A sodium chloride-sulfate spring rising on the Kannawa hillside above Beppu. Hot at 52 °C and weakly acidic at pH 4.4, the water carries a faint tang on contact with the skin. Moderately mineralized at 1,184 mg/kg, with chloride and sulfate sharing the anion side — chloride leads, sulfate adds quiet body. The water itself is colorless, clear, tasteless and odorless.
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
- Acute illness
- Active tuberculosis
- Malignant tumor
- 重い心臓病
- 呼吸不全
- Renal failure
- Bleeding disorders
- Severe anemia
- その他一般に病勢進行中の疾患
- 妊娠中(特に初期と末期)
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⁺290mgP61 national79%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺26.2mgP44 national8%
- Potassiumminor; matters for drinking limitsK⁺40.5mgP81 national7%
- Magnesiummild; a laxative if you drink itMg²⁺9.6mgP61 national4%
- LithiumLi⁺0.83mgP70 national2%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻417mgP72 national74%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻189mgP64 national25%
- Fluoridetrace; drinking limits applyF⁻0.19mgP10 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃177mgP78 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂18.7mgP73 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂11.0mgP42 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs0.23mgP82 national
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic digestive 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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- 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.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.