Why do sulfur springs smell like rotten eggs and turn cloudy with yukibana? Learn how hydrogen sulfide creates the odor, the two spring types, silver discoloration, and gas safety.
Published: Dec 24, 2025
Why do sulfur springs smell like rotten eggs and turn cloudy with yukibana? Learn how hydrogen sulfide creates the odor, the two spring types, silver discoloration, and gas safety.
Published: Dec 24, 2025
The “rotten egg” smell of sulfur springs comes from sulfur compounds in the water and gas, especially hydrogen sulfide. Many people feel it has the “classic hot spring” scent, while others find it overpowering and dislike it. Both reactions are natural.
In short, this smell and the milky water are the clearest signs of a sulfur spring’s character. At the same time, hydrogen sulfide can be dangerous in poorly ventilated places, and it can also turn silver accessories black. This article explains the source of the smell and color, the difference between two types, and safety precautions based on Ministry of the Environment standards. For an overview of all 10 spring types, see the Hot Spring Spring Types Guide; here, we focus on sulfur springs.
This article provides general information. Always follow warnings about restricted areas and fumarolic zones. If you feel unwell, leave the area immediately and do not push yourself.
According to the Ministry of the Environment’s “Guidelines for Mineral Spring Analysis,” a sulfur spring is defined as a therapeutic spring containing 2 mg or more of total sulfur per kilogram of hot spring water. If a bath facility displays “sulfur spring” or “sulfur-containing spring,” the water meets this standard.
Sulfur springs are broadly divided into two types depending on whether they contain hydrogen sulfide. The smell and color differ by type, so it helps to know the distinction first.
| Type | Main characteristics | Smell and color tendency | Water quality tendency |
|---|---|---|---|
| Simple sulfur type (sulfur spring) | Contains very little hydrogen sulfide | Mild odor | Often alkaline |
| Hydrogen sulfide type (hydrogen sulfide-bearing spring) | Contains a lot of hydrogen sulfide | Strong egg smell, often cloudy white | Often acidic |
If a sign says “hydrogen sulfide type,” you can expect a strong smell and cloudy white water. Famous sulfur springs include Manza, Takayu, Noboribetsu, Unzen, and Kusatsu, though each source within the same hot spring area can have different ingredients.
What people call a “sulfur smell” is not sulfur itself, but mainly hydrogen sulfide gas. Hydrogen sulfide has a distinct rotten-egg odor, and humans can detect it even at very low concentrations. If you notice the smell before arriving at a hot spring area, that is because of this high sensitivity.
In volcanic regions, hydrogen sulfide generated underground dissolves into spring water or gas and is released into the air as it reaches the surface. On cold, windless days, it disperses less easily, so the smell may seem stronger. Even at the same source, the impression can change greatly depending on how the water flows, the shape of the tub, and ventilation. So you cannot simply assume that a stronger smell means richer minerals or better benefits.
Also, while low concentrations can be detected by smell, hydrogen sulfide can numb the sense of smell at higher concentrations, making it harder to notice. You should not use smell alone as a safety indicator.
Many sulfur springs are nearly colorless and transparent when they first emerge. As the water comes into contact with air and oxidation progresses, dissolved sulfur compounds become tiny particles and precipitate out, scattering light and making the water look white or bluish-white.
The white sediment that collects in tubs or at the outlet, and the film that floats on the surface, is called yunohana. In sulfur springs, this is mainly made up of precipitated sulfur compounds. The amount of cloudiness and yunohana depends on how long the water has been exposed to air and on temperature, so even the same hot spring can look different depending on the time of day or the tub.
In sulfur springs, silver accessories can turn black in a short time. This happens because silver reacts with hydrogen sulfide and forms a black film of silver sulfide on the surface. In sulfur spring areas, hydrogen sulfide is also present in the air, so discoloration can happen even if you are just walking around without entering the bath.
This discoloration is a chemical reaction, so the silver itself is not damaged, but it does require cleaning to restore its original look. Rings, necklaces, earrings, and other silver items are best removed before bathing.
Hydrogen sulfide is the source of the familiar hot spring smell, but at high concentrations it is also harmful to the human body. Because it is heavier than air, it tends to accumulate in poorly ventilated indoor spaces and low-lying areas such as hollows or snow pits.
In normal operating facilities, ventilation and other safety measures are in place, so there is no need to worry excessively during ordinary bathing. In fact, the Hot Springs Act and Ministry of the Environment standards require bath facilities to install ventilation equipment and other measures to prevent poisoning accidents. On the other hand, there have been fatal accidents in the past where hydrogen sulfide accumulated in poorly ventilated outdoor hollows. In spring source areas and fumarolic zones, always follow all restricted-area signs and warnings.
| Situation | Level of caution | Key point |
|---|---|---|
| Normal commercial bathhouse | Low | Ventilation is managed. Leave early if you feel unwell |
| Indoor baths with few windows, small changing rooms | Medium | If it feels stuffy, avoid long bathing and check ventilation |
| Spring source areas, fumarolic zones, hollows | High | Follow restricted-area signs. Do not approach low places |
If you experience a headache, nausea, or dizziness while bathing, prioritize your health over enjoying the minerals, and leave the bath area at once for fresh air. Older adults and people with chronic conditions should also review Precautions Before Entering a Hot Spring.
If you do not like the egg smell, there is no need to force yourself to choose a famous sulfur spring. Japan has many hot springs with milder aromas, such as simple hot springs, bicarbonate springs, and chloride springs. You can compare the characteristics of each spring type in the Hot Spring Spring Types Guide.
Even among sulfur springs, the simple sulfur type tends to have a milder smell than the hydrogen sulfide type. Also, even within the same hot spring area, the source used by each inn and the ventilation of each bath facility differ, so you may find places that are easier to enjoy even in sulfur spring regions. When looking for a specific spring type or area, it helps to narrow your search and compare options from the facility list.
It is mainly hydrogen sulfide gas. It has a distinct rotten-egg odor, and humans can detect it even at very low concentrations. Sulfur itself is nearly odorless; what you smell is a gas component containing sulfur.
When the spring water emerges and comes into contact with air, oxidation causes dissolved sulfur compounds to precipitate as tiny particles, scattering light and making the water look white. The white sediment that collects in tubs or at the outlet is called yunohana.
Not necessarily. Smell intensity depends not only on mineral content, but also on temperature, ventilation, and tub design. What matters most is not how strong the smell is, but whether you can stay comfortable in that environment.
Silver accessories can turn black, so remove them beforehand. Also, if a bathhouse feels stuffy because of poor ventilation, avoid long bathing, and if you feel a headache or dizziness, leave the bath area immediately. It is important not to approach restricted areas such as spring source zones or hollows.
They are often said to be beneficial, but this article does not make any definitive claims. The effects of therapeutic springs are generally considered in the context of repeated use over a period of time, and individual differences are large, so do not expect too much.
The rotten-egg smell and cloudy water of sulfur springs are the clearest signs of a spring’s character, mainly caused by hydrogen sulfide. Sulfur springs include the milder simple sulfur type and the stronger-smelling, cloudier hydrogen sulfide type, which can be roughly identified from signs and analysis sheets.
At the same time, hydrogen sulfide can discolor silver products and become dangerous in poorly ventilated places. There is no need to worry excessively in normal commercial bathhouses, but remove accessories and always follow warnings in spring source areas and hollows. Since preferences for the smell differ naturally, enjoy sulfur springs as part of Japan’s volcanic hot spring culture, based on whether you personally find them comfortable.
The “rotten egg” smell of sulfur springs comes from sulfur compounds in the water and gas, especially hydrogen sulfide. Many people feel it has the “classic hot spring” scent, while others find it overpowering and dislike it. Both reactions are natural.
In short, this smell and the milky water are the clearest signs of a sulfur spring’s character. At the same time, hydrogen sulfide can be dangerous in poorly ventilated places, and it can also turn silver accessories black. This article explains the source of the smell and color, the difference between two types, and safety precautions based on Ministry of the Environment standards. For an overview of all 10 spring types, see the Hot Spring Spring Types Guide; here, we focus on sulfur springs.
This article provides general information. Always follow warnings about restricted areas and fumarolic zones. If you feel unwell, leave the area immediately and do not push yourself.
According to the Ministry of the Environment’s “Guidelines for Mineral Spring Analysis,” a sulfur spring is defined as a therapeutic spring containing 2 mg or more of total sulfur per kilogram of hot spring water. If a bath facility displays “sulfur spring” or “sulfur-containing spring,” the water meets this standard.
Sulfur springs are broadly divided into two types depending on whether they contain hydrogen sulfide. The smell and color differ by type, so it helps to know the distinction first.
| Type | Main characteristics | Smell and color tendency | Water quality tendency |
|---|---|---|---|
| Simple sulfur type (sulfur spring) | Contains very little hydrogen sulfide | Mild odor | Often alkaline |
| Hydrogen sulfide type (hydrogen sulfide-bearing spring) | Contains a lot of hydrogen sulfide | Strong egg smell, often cloudy white | Often acidic |
If a sign says “hydrogen sulfide type,” you can expect a strong smell and cloudy white water. Famous sulfur springs include Manza, Takayu, Noboribetsu, Unzen, and Kusatsu, though each source within the same hot spring area can have different ingredients.
What people call a “sulfur smell” is not sulfur itself, but mainly hydrogen sulfide gas. Hydrogen sulfide has a distinct rotten-egg odor, and humans can detect it even at very low concentrations. If you notice the smell before arriving at a hot spring area, that is because of this high sensitivity.
In volcanic regions, hydrogen sulfide generated underground dissolves into spring water or gas and is released into the air as it reaches the surface. On cold, windless days, it disperses less easily, so the smell may seem stronger. Even at the same source, the impression can change greatly depending on how the water flows, the shape of the tub, and ventilation. So you cannot simply assume that a stronger smell means richer minerals or better benefits.
Also, while low concentrations can be detected by smell, hydrogen sulfide can numb the sense of smell at higher concentrations, making it harder to notice. You should not use smell alone as a safety indicator.
Many sulfur springs are nearly colorless and transparent when they first emerge. As the water comes into contact with air and oxidation progresses, dissolved sulfur compounds become tiny particles and precipitate out, scattering light and making the water look white or bluish-white.
The white sediment that collects in tubs or at the outlet, and the film that floats on the surface, is called yunohana. In sulfur springs, this is mainly made up of precipitated sulfur compounds. The amount of cloudiness and yunohana depends on how long the water has been exposed to air and on temperature, so even the same hot spring can look different depending on the time of day or the tub.
In sulfur springs, silver accessories can turn black in a short time. This happens because silver reacts with hydrogen sulfide and forms a black film of silver sulfide on the surface. In sulfur spring areas, hydrogen sulfide is also present in the air, so discoloration can happen even if you are just walking around without entering the bath.
This discoloration is a chemical reaction, so the silver itself is not damaged, but it does require cleaning to restore its original look. Rings, necklaces, earrings, and other silver items are best removed before bathing.
Hydrogen sulfide is the source of the familiar hot spring smell, but at high concentrations it is also harmful to the human body. Because it is heavier than air, it tends to accumulate in poorly ventilated indoor spaces and low-lying areas such as hollows or snow pits.
In normal operating facilities, ventilation and other safety measures are in place, so there is no need to worry excessively during ordinary bathing. In fact, the Hot Springs Act and Ministry of the Environment standards require bath facilities to install ventilation equipment and other measures to prevent poisoning accidents. On the other hand, there have been fatal accidents in the past where hydrogen sulfide accumulated in poorly ventilated outdoor hollows. In spring source areas and fumarolic zones, always follow all restricted-area signs and warnings.
| Situation | Level of caution | Key point |
|---|---|---|
| Normal commercial bathhouse | Low | Ventilation is managed. Leave early if you feel unwell |
| Indoor baths with few windows, small changing rooms | Medium | If it feels stuffy, avoid long bathing and check ventilation |
| Spring source areas, fumarolic zones, hollows | High | Follow restricted-area signs. Do not approach low places |
If you experience a headache, nausea, or dizziness while bathing, prioritize your health over enjoying the minerals, and leave the bath area at once for fresh air. Older adults and people with chronic conditions should also review Precautions Before Entering a Hot Spring.
If you do not like the egg smell, there is no need to force yourself to choose a famous sulfur spring. Japan has many hot springs with milder aromas, such as simple hot springs, bicarbonate springs, and chloride springs. You can compare the characteristics of each spring type in the Hot Spring Spring Types Guide.
Even among sulfur springs, the simple sulfur type tends to have a milder smell than the hydrogen sulfide type. Also, even within the same hot spring area, the source used by each inn and the ventilation of each bath facility differ, so you may find places that are easier to enjoy even in sulfur spring regions. When looking for a specific spring type or area, it helps to narrow your search and compare options from the facility list.
It is mainly hydrogen sulfide gas. It has a distinct rotten-egg odor, and humans can detect it even at very low concentrations. Sulfur itself is nearly odorless; what you smell is a gas component containing sulfur.
When the spring water emerges and comes into contact with air, oxidation causes dissolved sulfur compounds to precipitate as tiny particles, scattering light and making the water look white. The white sediment that collects in tubs or at the outlet is called yunohana.
Not necessarily. Smell intensity depends not only on mineral content, but also on temperature, ventilation, and tub design. What matters most is not how strong the smell is, but whether you can stay comfortable in that environment.
Silver accessories can turn black, so remove them beforehand. Also, if a bathhouse feels stuffy because of poor ventilation, avoid long bathing, and if you feel a headache or dizziness, leave the bath area immediately. It is important not to approach restricted areas such as spring source zones or hollows.
They are often said to be beneficial, but this article does not make any definitive claims. The effects of therapeutic springs are generally considered in the context of repeated use over a period of time, and individual differences are large, so do not expect too much.
The rotten-egg smell and cloudy water of sulfur springs are the clearest signs of a spring’s character, mainly caused by hydrogen sulfide. Sulfur springs include the milder simple sulfur type and the stronger-smelling, cloudier hydrogen sulfide type, which can be roughly identified from signs and analysis sheets.
At the same time, hydrogen sulfide can discolor silver products and become dangerous in poorly ventilated places. There is no need to worry excessively in normal commercial bathhouses, but remove accessories and always follow warnings in spring source areas and hollows. Since preferences for the smell differ naturally, enjoy sulfur springs as part of Japan’s volcanic hot spring culture, based on whether you personally find them comfortable.