What is an iron spring? Learn the Ministry of the Environment standard of 20 mg/kg or more of total iron ions, why clear water turns reddish brown by oxidation, famous baths like Arima, Nagayu, and Ikaho, and the difference between bathing and drinking it.
Published: Dec 24, 2025
What is an iron spring? Learn the Ministry of the Environment standard of 20 mg/kg or more of total iron ions, why clear water turns reddish brown by oxidation, famous baths like Arima, Nagayu, and Ikaho, and the difference between bathing and drinking it.
Published: Dec 24, 2025
An iron spring is one of the 10 types of therapeutic hot springs, with a spring quality that contains a certain amount of iron in the water. It is also called an "iron spring." Known for its reddish brown or yellow-brown cloudy water, it often surprises people who are used to clear baths because of its striking appearance.
In short, this color is created when iron is exposed to air and oxidizes, and it is the most obvious sign of an iron spring. On the other hand, the common idea that "because it contains iron, it helps anemia" is too simplistic, and bathing and drinking the water have different meanings. This article explains the definition of iron springs, the mechanism behind the reddish brown color, representative famous baths, and precautions for bathing, based on the Ministry of the Environment's standards. For the full picture of the 10 spring types, see the Hot Spring Spring Quality Guide; here we focus only on iron springs.
This article is general information and does not guarantee any specific treatment or health benefits. Drinking the water is only allowed at facilities with permission. If you have a chronic condition or are not feeling well, do not push yourself and follow the guidance of a doctor or the facility.
Whether a spring is classified as an iron spring is determined by the amount of iron dissolved in the water. In the Ministry of the Environment's "Guidelines for Analysis of Mineral Springs," iron springs that carry a therapeutic hot spring classification are defined as hot springs containing 20 mg or more of total iron ions (the sum of ferrous and ferric ions) per 1 kg of hot spring water. If a bathhouse displays "iron spring" or "iron-... spring," that water meets this standard.
Here, "ferrous" and "ferric" refer to different states of iron ions. When dissolved in underground water, iron is mainly in the form of ferrous iron (Fe²⁺), which dissolves easily in water. When it rises to the surface and comes into contact with air, it oxidizes into ferric iron (Fe³⁺), as explained below. The standard for iron springs is based on the combined amount of both.
Also, hot springs that contain a lot of iron differ in character depending on the anion they are combined with. Some are strongly salty chloride-spring types, while others are combined with bicarbonate or sulfate ions, so even within iron springs there is a wide range of sensations and colors.
The color of an iron spring is created by oxidation of iron. Iron dissolved in water under the underground conditions of high heat and pressure is in the form of soluble ferrous iron (Fe²⁺), and at this stage the water is often almost colorless and clear. This is why spring water can look transparent at the source but turn brown and cloudy in the bath.
When the water comes to the surface and is exposed to air (oxygen), ferrous iron oxidizes into the more stable ferric iron (Fe³⁺). Ferric iron is less soluble in water, so it combines with water and precipitates as tiny particles such as iron hydroxide. These reddish brown to yellow-brown particles give the water its brown color and cloudiness. In other words, the color of an iron spring becomes deeper over time after it is exposed to air.
For this reason, the same source does not always look the same. Freshly drawn clear water, water that has just begun to color after exposure to air, and water that has been left longer and become darker and cloudier all give different impressions. It is easier to understand if you think of it not as "brown because it is old," but as a state in which iron oxidation has progressed. To better understand how spring water changes as it rises to the surface, also read How Do Hot Springs Emerge?.
Iron springs can be found across Japan, but if we list some well-known and distinctive famous baths, they are as follows. Even under the same label of "iron spring," the combined ingredients and colors can vary greatly.
| Hot spring area | Location | Features |
|---|---|---|
| Arima Onsen "Kinsen" | Hyogo Prefecture | Iron-rich sodium-chloride strong saline spring. It is almost colorless when it emerges, but turns reddish brown, or golden, as iron oxidizes in the air. It is also known for its high salt content |
| Nagayu Onsen | Oita Prefecture | Long introduced as an "iron-carbonated spring" because it contains a lot of carbon dioxide and also iron. You can enjoy carbon dioxide bubbles and the color of iron at the same time |
| Ikaho Onsen "Kogane no Yu" | Gunma Prefecture | Contains iron; it is almost colorless when it emerges, but turns brownish after exposure to air. A cloudy bath that has been loved for a long time |
The spring qualities and colors listed here are only examples, and even within the same hot spring area, the composition differs from source to source. For example, Arima has not only the iron-rich "Kinsen" but also the colorless "Ginsen," and Ikaho also has other sources with different colors. The most reliable way is to check the posted spring quality name or the hot spring analysis sheet to see which type the water belongs to.
At iron springs, drinking the water may be discussed in addition to bathing. Because it contains iron, some facilities that allow drinking may mention it in connection with "iron-deficiency anemia," but this is only within the general range of what is said about therapeutic hot springs, and this article does not claim any effect. The indications for therapeutic hot springs are based on repeated use over a certain period, and individual differences are large.
What is important is that bathing and drinking the water are completely different uses. It is unrealistic to think that anemia will improve by absorbing iron through the skin while bathing. Do not confuse oral intake of iron-rich water with soaking in the bath.
If you want to drink the water, you must always confirm whether drinking is permitted at that facility and whether a guideline for the amount is provided. Not all iron springs can be drunk, and it is not appropriate to drink the source water on your own judgment. Some people may find iron-rich water hard to drink. For general ideas about benefits and precautions if you have an existing condition, also check Precautions Before Entering a Hot Spring.
In iron springs, water containing iron may transfer color to towels and light-colored clothing. If you can use the facility's provided towel, prioritize that, and when using your own white towel or swimsuit, keep in mind that it may stain.
Also, oxidized iron may settle on the edge of the bathtub, the floor, or around the outlet, making surfaces slippery. This is a spring quality that draws the eye with its dramatic color, but when getting in and out, pay attention to your footing. Cloudy baths make the bottom and steps hard to see, so it is safest to move slowly.
In addition, strong saline springs common among iron springs, or water that also contains carbon dioxide, may warm the body quickly or have concentrated components, so it is better to start with a short bath rather than staying in too long at first. For trends by spring type and comparisons with other distinctive waters, The Smell and Color of Sulfur Springs and Radioactive Springs (Radon Springs): Standards and Safety are also helpful. You can search for facilities with iron springs in the facility list.
Because the iron dissolved in the water is exposed to air (oxygen) and oxidizes. Underground, it is often in the form of ferrous iron that dissolves easily in water and is nearly colorless, but at the surface it oxidizes into ferric iron and precipitates as tiny particles such as iron hydroxide. These particles color the water reddish brown to yellow-brown.
Right after emerging, oxidation has not progressed, so the water is nearly clear. The longer it is exposed to air, the more the iron oxidizes and the deeper the color becomes. The depth of color indicates not age, but how far oxidation has progressed after exposure to air.
It is not realistic to think that anemia improves by absorbing iron through the skin while bathing. Iron-deficiency anemia is sometimes mentioned as an indication for drinking the water (oral intake), but this article does not claim any effect. Drinking is only allowed at facilities with permission, and depending on your constitution or medical history, it may not be suitable.
In iron-rich water, color may transfer to pale towels or clothing. Using the facility's towel or bringing items you do not mind staining is reassuring. Since iron may also deposit on bathtub edges and make them slippery, please watch your footing.
Reddish brown or yellow-brown cloudy water is a sign of an iron spring, but freshly drawn water may be nearly clear, so appearance alone is not enough. The most reliable method is to check the spring quality name on the analysis sheet posted in the changing room or bath area.
An iron spring is a therapeutic hot spring containing 20 mg/kg or more of total iron ions (Fe²⁺ + Fe³⁺), and it is a spring quality whose reddish brown or yellow-brown cloudy water is easy to recognize. This color is created when iron, dissolved underground as ferrous iron, comes into contact with air at the surface, oxidizes into ferric iron, and precipitates as particles such as iron hydroxide. Famous baths with different combinations of components can be found throughout Japan, such as Arima's Kinsen, Nagayu, and Ikaho's Kogane no Yu.
As a traveler, it is enough to understand why it turns reddish brown, that bathing and drinking are separate things, and the need to be careful about staining and footing. Without making strong claims about benefits, and without being overly cautious, one of the charms of an iron spring is simply being able to enjoy the color of the water itself. For the full picture of each spring type, please see the Hot Spring Spring Quality Guide.
An iron spring is one of the 10 types of therapeutic hot springs, with a spring quality that contains a certain amount of iron in the water. It is also called an "iron spring." Known for its reddish brown or yellow-brown cloudy water, it often surprises people who are used to clear baths because of its striking appearance.
In short, this color is created when iron is exposed to air and oxidizes, and it is the most obvious sign of an iron spring. On the other hand, the common idea that "because it contains iron, it helps anemia" is too simplistic, and bathing and drinking the water have different meanings. This article explains the definition of iron springs, the mechanism behind the reddish brown color, representative famous baths, and precautions for bathing, based on the Ministry of the Environment's standards. For the full picture of the 10 spring types, see the Hot Spring Spring Quality Guide; here we focus only on iron springs.
This article is general information and does not guarantee any specific treatment or health benefits. Drinking the water is only allowed at facilities with permission. If you have a chronic condition or are not feeling well, do not push yourself and follow the guidance of a doctor or the facility.
Whether a spring is classified as an iron spring is determined by the amount of iron dissolved in the water. In the Ministry of the Environment's "Guidelines for Analysis of Mineral Springs," iron springs that carry a therapeutic hot spring classification are defined as hot springs containing 20 mg or more of total iron ions (the sum of ferrous and ferric ions) per 1 kg of hot spring water. If a bathhouse displays "iron spring" or "iron-... spring," that water meets this standard.
Here, "ferrous" and "ferric" refer to different states of iron ions. When dissolved in underground water, iron is mainly in the form of ferrous iron (Fe²⁺), which dissolves easily in water. When it rises to the surface and comes into contact with air, it oxidizes into ferric iron (Fe³⁺), as explained below. The standard for iron springs is based on the combined amount of both.
Also, hot springs that contain a lot of iron differ in character depending on the anion they are combined with. Some are strongly salty chloride-spring types, while others are combined with bicarbonate or sulfate ions, so even within iron springs there is a wide range of sensations and colors.
The color of an iron spring is created by oxidation of iron. Iron dissolved in water under the underground conditions of high heat and pressure is in the form of soluble ferrous iron (Fe²⁺), and at this stage the water is often almost colorless and clear. This is why spring water can look transparent at the source but turn brown and cloudy in the bath.
When the water comes to the surface and is exposed to air (oxygen), ferrous iron oxidizes into the more stable ferric iron (Fe³⁺). Ferric iron is less soluble in water, so it combines with water and precipitates as tiny particles such as iron hydroxide. These reddish brown to yellow-brown particles give the water its brown color and cloudiness. In other words, the color of an iron spring becomes deeper over time after it is exposed to air.
For this reason, the same source does not always look the same. Freshly drawn clear water, water that has just begun to color after exposure to air, and water that has been left longer and become darker and cloudier all give different impressions. It is easier to understand if you think of it not as "brown because it is old," but as a state in which iron oxidation has progressed. To better understand how spring water changes as it rises to the surface, also read How Do Hot Springs Emerge?.
Iron springs can be found across Japan, but if we list some well-known and distinctive famous baths, they are as follows. Even under the same label of "iron spring," the combined ingredients and colors can vary greatly.
| Hot spring area | Location | Features |
|---|---|---|
| Arima Onsen "Kinsen" | Hyogo Prefecture | Iron-rich sodium-chloride strong saline spring. It is almost colorless when it emerges, but turns reddish brown, or golden, as iron oxidizes in the air. It is also known for its high salt content |
| Nagayu Onsen | Oita Prefecture | Long introduced as an "iron-carbonated spring" because it contains a lot of carbon dioxide and also iron. You can enjoy carbon dioxide bubbles and the color of iron at the same time |
| Ikaho Onsen "Kogane no Yu" | Gunma Prefecture | Contains iron; it is almost colorless when it emerges, but turns brownish after exposure to air. A cloudy bath that has been loved for a long time |
The spring qualities and colors listed here are only examples, and even within the same hot spring area, the composition differs from source to source. For example, Arima has not only the iron-rich "Kinsen" but also the colorless "Ginsen," and Ikaho also has other sources with different colors. The most reliable way is to check the posted spring quality name or the hot spring analysis sheet to see which type the water belongs to.
At iron springs, drinking the water may be discussed in addition to bathing. Because it contains iron, some facilities that allow drinking may mention it in connection with "iron-deficiency anemia," but this is only within the general range of what is said about therapeutic hot springs, and this article does not claim any effect. The indications for therapeutic hot springs are based on repeated use over a certain period, and individual differences are large.
What is important is that bathing and drinking the water are completely different uses. It is unrealistic to think that anemia will improve by absorbing iron through the skin while bathing. Do not confuse oral intake of iron-rich water with soaking in the bath.
If you want to drink the water, you must always confirm whether drinking is permitted at that facility and whether a guideline for the amount is provided. Not all iron springs can be drunk, and it is not appropriate to drink the source water on your own judgment. Some people may find iron-rich water hard to drink. For general ideas about benefits and precautions if you have an existing condition, also check Precautions Before Entering a Hot Spring.
In iron springs, water containing iron may transfer color to towels and light-colored clothing. If you can use the facility's provided towel, prioritize that, and when using your own white towel or swimsuit, keep in mind that it may stain.
Also, oxidized iron may settle on the edge of the bathtub, the floor, or around the outlet, making surfaces slippery. This is a spring quality that draws the eye with its dramatic color, but when getting in and out, pay attention to your footing. Cloudy baths make the bottom and steps hard to see, so it is safest to move slowly.
In addition, strong saline springs common among iron springs, or water that also contains carbon dioxide, may warm the body quickly or have concentrated components, so it is better to start with a short bath rather than staying in too long at first. For trends by spring type and comparisons with other distinctive waters, The Smell and Color of Sulfur Springs and Radioactive Springs (Radon Springs): Standards and Safety are also helpful. You can search for facilities with iron springs in the facility list.
Because the iron dissolved in the water is exposed to air (oxygen) and oxidizes. Underground, it is often in the form of ferrous iron that dissolves easily in water and is nearly colorless, but at the surface it oxidizes into ferric iron and precipitates as tiny particles such as iron hydroxide. These particles color the water reddish brown to yellow-brown.
Right after emerging, oxidation has not progressed, so the water is nearly clear. The longer it is exposed to air, the more the iron oxidizes and the deeper the color becomes. The depth of color indicates not age, but how far oxidation has progressed after exposure to air.
It is not realistic to think that anemia improves by absorbing iron through the skin while bathing. Iron-deficiency anemia is sometimes mentioned as an indication for drinking the water (oral intake), but this article does not claim any effect. Drinking is only allowed at facilities with permission, and depending on your constitution or medical history, it may not be suitable.
In iron-rich water, color may transfer to pale towels or clothing. Using the facility's towel or bringing items you do not mind staining is reassuring. Since iron may also deposit on bathtub edges and make them slippery, please watch your footing.
Reddish brown or yellow-brown cloudy water is a sign of an iron spring, but freshly drawn water may be nearly clear, so appearance alone is not enough. The most reliable method is to check the spring quality name on the analysis sheet posted in the changing room or bath area.
An iron spring is a therapeutic hot spring containing 20 mg/kg or more of total iron ions (Fe²⁺ + Fe³⁺), and it is a spring quality whose reddish brown or yellow-brown cloudy water is easy to recognize. This color is created when iron, dissolved underground as ferrous iron, comes into contact with air at the surface, oxidizes into ferric iron, and precipitates as particles such as iron hydroxide. Famous baths with different combinations of components can be found throughout Japan, such as Arima's Kinsen, Nagayu, and Ikaho's Kogane no Yu.
As a traveler, it is enough to understand why it turns reddish brown, that bathing and drinking are separate things, and the need to be careful about staining and footing. Without making strong claims about benefits, and without being overly cautious, one of the charms of an iron spring is simply being able to enjoy the color of the water itself. For the full picture of each spring type, please see the Hot Spring Spring Quality Guide.