A hot 58.5 degree C blended sodium chloride, bicarbonate and sulfate spring. Weakly alkaline and soft on the skin, it sits in Japan's beauty-water family — the chloride base brings a salty edge, the bicarbonate adds softness, and the sulfate lends quiet body. Moderate mineralization (1,257 mg/kg dissolved solids) plus a generous 162 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel noticeably smoother afterwards.
Good for
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, nettō 熱の湯).
cold sensitivity
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
- 急性疾患（とくに熱のある場合）
- Active tuberculosis
- Malignant tumor
- 重い心臓病
- 呼吸不全
- Renal failure
- Bleeding disorders
- Severe anemia
- その他一般に病勢進行中の疾患
- 妊娠中（とくに初期と末期）
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⁺260mgP54 national58%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺62.3mgP61 national19%
- LithiumLi⁺0.40mgP47 national0.3%
- AluminumAl³⁺0.40mgP76 national0.1%
- ManganeseMn²⁺0.40mgP65 national0.1%
- Iron (II)Fe²⁺0.20mgP41 national0.0%
- CopperCu²⁺< —mg
- Potassiumminor; matters for drinking limitsK⁺—mg
- Magnesiummild; a laxative if you drink itMg²⁺—mg
- AmmoniumNH₄⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻327mgP65 national47%
- FluorideF⁻0.40mgP32 national0.2%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻192mgP65 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻—mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃162mgP77 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.05mgP4 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂349mgP93 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—mg
Measured at the source
On the certificate
- Chronic digestive disorders
- 慢性皮膚病
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.
- 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.