A sodium- and calcium-led chloride spring with bicarbonate softness and a quiet sulfate body layered in — a true compound water rather than a single-axis source. Neutral pH keeps the touch gentle, while 1,728 mg/kg of dissolved solids gives enough mineral body to be noticeable on the skin. A generous 241 mg/kg of metasilicic acid puts it squarely in the Japanese 'beauty-water' family — the silica leaves a thin film that makes skin feel noticeably smoother afterwards. The water carries a faint yellow tint with slight turbidity at the source, with no perceptible taste or smell.
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, netto 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
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⁺277mgP61 national55%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺116mgP77 national27%
- Magnesiummild; a laxative if you drink itMg²⁺35.2mgP80 national13%
- Potassiumminor; matters for drinking limitsK⁺32.0mgP77 national4%
- Iron (II)Fe²⁺2.9mgP77 national0.5%
- AmmoniumNH₄⁺0.80mgP59 national0.2%
- ManganeseMn²⁺0.50mgP66 national0.1%
- AluminumAl³⁺0.10mgP55 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻360mgP71 national47%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻379mgP70 national29%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻248mgP70 national24%
- FluorideF⁻0.80mgP41 national0.2%
- BromideBr⁻0.90mgP57 national0.1%
- CarbonateCO₃²⁻0.10mgP10 national0.0%
- H2PO4H₂PO₄⁻0.30mgP73 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃241mgP94 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂34.4mgP82 national
- Metaarsenious acidHAsO₂0.70mgP84 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂104mgP79 national
- CopperCuNDmg
- 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.
- 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.