After soaking in a hot spring, you might notice your fingertips wrinkling. Conversely, your skin might feel smooth after a bath. What causes this difference? The answer lies in osmotic pressure.
Osmotic pressure is one of the scientific indicators listed in hot spring analysis reports, but it is often overlooked. Understanding osmotic pressure can enhance your appreciation of the benefits of hot springs.
What is Osmotic Pressure?
Osmotic pressure is the force that causes water to move through a semipermeable membrane.
In simpler terms, it indicates how easily water can enter or exit the body.
Human body fluids (such as blood) have a certain osmotic pressure. The effect on the body varies depending on whether the osmotic pressure of the hot spring is the same as, higher, or lower than that of body fluids.
Classification of Hot Springs by Osmotic Pressure
Hot springs can be classified into three categories based on osmotic pressure.
Hypotonic: Lower osmotic pressure than body fluids (less than 8g/kg) Isotonic: Osmotic pressure similar to body fluids (8-10g/kg) Hypertonic: Higher osmotic pressure than body fluids (more than 10g/kg)
The "g/kg" indicates the amount of dissolved substances in 1 kg of hot spring water.
Hypotonic Hot Springs
Hypotonic hot springs have lower osmotic pressure than body fluids. Many Japanese hot springs fall into this category.
Characteristics of Hypotonic Hot Springs
Amount of Dissolved Substances: Less than 8g/kg. The components are diluted.
Effects on the Body: Water penetrates the body. The skin wrinkles.
Representative Spring Qualities: Simple hot springs, some bicarbonate springs.
What Happens When You Enter a Hypotonic Hot Spring?
When soaking in a hypotonic hot spring for a long time, the water from the spring penetrates the skin, causing it to wrinkle.
This is the same phenomenon as when your fingertips wrinkle after a long bath, indicating that water has penetrated the skin.
Benefits of Hypotonic Hot Springs
Moisturizing: Water penetrates the skin, effective for dry skin.
Easily Absorbed: Hot spring components penetrate the skin easily, offering potential beauty benefits.
Soft Skin Feel: Water entering the skin makes it feel soft.
Isotonic Hot Springs
Isotonic hot springs have osmotic pressure similar to body fluids, offering the most balanced experience.
Characteristics of Isotonic Hot Springs
Amount of Dissolved Substances: 8-10g/kg. Similar to body fluids.
Effects on the Body: Minimal water exchange. Little strain on the skin.
Representative Spring Qualities: Some chloride springs, sulfate springs.
What Happens When You Enter an Isotonic Hot Spring?
In isotonic hot springs, the skin does not wrinkle due to minimal water exchange.
Even with prolonged soaking, there is little strain on the skin, making it the safest type of hot spring.
Benefits of Isotonic Hot Springs
Gentle on the Skin: Minimal water exchange means minimal skin irritation.
Prolonged Soaking Possible: Less fatigue, allowing for leisurely soaking.
Well-Balanced: No strain on the body due to similar osmotic pressure to body fluids.
Hypertonic Hot Springs
Hypertonic hot springs have higher osmotic pressure than body fluids, often found in springs with concentrated components.
Characteristics of Hypertonic Hot Springs
Amount of Dissolved Substances: More than 10g/kg. Components are concentrated.
Effects on the Body: Water is drawn out from the body, making the skin smooth.
Representative Spring Qualities: Chloride springs, sulfate springs, some bicarbonate springs.
What Happens When You Enter a Hypertonic Hot Spring?
In hypertonic hot springs, body water is drawn out, resulting in smooth skin.
This is due to the principle of osmotic pressure, where hypertonic liquid (hot spring) draws water from hypotonic liquid (body fluids).
Benefits of Hypertonic Hot Springs
Heat Retention: Hot spring components adhere to the skin, enhancing heat retention and reducing post-bath chills.
Beauty Benefits: Easier removal of dead skin cells, resulting in smooth skin.
Concentrated Components: Rich in hot spring components, offering high efficacy.
Caution with Hypertonic Hot Springs
Due to water loss from the body, be cautious of dehydration when using hypertonic hot springs.
Ensure adequate hydration before and after bathing, and avoid prolonged soaking.
Relationship Between Osmotic Pressure and Spring Quality
Osmotic pressure is closely related to spring quality.
Simple Hot Springs: Mostly hypotonic due to diluted components.
Chloride Springs: Often hypertonic due to high salt content from seawater origins.
Sulfate Springs: Often hypertonic due to abundant sulfates.
Bicarbonate Springs: Vary from hypotonic to hypertonic.
Sulfur Springs: Mostly hypotonic to isotonic.
Choosing Hot Springs by Osmotic Pressure
Knowing osmotic pressure helps you choose the right hot spring for you.
For Dry Skin: Hypotonic hot springs, as water penetrates the skin.
For Sensitive Skin: Isotonic hot springs, as they are gentle on the skin.
For Heat Retention: Hypertonic hot springs, as they reduce post-bath chills.
Check the Hot Spring Analysis Report
Osmotic pressure is listed in the hot spring analysis report.
Look at the "Total Dissolved Substances" section. This value indicates osmotic pressure.
- Less than 8g/kg: Hypotonic
- 8-10g/kg: Isotonic
- More than 10g/kg: Hypertonic
Check the hot spring analysis report before entering a hot spring.
Conclusion
Osmotic pressure significantly affects the body's response to hot springs. Hypotonic springs allow water to enter the body, while hypertonic springs draw water out.
Hypotonic springs cause skin to wrinkle, while hypertonic springs make it smooth. Isotonic springs offer the most balanced experience.
Understanding osmotic pressure helps you choose the right hot spring. Check the "Total Dissolved Substances" in the hot spring analysis report to confirm osmotic pressure. Enjoying hot springs scientifically is part of Japanese onsen culture.
