A sodium-led bicarbonate spring with chloride and sulfate layered in close behind — the jūsō-sen soda-water style, soft on the skin. It runs neutral at the wellhead and moderately mineralized at 1,313 mg/kg of dissolved solids, giving the water noticeable body without any sharpness. A generous 118.3 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel noticeably smoother afterwards.
Good for
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
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
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
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⁺224mgP53 national59%
- Magnesiummild; a laxative if you drink itMg²⁺34.9mgP81 national18%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺49.8mgP59 national15%
- Potassiumminor; matters for drinking limitsK⁺49.3mgP84 national8%
- ManganeseMn²⁺0.70mgP77 national0.2%
- Iron (II)Fe²⁺0.60mgP58 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺0.02mgP22 national
- CopperCu²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻391mgP69 national38%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻202mgP55 national34%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻230mgP72 national28%
- BromideBr⁻0.80mgP55 national0.1%
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- SS²⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃118mgP62 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.8mgP58 national
- Metaarsenious acidHAsO₂0.08mgP43 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂88.1mgP79 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs0.05mgP57 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.