A sodium chloride spring with sulfate adding quiet body, drawn from one of Japan's hottest natural sources at 92.9 °C. Weakly alkaline and soft on the skin, it sits in the moderately mineralized range with noticeable body. A generous 197.5 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably 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, nettō 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⁺634mgP79 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺118mgP76 national
- Potassiumminor; matters for drinking limitsK⁺63.8mgP87 national
- Magnesiummild; a laxative if you drink itMg²⁺5.9mgP54 national
- StrontiumSr²⁺1.3mgP80 national
- LithiumLi⁺1.2mgP79 national
- Manganesetrace; companion to iron springsMn²⁺0.20mgP45 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻829mgP79 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻479mgP87 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻182mgP55 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻33.0mgP85 national
- Fluoridetrace; drinking limits applyF⁻0.60mgP38 national
- Hydrogen phosphateHPO₄²⁻0.30mgP60 national
- Bromidetrace, sea-originBr⁻0.20mgP30 national
- Iodideyellows in air; helps blood lipidsI⁻0.20mgP52 national
- HydroxideOH⁻0.04mgP35 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃198mgP84 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂22.1mgP76 national
- Metaarsenious acidHAsO₂0.50mgP83 national
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.