A hot 49.4 °C bicarbonate-chloride spring led by sodium and magnesium. Neutral pH gives a soft, gentle feel; the water is faintly yellow and clear at the source but throws a reddish-brown sediment as it oxidizes, betraying a touch of iron. A generous 201.8 mg/kg of metasilicic acid layers in the smoothing "beauty-water" film for which bicarbonate springs are loved.
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
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⁺392mgP69 national57%
- Magnesiummild; a laxative if you drink itMg²⁺84.3mgP90 national23%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺92.6mgP71 national15%
- Potassiumminor; matters for drinking limitsK⁺34.8mgP78 national3%
- AmmoniumNH₄⁺6.6mgP93 national1%
- Iron (II)Fe²⁺1.9mgP71 national0.2%
- ManganeseMn²⁺0.60mgP68 national0.1%
- ZincZn²⁺0.0030mgP35 national
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,274mgP93 national67%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻333mgP67 national30%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻34.4mgP35 national2%
- CarbonateCO₃²⁻0.50mgP16 national0.1%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.0%
- FluorideF⁻0.20mgP17 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃202mgP86 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂30.5mgP80 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂266mgP91 national
- CadmiumCdNDmg
- CopperCuNDmg
- 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- 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.