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Katsuragionsen Happunoyu

Water profile

Katsuragionsen Happunoyu

Spring chemistry and what it does, with the research behind it.

第二源泉Spring No. 2

塩ChlorideSodium Chloride-Bicarbonate spring · +Bicarbonate

A sodium-led chloride spring with a bicarbonate undertone — salty and heat-retaining, softened by the bicarbonate component. Hypotonic and neutral, it is classified as a cold-mineral spring (under 25 °C) and reheated for bathing.

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

On the certificate:Cold sensitivity

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

On the certificate:CutsBurns

Recovery

Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.

fatigue, stress

On the certificate:ConvalescenceFatigue recoveryGeneral health improvementFrail children

Take care

Contraindications
  • Acute illness (especially when fever is present)
  • Active tuberculosis
  • Malignant tumor
  • Severe heart disease
  • Respiratory failure
  • Renal failure
  • Bleeding disorders
  • Severe anemia
  • Any illness in an actively progressing stage
  • Pregnancy (especially the early and late stages)
  • Kidney disease
  • Hypertension
  • Any condition with edema
  • Hyperthyroidism (iodine-containing springs are contraindicated)

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.

Cationsshare of charge (mval%)
  • Sodium667mgP81 national
  • Calcium106mgP75 national
  • Magnesium63.7mgP88 national
  • Iron (II)8.5mgP88 national
  • Potassium7.8mgP41 national
  • Manganese0.40mgP61 national
  • Aluminum< —mg
Anionsshare of charge (mval%)
  • Chloride1,131mgP82 national
  • Bicarbonate503mgP76 national
  • Sulfate36.8mgP37 national
  • Nitrate1.0mgP76 national
Non-dissociatedmeasured by mass
  • Metasilicic acid72.3mgP45 national
  • Metaboric acid16.5mgP70 national
Dissolved gasesmeasured by mass
  • Free carbon dioxide484mgP96 national
  • Free hydrogen sulfide< —mg
Trace metalsmeasured by mass
  • Cadmium< —mg
  • Copper< —mg
  • Lead< —mg

Measured at the source

Tonicity
Hypotonic
Free CO₂
484mg/kg
Typical

On the certificate

  • Neuralgia
  • Muscle pain
  • Joint pain
  • Frozen shoulder
  • Motor paralysis
  • Joint stiffness
  • Bruises
  • Sprains
  • Chronic digestive disorders
  • Hemorrhoids
  • Chronic gynecological conditions
  • Chronic skin conditions
  • Diabetes
  • Gout
  • Liver disorders
  • Chronic digestive disorders
  • Chronic constipation

Evidence

  1. 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.
  2. Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
  3. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  4. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  5. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

第三源泉Spring No. 3

湯SimpleSodium-Calcium-Magnesium Bicarbonate-Chloride spring

A gentle, lightly mineralized cold-mineral spring that qualifies as 'onsen' under Article 2 by virtue of its metasilicic-acid and metaboric-acid content. Hypotonic and neutral, with around 59 mg/kg of metasilicic acid — enough to place it in the Japanese 'beauty-water' family. Reheated for bathing, as the source is below 25 °C.

Good for

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

Recovery

Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.

fatigue, stress

On the certificate:ConvalescenceFatigue recoveryGeneral health improvement

Take care

Contraindications
  • Acute illness (especially when fever is present)
  • Active tuberculosis
  • Malignant tumor
  • Severe heart disease
  • Respiratory failure
  • Renal failure
  • Bleeding disorders
  • Severe anemia
  • Any illness in an actively progressing stage
  • Pregnancy (especially the early and late stages)

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.

Cationsshare of charge (mval%)
  • Sodium19.8mgP4 national
  • Calcium13.9mgP30 national
  • Magnesium5.6mgP53 national
  • Potassium0.60mgP7 national
  • Aluminum< —mg
  • Iron (II)< —mg
  • Hydrogen< —mg
  • Manganese< —mg
Anionsshare of charge (mval%)
  • Bicarbonate41.5mgP27 national
  • Chloride21.5mgP16 national
  • Sulfate1.5mgP9 national
  • Nitrate0.40mgP55 national
Non-dissociatedmeasured by mass
  • Metasilicic acid59.2mgP35 national
  • Metaboric acid1.6mgP15 national
Dissolved gasesmeasured by mass
  • Free carbon dioxide33.0mgP61 national
  • Free hydrogen sulfide< —mg
Trace metalsmeasured by mass
  • Cadmium< —mg
  • Copper< —mg
  • Lead< —mg

Measured at the source

Tonicity
Hypotonic
Free CO₂
33mg/kg
Low

On the certificate

  • Neuralgia
  • Muscle pain
  • Joint pain
  • Frozen shoulder
  • Motor paralysis
  • Joint stiffness
  • Bruises
  • Sprains
  • Chronic digestive disorders
  • Hemorrhoids
  • Cold sensitivity

Evidence

  1. 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.
  2. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  3. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  4. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

第四源泉Spring No. 4

炭CarbonatedSodium Chloride-Bicarbonate spring · +Chloride · Bicarbonate

A rare carbonated chloride-bicarbonate cold-mineral spring — free CO₂ at 1,094 mg/kg easily clears the 'nisanka-tanso-sen' threshold, so fine bubbles cling to the skin on entry. Sodium-led, salt-tinged and softened by bicarbonate; hypotonic and neutral, and reheated for bathing as the source sits below 25 °C.

Good for

Circulation

Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.

tired legs

On the certificate:HemorrhoidsArteriosclerosisChronic gynecological conditions

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

On the certificate:Cold sensitivity

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

On the certificate:CutsBurns

Recovery

Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.

fatigue, stress

On the certificate:ConvalescenceFatigue recoveryGeneral health improvementFrail children

Take care

Contraindications
  • Acute illness (especially when fever is present)
  • Active tuberculosis
  • Malignant tumor
  • Severe heart disease
  • Respiratory failure
  • Renal failure
  • Bleeding disorders
  • Severe anemia
  • Any illness in an actively progressing stage
  • Pregnancy (especially the early and late stages)
  • Diarrhea
  • Kidney disease
  • Hypertension
  • Any condition with edema
  • Hyperthyroidism (iodine-containing springs are contraindicated)

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.

Cationsshare of charge (mval%)
  • Sodium1,594mgP91 national
  • Magnesium98.2mgP92 national
  • Calcium157mgP82 national
  • Potassium20.8mgP64 national
  • Iron (II)2.0mgP73 national
  • Manganese0.50mgP66 national
  • Aluminum< —mg
Anionsshare of charge (mval%)
  • Chloride2,164mgP88 national
  • Bicarbonate1,268mgP93 national
  • Sulfate106mgP55 national
  • Nitrate2.8mgP94 national
Non-dissociatedmeasured by mass
  • Metaboric acid202mgP98 national
  • Metasilicic acid59.4mgP35 national
Dissolved gasesmeasured by mass
  • Free carbon dioxide1,094mgP100 national
  • Free hydrogen sulfide< —mg
Trace metalsmeasured by mass
  • Cadmium< —mg
  • Copper< —mg
  • Lead< —mg

Measured at the source

Tonicity
Hypotonic
Free CO₂
1094mg/kg
Very high

On the certificate

  • Neuralgia
  • Muscle pain
  • Joint pain
  • Frozen shoulder
  • Motor paralysis
  • Joint stiffness
  • Bruises
  • Sprains
  • Chronic digestive disorders
  • Hypertension
  • Chronic skin conditions
  • Chronic digestive disorders
  • Chronic constipation

Evidence

  1. 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.
  2. Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
  3. Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
  4. Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
  5. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  6. Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
  7. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  8. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

第五源泉Spring No. 5

鉄Iron-bearingSodium Chloride spring · +Chloride

An iron-rich, hypersaline sodium-chloride spring — chloride above 14,000 mg/kg and total dissolved solids in the 20-g/kg range place it well above ocean salinity, in the 'kyoen-sen' (strong-salt) tier. Hypertonic and neutral, it carries enough iron (around 20 mg/kg) to oxidize reddish-brown on air contact, and bathing leaves a thick mineral coating on the skin.

Good for

Skin conditions

Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.

atopic skin, eczema

On the certificate:Chronic skin conditions

Circulation

Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.

tired legs

On the certificate:HemorrhoidsArteriosclerosisChronic gynecological conditions

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

On the certificate:Cold sensitivity

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

On the certificate:CutsBurns

Recovery

Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.

fatigue, stress

On the certificate:ConvalescenceFatigue recoveryGeneral health improvementFrail children

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

Contraindications
  • Acute illness (especially when fever is present)
  • Active tuberculosis
  • Malignant tumor
  • Severe heart disease
  • Respiratory failure
  • Renal failure
  • Bleeding disorders
  • Severe anemia
  • Any illness in an actively progressing stage
  • Pregnancy (especially the early and late stages)
  • Kidney disease
  • Hypertension
  • Any condition with edema
  • Hyperthyroidism (iodine-containing springs are contraindicated)

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.

Cationsshare of charge (mval%)
  • Sodium9,399mgP99 national
  • Magnesium484mgP99 national
  • Calcium611mgP94 national
  • Potassium152mgP95 national
  • Iron (II)20.1mgP96 national
  • Manganese9.2mgP98 national
  • Aluminum< —mg
  • Hydrogen< —mg
Anionsshare of charge (mval%)
  • Chloride14,450mgP98 national
  • Bicarbonate4,080mgP100 national
Non-dissociatedmeasured by mass
  • Metaboric acid1,317mgP100 national
  • Metasilicic acid114mgP59 national
Dissolved gasesmeasured by mass
  • Free carbon dioxide948mgP99 national
  • Free hydrogen sulfide21.5mgP97 national
Trace metalsmeasured by mass
  • Cadmium< —mg
  • Copper< —mg
  • Lead< —mg

Measured at the source

Tonicity
Hypertonic
Free CO₂
948mg/kg
Typical
Free H₂S
22mg/kg
Very high

On the certificate

  • Neuralgia
  • Muscle pain
  • Joint pain
  • Frozen shoulder
  • Motor paralysis
  • Joint stiffness
  • Bruises
  • Sprains
  • Chronic digestive disorders
  • Anemia
  • Chronic constipation

Evidence

  1. 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.
  2. Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
  3. Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
  4. Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
  5. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  6. Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
  7. Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
  8. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  9. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.
  10. 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

Educational, not medical advice. If you have a health condition, consult a physician before onsen therapy.