A blended source — Ikaho Onsen's communal Sogo-yu pool of wells — led by calcium and sodium with sulfate as the dominant anion, layered with bicarbonate softness and a salty chloride edge. Neutral and hypotonic, moderately mineralized at about 1.2 g/kg, it sits gently on the skin; the water is colorless and clear but carries a few brown flecks, the visible trace of its dissolved iron. A generous 181 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving the kind of silica film that makes skin feel noticeably smoother afterwards. The posted certificate lists cuts, burns, chronic skin conditions and cold sensitivity among its documented indications.
Good for
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
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
From a certificate analyzed in 2017.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate.
- Acute illness (especially with fever)
- Active tuberculosis
- Malignant tumor
- Severe heart disease
- Respiratory failure
- Renal failure
- Bleeding disorders
- Severe anemia
- Any other illness in a progressive stage
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺139mgP81 national48%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺101mgP28 national30%
- Magnesiummild; a laxative if you drink itMg²⁺32.2mgP78 national18%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺7.2mgP86 national2%
- Potassiumminor; matters for drinking limitsK⁺8.7mgP43 national2%
- ManganeseMn²⁺1.4mgP84 national0.3%
- AluminumAl³⁺< —mg0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻284mgP72 national41%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻275mgP64 national31%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻146mgP50 national28%
- CarbonateCO₃²⁻< —mg0.0%
- FluorideF⁻< —mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃181mgP81 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂8.3mgP50 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂172mgP87 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.02mgP32 national
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
Measured at the source
On the certificate
From a certificate analyzed in 2017.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were dropped from the official list then.
- Chronic digestive disorders
- Chronic skin conditions (dropped from the official list in 2014)
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.