A sodium-led spring blending bicarbonate, chloride and sulfate anions, emerging at a hot 68 °C and faintly yellow with slight turbidity at the source. Neutral pH and moderate mineralization (about 1.52 g/kg) give it noticeable body without heaviness. A generous 313.6 mg/kg of metasilicic acid leaves the kind of fine silica film that makes skin feel noticeably smoother after the soak.
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⁺221mgP52 national59%
- Magnesiummild; a laxative if you drink itMg²⁺33.2mgP78 national17%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺46.0mgP55 national14%
- Potassiumminor; matters for drinking limitsK⁺55.8mgP85 national9%
- LithiumLi⁺1.2mgP79 national1%
- AmmoniumNH₄⁺0.50mgP45 national0.2%
- Iron (II)Fe²⁺0.30mgP45 national0.1%
- ManganeseMn²⁺0.30mgP57 national0.1%
- StrontiumSr²⁺0.40mgP58 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- BariumBa²⁺NDmg
- CopperCu²⁺NDmg
- Iron (III)Fe³⁺NDmg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻412mgP68 national40%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻206mgP54 national34%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻210mgP68 national26%
- FluorideF⁻0.30mgP25 national0.1%
- ThiosulfateS₂O₃²⁻0.60mgP63 national0.1%
- NitrateNO₃⁻0.70mgP78 national0.0%
- Bromidetrace, sea-originBr⁻0.70mgP50 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- HydroxideOH⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃314mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂16.1mgP67 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂153mgP85 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn1.1mgP36 national
- ArsenicAs0.01mgP32 national
- ChromiumCrNDmg
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.