A sodium chloride spring rising in the Kofu basin at 43 degrees Celsius. The water is weakly alkaline and soft on the skin, salty enough to leave noticeable warmth long after the bath, and carries a faint hydrogen sulfide note at the wellhead. Total dissolved solids sit at 1,165 mg/kg — moderately mineralized, with chloride dominant and a quiet sulfur edge.
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
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⁺391mgP69 national86%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺8.4mgP23 national12%
- Magnesiummild; a laxative if you drink itMg²⁺0.30mgP16 national1%
- PotassiumK⁺3.4mgP28 national0.3%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻641mgP77 national95%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻35.4mgP20 national3%
- Fluoridetrace; drinking limits applyF⁻4.4mgP75 national1%
- BromideBr⁻1.7mgP69 national0.1%
- BO2BO₂⁻—mg
- H2SO4FreeH2SO4Free35.2mgP97 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃27.9mgP11 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.3mgP47 national
- CadmiumCdNDmg
- ChromiumCrNDmg
- 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.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- 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.