A sodium- and calcium-led chloride-sulfate spring rising hot at 56.5 °C from the wellhead, with sulfate adding quiet body to a salt-led mineralization. Alkaline pH 9.0 gives the slippery, beauty-water feel on the skin, while the moderate dissolved load (about 2.6 g/kg) lands well above a plain simple-thermal source — there is enough chloride and sulfate to leave a faint mineral film without crossing into a heavy salt coating.
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⁺537mgP76 national56%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺365mgP89 national44%
- PotassiumK⁺9.0mgP44 national0.6%
- Magnesiummild; a laxative if you drink itMg²⁺0.07mgP6 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.05mgP30 national
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,021mgP79 national69%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻603mgP92 national30%
- CarbonateCO₃²⁻11.5mgP51 national0.9%
- BromideBr⁻0.60mgP50 national0.0%
- HydroxideOH⁻0.20mgP65 national0.0%
- Iodideyellows in air; helps blood lipidsI⁻0.60mgP73 national
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃54.3mgP34 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.3mgP46 national
- Metaarsenious acidHAsO₂0.03mgP37 national
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.