A simple acidic sulfur spring (hydrogen sulfide type) drawn from the communal steam vent 'Naka-no-yu' on the Maeguro slopes of Nasu-Shiobara. The wellhead runs at 79.2 °C — among Japan's hottest natural sources — and at pH 2.6 it is sharply acidic enough to sting slightly on contact. The water is colorless and clear at the source but carries a strong rotten-egg note of H₂S along with a sour, bitter taste; with 55.4 mg/kg of free hydrogen sulfide it has the unmistakable bite and skin feel of an acidic sulfur spring, long known for stubborn skin conditions.
Good for
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
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
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
- 急性疾患(とくに熱のある場合)
- 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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺8.8mgP24 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺4.8mgP1 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺2.5mgP72 national
- Magnesiummild; a laxative if you drink itMg²⁺2.5mgP36 national
- Potassiumminor; matters for drinking limitsK⁺1.7mgP19 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺1.4mgP67 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.10mgP54 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- HSO4HSO4212mgP81 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻17.9mgP30 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻10.7mgP13 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂55.4mgP71 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S55.4mgP99 national
Measured at the source
On the certificate
- Chronic digestive disorders
- 冷え症
- 高血圧症
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.
- 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.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- 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.
- 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