Japan is home to an incredible variety of hot springs, each offering unique mineral compositions and therapeutic benefits. While all onsen feature naturally heated water from underground, the dissolved minerals present in the water can influence its appearance, scent, and healing properties. Combined with the different types of baths and scenic surroundings, this creates a nearly endless array of ways to experience and enjoy hot springs.
What Defines an Onsen?
Under Japan’s hot spring law, an onsen is defined as water, steam, or gas that emerges from the earth at a temperature of at least 25 degrees Celsius (77 degrees Fahrenheit) or contains a minimum concentration of specific minerals.
Types of Hot Spring Sources
Traditionally, hot springs were naturally occurring where geothermal water bubbled up from the earth. However, modern technology has enabled the drilling of deep wells to extract hot spring water, leading to the rise of onsen facilities even in urban centers where natural hot spring sources are scarce. Generally, hot spring sources can be categorized into three types:
Natural springs – Water naturally flows from the ground without any artificial intervention.
Dug or bored wells – Hot spring water is extracted from underground but does not require pumping.
Pumped sources – Water is drawn from deep underground using pumps to bring it to the surface.
Natural onsen spring of Yubara Onsen in Japan
Hot Spring Water Types
Below are some of the most common types of hot spring waters found in Japan, each with distinct characteristics and health benefits:
Simple – These onsen contain naturally warm water (minimum 25°C) but lack sufficient mineral content to fall into other categories. They are gentle on the skin and beneficial for relieving fatigue, nerve pain, insomnia, and high blood pressure.
Chloride – Similar to seawater, chloride-rich hot springs help the body retain heat and are effective for treating muscle pain, joint pain, skin conditions, and burns.
Sulfur – Recognizable by its strong rotten egg-like odor, sulfurous water promotes blood circulation and skin health. It is beneficial for acne, skin spots, diabetes, high blood pressure, and joint pain, but prolonged exposure can be stimulating.
Sulphate – Containing calcium, sodium, or magnesium sulfates, these waters have a bitter taste but are valued for their healing effects on skin ailments, cuts, bruises, and constipation.
Carbonated – Known for their skin-smoothing properties, carbonated hot springs create a silky effect on the skin and are used to treat various dermatological issues.
Iron – Initially clear, iron-rich waters turn brown upon exposure to oxygen. These onsen help the body retain warmth and are beneficial for anemia and menstrual disorders.
Radioactive – Containing small amounts of radium or radon, these waters are believed to aid in lowering blood pressure and treating circulatory diseases, rheumatism, and gout.
Acidic – Found in highland regions, these hot springs have strong sterilizing properties, making them effective for chronic skin diseases. However, those with sensitive skin should limit exposure.
Carbon Dioxide – These rare hot springs contain dissolved CO2 gas, creating a natural effervescence. They help improve circulation, keep the body warm, and lower blood pressure.
Brown iron water of Arima Onsen in Japan
Onsen Water Treatments
For onsen purists, the ideal hot spring experience is bathing in untreated water that flows directly from the source. However, various treatments are often applied to ensure comfort and maintain water quality:
Untreated Water (Gensen Kakenagashi) – This refers to onsen where the water flows directly from the source into the bath without alterations, offering the most authentic experience.
Cooling the Water – Some hot springs emerge at near-boiling temperatures and require cooling before they can be used. Natural cooling methods include cascading water through fountains, waterfalls, or open-air channels.
Heating the Water – In cases where the water temperature is too low for bathing, it can be artificially heated, preferably without dilution.
Adding Water – Water may be added to adjust temperature, increase volume, or dilute excessively acidic springs.
Recirculating Water – Some facilities, particularly those with limited spring water supply, filter and recirculate their bathwater. While practical, this is not considered the highest-quality onsen experience.
Adding Chlorine – To maintain hygiene, particularly in modern and urban facilities, chlorine is often added to the water. However, this can affect the natural feel and scent of the onsen.
