A sulfur-bearing sodium spring that stacks sulfate, bicarbonate and chloride in that order. The water itself runs colorless and clear, but a moderate hydrogen sulfide note rises off it and a sulfurous taste lingers on the tongue — the unmistakable rotten-egg signature of H₂S. It is weakly alkaline and soft on the skin, moderately mineralized with noticeable body, and carries enough metasilicic acid to sit in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel smoother afterwards. At the source it also ranks among Japan's hottest natural waters.
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
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, netto 熱の湯).
cold sensitivity
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⁺271mgP59 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺50.7mgP57 national17%
- Potassiumminor; matters for drinking limitsK⁺5.8mgP36 national1%
- AmmoniumNH₄⁺1.6mgP77 national0.6%
- StrontiumSr²⁺0.40mgP62 national0.1%
- Iron (II)Fe²⁺0.20mgP39 national0.1%
- AluminumAl³⁺0.10mgP53 national0.1%
- MagnesiumMg²⁺0.10mgP11 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻317mgP74 national43%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻244mgP62 national26%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻115mgP44 national21%
- Hydrogen sulfidesulfur character — antibacterialHS⁻46.6mgP100 national9%
- FluorideF⁻1.1mgP49 national0.4%
- BromideBr⁻0.50mgP47 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃85.1mgP47 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂10.6mgP58 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂18.4mgP51 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S6.1mgP86 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0006mgP53 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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