A heat-retaining chloride spring in which sodium and calcium share the lead almost equally. At roughly 10 g/kg of dissolved minerals it is hypertonic and richly mineralized, leaving a mineral film on the skin after you soak. The feel is mildly alkaline and soft, yet the water is strongly salty on the lips. Its certificate lists cold sensitivity, peripheral circulation disorders and dry skin among the indications specific to this spring type.
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
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
Take care
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor, severe anemia or other marked physical weakness
- Severe heart or lung disease causing shortness of breath on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- Visible bleeding
- Acute exacerbation of a chronic illness
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⁺2,455mgP93 national54%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,789mgP98 national45%
- PotassiumK⁺18.0mgP59 national0.2%
- StrontiumSr²⁺17.7mgP97 national0.2%
- MagnesiumMg²⁺2.9mgP40 national0.1%
- ManganeseMn²⁺0.20mgP47 national0.0%
- LithiumLi⁺0.10mgP27 national0.0%
- BariumBa²⁺0.10mgP49 national0.0%
- FeTotalFe0.10mgP42 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺0.0050mgP15 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,111mgP93 national93%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻528mgP88 national7%
- BromideBr⁻12.4mgP90 national0.1%
- CarbonateCO₃²⁻4.8mgP32 national0.1%
- BicarbonateHCO₃⁻2.4mgP2 national0.0%
- IodideI⁻1.3mgP84 national0.0%
- Hydrogen phosphateHPO₄²⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃34.2mgP16 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂30.4mgP80 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂13.1mgP44 national
- ArsenicAs0.02mgP32 national
- CopperCu0.0070mgP54 national
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic muscle or joint pain or stiffness (the chronic phase of rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains and the like)
- Peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Stress-related symptoms (sleep disorders, depressed mood)
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.