A sulfur-bearing sodium-calcium sulfate spring, blended from five wellheads — Marugama, Yudegama, Takenoshigama, Shimogama and Ontake-no-yu — and sampled where that mix is piped out for distribution. The water runs colorless and clear yet carries a strong hydrogen sulfide note, and it emerges among Japan's hottest natural sources, with sulfate lending the bath a quiet mineral body. Alkalinity gives the slippery touch that marks Japan's beauty-water family, and a generous 110 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother.
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
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⁺191mgP49 national62%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺98.5mgP73 national37%
- Potassiumminor; matters for drinking limitsK⁺6.3mgP37 national1%
- AmmoniumNH₄⁺1.1mgP68 national0.5%
- StrontiumSr²⁺0.50mgP70 national0.1%
- AluminumAl³⁺0.10mgP53 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻494mgP87 national73%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻92.4mgP39 national19%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻16.2mgP62 national4%
- Thiosulfateoxidized sulfurS₂O₃²⁻13.5mgP96 national2%
- Hydrogen sulfidesulfur character — antibacterialHS⁻6.4mgP88 national1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻9.8mgP6 national1%
- FluorideF⁻1.0mgP47 national0.4%
- HydroxideOH⁻0.20mgP66 national0.1%
- Bromidetrace, sea-originBr⁻0.30mgP37 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃110mgP58 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.1mgP45 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.
- 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