A sodium-led bicarbonate spring with chloride and sulfate layered in — the classic jūsō-sen profile, here served hot at 56 °C. Neutral pH keeps it soft on the skin, placing it in the Japanese 'beauty-water' family. A generous 253 mg/kg of metasilicic acid leaves the skin noticeably smoother, and the water carries a faint yellow-brown tint at the source with light notes of hydrogen sulfide and iron.
Good for
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
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, nettō 熱の湯).
cold sensitivity
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
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⁺187mgP43 national67%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺28.5mgP44 national12%
- Potassiumminor; matters for drinking limitsK⁺36.2mgP78 national8%
- MagnesiumMg²⁺1.0mgP28 national0.7%
- Manganesetrace; companion to iron springsMn²⁺1.0mgP80 national
- LithiumLi⁺0.40mgP47 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.30mgP45 national
- AmmoniumNH₄⁺0.30mgP37 national
- StrontiumSr²⁺0.30mgP50 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻312mgP62 national45%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻113mgP42 national28%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻148mgP62 national27%
- FluorideF⁻0.40mgP32 national0.2%
- Hydrogen sulfideHS⁻0.30mgP46 national0.1%
- Bromidetrace, sea-originBr⁻0.30mgP35 national
- Hydrogen phosphateHPO₄²⁻0.30mgP60 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃253mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.4mgP55 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂158mgP86 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.50mgP69 national
- ArsenicAs0.03mgP50 national
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—mg
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.
- 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.