An alkaline simple sulfur spring. The water runs colorless and clear, yet it carries an unmistakable faint rotten-egg note of H₂S and a sulfurous taste on the lips, with enough hydrogen sulfide ion to place it squarely in the sulfur-bearing family. The alkalinity gives it the slippery, soft feel of Japan's beauty-water springs, reinforced by a generous load of metasilicic acid that leaves skin noticeably smoother. Overall mineralization stays light, so despite the sulfur the bath reads gentle rather than aggressive.
Good for
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Take care
Sensitive-skin caution
Strongly acidic or sulfur-rich water can sting broken, atopic-flare, or dry elderly skin. Ease in slowly and rinse off if it stings.
broken or dry skin
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺88.4mgP25 national87%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺10.8mgP27 national12%
- PotassiumK⁺1.4mgP17 national0.9%
- AmmoniumNH₄⁺0.40mgP43 national0.5%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻113mgP57 national50%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻42.6mgP25 national25%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻30.5mgP19 national11%
- Hydrogen sulfidesulfur character — antibacterialHS⁻11.4mgP94 national7%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.0mgP46 national6%
- FluorideF⁻0.60mgP35 national0.6%
- HydroxideOH⁻0.10mgP49 national0.2%
- Bromidetrace, sea-originBr⁻0.20mgP28 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃90.9mgP52 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.8mgP27 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP65 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.
- Ministry of the Environment, Japan. 禁忌症及び入浴又は飲用上の注意事項 (2014 notice).official