A hot (45.5 °C) calcium-sodium chloride spring at neutral pH 7.3. At the wellhead it carries a strongly salty taste and a faint hydrogen sulfide note; the sulfide is volatile and the lab sample 24 hours later is odorless while the salt remains. Moderately mineralized (5.37 g/kg dissolved solids) with noticeable body on the skin.
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
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
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⁺930mgP95 national51%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺1,000mgP85 national48%
- StrontiumSr²⁺16.0mgP97 national0.4%
- PotassiumK⁺10.0mgP46 national0.3%
- LithiumLi⁺1.2mgP79 national0.2%
- MagnesiumMg²⁺1.0mgP27 national0.1%
- Manganesetrace; companion to iron springsMn²⁺0.06mgP23 national
- ZincZn²⁺0.02mgP46 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3,100mgP91 national94%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻250mgP70 national6%
- BromideBr⁻11.0mgP90 national0.1%
- BicarbonateHCO₃⁻7.6mgP4 national0.1%
- FluorideF⁻0.60mgP35 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃39.0mgP20 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.3mgP20 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.6mgP23 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn0.57mgP27 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.