A hot spring with a metallic smell is one that gives off an iron-like scent, or an odor reminiscent of blood or rust. In Japan, this may be described as kanake-shu. Such a smell is often noticeable in iron-rich springs, but odor alone cannot determine the spring type.

In short, kanake-shu is linked to the oxidation of iron as it comes into contact with air, and it serves as a clue to the character of an iron-rich spring from a different angle than color. On the other hand, it is completely different from the sulfur odor often discussed in hot springs, and the two are easy to confuse. This article explains the source of the metallic smell, why it is noticeable, how it differs from sulfur odor, what can and cannot be learned from smell, and how people who dislike it can choose a spring, all based on the Ministry of the Environment's standards. Please read with the understanding that odor perception varies greatly from person to person.

This article provides general information and does not guarantee any specific therapeutic or health benefits. Drinking the spring water is only allowed at facilities that are licensed for it. If you have a chronic condition or are feeling unwell, do not push yourself and follow the guidance of a doctor or the facility.

What is a metallic smell (kanake-shu)?

A metallic smell does not mean the metal itself has a strong fragrance. It usually refers to a distinctive iron-like or rust-like odor that appears when water containing iron changes after coming into contact with air. Some people describe it as the impression of licking blood, or the smell on their hands after holding an old iron nail or coin.

This smell is usually not so strong that you can notice it from far away before arriving at a hot spring area, as is often the case with sulfur odor. More often, you only begin to notice it near the edge of the bath or close to the outlet where the water enters. The strength of the smell changes not only with the amount of iron, but also with the freshness of the water, how much it has been exposed to air, and the ventilation in the bath area. Even with the same source, freshly drawn water and water that has been exposed to air for a while can feel quite different.

Kanake-shu often appears together with reddish-brown cloudiness or red-brown deposits around the outlet and bath edges. Because color and odor provide clues at the same time, observing them together makes it easier to understand the character of the spring. The mechanism behind the color itself is covered in detail in Brown and Reddish-Brown Hot Springs.

Why does it smell metallic? The background of iron oxidation

The background of kanake-shu is the phenomenon of iron oxidation. Water rich in iron dissolves underground under high-temperature, high-pressure conditions in the form of ferrous iron (iron II / Fe²⁺), which dissolves more easily in water. At this stage, the water is often nearly colorless and odor is usually mild.

When the water emerges at the surface and comes into contact with air (oxygen), ferrous iron oxidizes into the more stable ferric iron (iron III / Fe³⁺). Ferric iron is less soluble in water, so it precipitates as tiny particles such as iron hydroxide, creating reddish-brown cloudiness and sediment. Kanake-shu is easier to understand if you see it as the smell associated with this process of iron changing state through oxidation. That is why it is more noticeable near the outlet or beside water that has begun to change color after contact with air.

However, odor perception varies greatly from person to person. Some people smell iron, while others hardly notice anything at all. For that reason, you cannot conclude that a spring is not iron-rich just because it does not smell metallic. Conversely, even if it does seem metallic, other components or the plumbing and bath environment may also be affecting the smell. The oxidation process of iron and the definition of iron-rich springs are explained in more detail in What Is an Iron-Rich Spring?, so this article focuses on the smell itself.

How it differs from sulfur odor: two distinct scents in different directions

The most famous hot spring smell is sulfur odor, but it is completely different from kanake-shu in both origin and impression. Sulfur odor is often perceived as the smell of rotten eggs, and its main source is hydrogen sulfide gas. Kanake-shu, by contrast, has an impression closer to iron, blood, or rust. Because the two are easy to confuse, the comparison below helps clarify the difference.

The two odors are different, but in practice you may sense both in the same bath. In complex springs that contain both iron and sulfur components, the impressions can blend together. In such cases, it is more accurate to check the spring analysis report and facility notices rather than trying to identify the ingredients from smell alone. The source of sulfur odor itself, along with the mechanism behind cloudy water, mineral deposits, and ventilation precautions, is covered in Hot Springs with a Sulfur Smell.

Which hot springs make you notice kanake-shu more easily?

Kanake-shu is easiest to imagine in iron-rich springs. Especially in types that change color easily after contact with air, the smell impression appears alongside the cloudy color. It is also easy to notice near the outlet or around the edge of a bath where the water has been sitting for some time, because that is where iron oxidation advances.

A famous example is Kin-no-yu at Arima Onsen in Hyogo Prefecture. It is a type of iron-rich sodium-chloride strong saline spring. When it emerges, it is nearly colorless, but after contact with air the iron oxidizes and turns reddish-brown or golden. It is also known for its high salt content, and it is often introduced as a hot spring with a strong identity in both appearance and smell. However, even among iron-rich springs, the smell can vary greatly depending on the other ingredients, how the facility uses the water, and the condition of the bath. The role of famous waters like Kin-no-yu at Arima and other renowned iron-rich springs is summarized in What Is an Iron-Rich Spring?.

It should also be noted that water with a metallic smell often comes with reddish-brown cloudiness, but the color and smell are simply different ways of perceiving the same iron oxidation. A darker color does not always mean a stronger smell.

What you can and cannot learn from smell

What smell can tell you is the possibility that the hot spring contains characteristic components or that changes are occurring as the water contacts air. For people who are newly becoming aware of hot spring differences, smell is a very accessible way to experience the uniqueness of a spring. Even knowing the difference in direction between kanake-shu and sulfur odor makes it easier to organize your impression of the water.

On the other hand, there are many things smell cannot tell you or confirm.

• Strong odor does not mean stronger benefits. Odor intensity depends not only on ingredient levels, but also on freshness, ventilation, and the structure of the bath.
• The presence or absence of smell does not determine the spring type. A spring may be iron-rich even if it does not smell metallic, and a metallic impression may also be mixed with other factors.
• Odor perception varies greatly from person to person. It is natural for the same water to be experienced differently by different people.

If you want to know the spring type accurately, the best method is to check the spring analysis report posted in the changing room or bath area rather than judging by smell alone. If you want an overview of the 10 main spring types, Hot Spring Types for Beginners is also useful.

Notes on drinking spring water and health benefits

In iron-rich springs, drinking the water may be recommended at facilities where it is officially permitted, with reference to oral iron intake. However, it is not appropriate to connect the metallic smell too directly with health benefits. Smell only reflects the oxidation of iron through the sense of smell; it does not indicate the strength of any effect.

It is also unrealistic to think that large amounts of iron are absorbed through the skin during bathing, and bathing and drinking the spring water are completely different uses. Drinking the water should only be done at facilities with designated drinking stations that clearly permit it, and you must follow the recommended amount and frequency. Not all iron-rich springs can be drunk, and it is best not to casually taste source water during sightseeing. Some people may also find iron-rich water difficult to drink. For more detail on drinking the water and the idea of indications, see What Is an Iron-Rich Spring?, and for general precautions before bathing, also check Precautions Before Entering a Hot Spring.

How to choose if you dislike metallic smells

If you dislike kanake-shu, there is no need to choose a strongly distinctive iron-rich spring. You can simply choose water with a milder fragrance. For example, simple springs and bicarbonate springs often have a less assertive, gentler impression. When choosing a hot spring, you do not need to base your decision on fame alone; it is fine to use your own sense of comfort as the standard.

Conversely, for people who enjoy the character of a hot spring, including its smell, iron-rich springs are highly memorable. Observing the reddish-brown cloudiness, the deposits around the outlet, and kanake-shu gives a three-dimensional sense of how different they are from other spring types. Comparing them with sulfur springs, which have a very different scent direction, makes each type's character even clearer. You can search for springs and facilities in the region that interest you from the Facility List.

Frequently asked questions

Does a hot spring with a metallic smell contain a lot of iron?

It is true that kanake-shu is easier to notice in iron-rich springs, but you cannot determine the amount of iron from smell alone. The strength of the odor depends not only on the amount of iron, but also on the freshness of the water, how much it has contacted air, and the ventilation of the bath area. If you want to know accurately, it is best to check the posted spring analysis report.

How is metallic smell different from sulfur odor?

They differ in both origin and impression. Sulfur odor is a rotten-egg-like smell, mainly caused by sulfur components such as hydrogen sulfide. Metallic smell, or kanake-shu, has an impression close to iron, blood, or rust, and is associated with the oxidation of iron. The spring types where each is commonly noticed also differ: sulfur odor in sulfur springs, metallic smell in iron-rich springs. However, both may sometimes be sensed in the same bath.

Is a metallic smell a sign that the water is old?

It is less about age and more about the state in which iron has been exposed to air and oxidized. Freshly drawn water is often dissolved as ferrous iron, so smell and color may be mild at first. As the water is exposed to air for longer, oxidation progresses and kanake-shu becomes more noticeable along with reddish-brown cloudiness. That is why it is easy to notice near the outlet.

Does a stronger smell mean stronger benefits?

Not necessarily. Odor strength depends not only on ingredient levels, but also on freshness, ventilation, and the structure of the bath. Color and smell are simply entry points for understanding the hot spring; they do not indicate the strength of any benefits. If you want to know the spring's type or properties, check the facility notice or the spring analysis report rather than relying on appearance or smell.

I dislike metallic smells. Are there easier springs to enter?

Choose a spring type with a gentler scent. Simple springs and bicarbonate springs often have a mild, less distinctive impression. Even among iron-rich springs, the smell can vary depending on the facility and the condition of the water, so you may be able to choose one that is relatively easy to enjoy. It is perfectly fine to base your choice on what suits your own senses.

Summary

A hot spring with a metallic smell is one of the characteristic features often noticed in iron-rich springs. In Japan, it may be called kanake-shu. It is an iron-like odor, or one reminiscent of blood or rust, and it is linked to the process in which iron oxidizes from ferrous iron to ferric iron after contact with air. It is completely different from the egg-like sulfur odor, though both may sometimes be sensed in the same bath.

For travelers, it is enough to remember that kanake-shu is different from sulfur odor, that you cannot determine the spring type or benefits from smell alone, and that odor perception varies greatly from person to person. Enjoy the smell as an entry point to the unique character of a hot spring, while confirming the exact spring type through the analysis report or on-site information. Then you can safely appreciate the metallic smell as another clue, separate from color.

Sources

• Ministry of the Environment, "Guidelines for Mineral Spring Analysis" (Revised in 2014)
• Ministry of the Environment, "Protection and Use of Hot Springs: Definition and Classification of Therapeutic Springs"
• Japan Spa Association