A sodium-calcium chloride-sulfate spring: the chloride side gives the water its salty, heat-retaining character, while sulfate adds a quiet mineral body underneath. It runs neutral in pH and is moderately mineralized at roughly 1,500 mg/kg of dissolved solids, so it has noticeable substance without feeling harsh. Metasilicic acid is generous at 107 mg/kg, leaving the kind of thin silica film that places this water squarely in Japan's beauty-water family — skin feels distinctly 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, 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⁺293mgP62 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺164mgP83 national
- Potassiumminor; matters for drinking limitsK⁺20.1mgP63 national
- Magnesiummild; a laxative if you drink itMg²⁺2.3mgP36 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.18mgP30 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻501mgP75 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻290mgP71 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻79.4mgP40 national
- Bromidetrace, sea-originBr⁻3.0mgP77 national
- Fluoridetrace; drinking limits applyF⁻0.60mgP36 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃107mgP56 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂36.9mgP83 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.4mgP28 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.0mgP94 national
- 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.
- 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.