Where is total dissolved solids written?
Does a simple spring have few minerals?
Why 1 g/kg matters
At 1 g/kg or more, water meets one of the Hot Spring Act's criteria, and therapeutic springs are named after their main components — chloride, bicarbonate, sulfate and so on. Water below 1 g/kg that is 25°C or warmer is a "simple spring".
| Total dissolved solids | How the spring type is named |
|---|---|
| below 1 g/kg (25°C or warmer) | Simple spring (unless a specific component exceeds its threshold) |
| 1 g/kg or more | Named after the main components: chloride, bicarbonate, sulfate, etc. |
The three osmotic bands
Total dissolved solids also sets the osmotic band shown in brackets in the spring type name (e.g. "Chloride spring (hypertonic, neutral, hot)"). "Isotonic" is close to the concentration of body fluids.
| Band | Total dissolved solids | Freezing point |
|---|---|---|
| Hypotonic | below 8 g/kg | -0.55°C or higher |
| Isotonic | 8 to below 10 g/kg | below -0.55°C, -0.58°C or higher |
| Hypertonic | 10 g/kg or more | below -0.58°C |
Bathing in strong water
- Strong, hypertonic water is said to make "yuatari" (bath fatigue) more likely. Avoid long soaks and take breaks
- Minerals tend to stay on the skin after strong water; people with sensitive skin may want to rinse before leaving
- Total dissolved solids measures concentration, not how effective a spring is
Find onsen by concentration
On Explore, use "Sort by component" to order onsen from the strongest or the lightest water 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
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