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Frequently Asked Questions
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How is total sulfur different from the "hydrogen sulfide and sulfides" sum?
Can a sulfur spring have no smell?
Three components counted in total sulfur
On the analysis sheet, sulfur components are listed in different rows depending on their form. Total sulfur adds up the following three after converting each to sulfur (S).
| Component | Row on the sheet | Notes |
|---|---|---|
| Free hydrogen sulfide (H₂S) | Dissolved gases | Source of the boiled-egg smell; common in acidic to neutral water |
| Hydrosulfide ion (HS⁻) | Anions | Common in alkaline water |
| Thiosulfate ion (S₂O₃²⁻) | Anions | Forms when hydrogen sulfide reacts with air |
Thresholds
- 1 mg/kg or more: one of the Hot Spring Act's criteria for an onsen
- 2 mg/kg or more: a therapeutic "sulfur spring". Springs mainly containing free hydrogen sulfide are labelled "sulfur spring (hydrogen sulfide type)"
Two springs with the same total sulfur can feel quite different depending on which component dominates. Water rich in hydrogen sulfide is said to smell stronger and often turns milky white. See the columns on sulfide ions and free hydrogen sulfide for details.
Find onsen by sulfur
On Explore, use "Sort by component" to order onsen by total sulfur, or by the sum of hydrogen sulfide and sulfides, from 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.
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