A sodium-calcium spring blending bicarbonate, sulfate, and chloride character, emerging hot at 60.5 C with neutral pH. Metasilicic acid is unusually generous at 261 mg/kg, leaving a thin silica film that smooths the skin afterwards. The water carries a faint yellow tint, a moderate astringent edge on the tongue, and the unmistakable whiff of hydrogen sulfide typical of Kurokawa Onsen.
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⁺161mgP42 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺54.0mgP58 national20%
- Magnesiummild; a laxative if you drink itMg²⁺27.5mgP74 national17%
- Potassiumminor; matters for drinking limitsK⁺42.6mgP81 national8%
- LithiumLi⁺0.40mgP50 national0.5%
- Iron (II)Fe²⁺0.60mgP56 national0.2%
- AmmoniumNH₄⁺0.40mgP43 national0.2%
- StrontiumSr²⁺0.20mgP44 national0.0%
- BariumBa²⁺0.10mgP53 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻289mgP64 national37%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻197mgP65 national32%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻139mgP46 national31%
- ThiosulfateS₂O₃²⁻0.90mgP70 national0.1%
- FluorideF⁻0.30mgP24 national0.1%
- BromideBr⁻0.80mgP54 national0.1%
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃261mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.7mgP54 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂104mgP79 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.03mgP40 national
- ChromiumCr< —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.