A blended, steam-augmented source drawing from Gora Onsen wells #5 and #6. The wellhead runs hot at 56.2 °C and is delivered to the bath at 40–41 °C. The water is a neutral-pH sodium chloride spring with a faint hydrogen sulfide note and an unusually generous 161 mg/kg of metasilicic acid, which leaves skin feeling smooth. Salty enough for the heat-retaining 'warming' style of chloride bathing without the sting of a strong-salt source.
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
From a certificate analyzed in 2018.
- 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
- Renal failure 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⁺342mgP66 national
- Potassiumminor; matters for drinking limitsK⁺46.6mgP83 national
- Magnesiummild; a laxative if you drink itMg²⁺28.5mgP75 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.1mgP7 national
- StrontiumSr²⁺0.17mgP27 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.07mgP35 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.01mgP12 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻559mgP77 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻68.0mgP36 national
- NitrateNO₃⁻21.2mgP100 national
- Bromidetrace, sea-originBr⁻1.2mgP62 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻0.64mgP5 national
- Fluoridetrace; drinking limits applyF⁻0.60mgP35 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃161mgP74 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂30.4mgP80 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂5.8mgP30 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.34mgP66 national
Measured at the source
On the certificate
From a certificate analyzed in 2018.
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Muscle stiffness associated with motor paralysis
- Peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Peripheral circulation disorders
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.