A blend drawn from five wells along the Izu coast — Katase, Shirata and the Atagawa-Hokkawa line among them — yielding a sodium-calcium chloride-sulfate water. It is weakly alkaline and soft on the skin, chloride giving the salt-tinged, heat-retaining body and sulfate adding quiet mineral weight, with dissolved solids at a moderate 1,585 mg/kg. Metasilicic acid at 110.4 mg/kg is generous enough to place it in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel noticeably smoother. The posting lists cuts, burns, chronic skin conditions and chronic gynecological complaints among its spring-type indications, and notes that four of the five wells are diluted with water at the wellhead.
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
Take care
- Acute illness (especially with fever)
- Active tuberculosis
- Malignant tumor
- Severe heart disease
- Heart disease (when bathing in water at 42 °C or above)
- Respiratory failure
- Renal failure
- Bleeding disorders
- Severe anemia
- Advanced arteriosclerosis (when bathing in hot water)
- Hypertension (when bathing in hot water)
- Any other condition in an actively progressing phase
- Pregnancy (especially the early and late stages)
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⁺387mgP70 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺98.9mgP73 national
- Potassiumminor; matters for drinking limitsK⁺24.3mgP69 national
- Magnesiummild; a laxative if you drink itMg²⁺3.3mgP40 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.03mgP25 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⁻471mgP74 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻367mgP78 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻108mgP44 national
- Bromidetrace, sea-originBr⁻0.20mgP29 national
- HydroxideOH⁻0.02mgP28 national
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃110mgP58 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂12.3mgP61 national
- Metaarsenious acidHAsO₂0.30mgP73 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.5mgP24 national
Measured at the source
On the certificate
- Chronic digestive disorders
- Chronic skin conditions
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.