The hotel's own spring, a sodium chloride water served from a storage tank near the wellhead. At pH 3.9 it sits in acidic territory with a faintly tangy bite on the skin, and the 84.8 degree Celsius source temperature puts it among Japan's hottest natural sources. Total dissolved solids of 3.5 g/kg give it moderate body, and at the inlet the water carries a slight salty edge with a faint metallic note from dissolved iron.
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
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
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
Take care
Sensitive-skin caution
Strongly acidic or sulfur-rich water can sting broken, atopic-flare, or dry elderly skin. Ease in slowly and rinse off if it stings.
broken or dry skin
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⁺965mgP83 national84%
- Potassiumminor; matters for drinking limitsK⁺147mgP94 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.0mgP55 national4%
- Magnesiummild; a laxative if you drink itMg²⁺9.0mgP60 national1%
- HydrogenH⁺0.10mgP59 national0.1%
- Manganesetrace; companion to iron springsMn²⁺0.60mgP72 national
- BariumBa²⁺—mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺—mg
- LithiumLi⁺—mg
- AmmoniumNH₄⁺—mg
- StrontiumSr²⁺—mg
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,446mgP83 national81%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻—mg19%
- Fluoridetrace; drinking limits applyF⁻1.8mgP58 national
- Bromidetrace, sea-originBr⁻—mg
- Hydrogen phosphateHPO₄²⁻—mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂387mgP99 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃—mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂41.7mgP66 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S—mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —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.
- 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.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- 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.
- Ministry of the Environment, Japan. 禁忌症及び入浴又は飲用上の注意事項 (2014 notice).official