A sulfate hot spring drawn from Oka Onsen Well No. 275 in Ito City, Shizuoka. The wellhead is genuinely hot at 47.0 °C with a generous 141 L/min flow, and at pH 8.4 the water sits in the mildly alkaline beauty-water family — soft on the skin without being slippery. Sensory profile is the boring-in-a-good-way trio of colorless, tasteless, and odorless, and the panel lists neuralgia, muscle pain, and joint pain among the approved bathing indications.
Good for
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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 2015.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate.
- Acute illness
- Tuberculosis
- Malignant tumor
- Heart disease
- Renal failure
- Bleeding disorders
- Severe anemia
- Hypertension
- Severe arteriosclerosis
Measured at the source
On the certificate
From a certificate analyzed in 2015.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current 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.
- 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.