The bath draws Ikaho Onsen's Sōgōyu — a blend of the resort's wells — as a calcium-sodium sulfate/bicarbonate/chloride water. Sulfate gives the quiet body, with bicarbonate softness and a salty chloride edge layered over it; at roughly 1,180 mg/kg of dissolved solids it is moderately mineralized with noticeable substance. A generous 177 mg/kg of metasilicic acid places it in Japan's beauty-water family — silica of that order leaves the kind of thin film that makes skin feel smoother afterwards. The posting states the water is served undiluted and without recirculation, reheated only when it cools, and sterilized with silver ions and ozone for hygiene.
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
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⁺142mgP80 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺105mgP30 national
- Magnesiummild; a laxative if you drink itMg²⁺29.5mgP75 national
- Potassiumminor; matters for drinking limitsK⁺10.2mgP45 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺8.3mgP87 national
- Manganesetrace; companion to iron springsMn²⁺1.4mgP85 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻301mgP73 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻282mgP64 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻118mgP44 national
- Fluoridetrace; drinking limits applyF⁻0.30mgP22 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃177mgP79 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.7mgP48 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂98.6mgP78 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.01mgP29 national
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.