Why Do Hot Springs Smell Bad? Hydrogen Sulfide is to Blame

By Published On: September 14, 2020Last Updated: November 23, 2022

Hot springs are popular throughout the world, especially in colder climates. There’s nothing like soaking your body in steaming hot water surrounded by picturesque views of snow-capped mountains. But hang on. Why is there an odor of rotten eggs coming from the water? Hydrogen sulfide is the culprit, thanks to sulfur-containing rocks deep underground. We look at how this molecule is formed and whether it poses any health risks.

Hot springs smell bad because of hydrogen sulfide, a gas produced from sulfur-containing minerals deep within the Earth. The gas dissolves well in hot water found deep underground. As this water rises to the surface of hot springs, it cools down. Hydrogen sulfide doesn’t dissolve as well in colder water and escapes, giving us the smell of rotten eggs.

How Hot Springs Get Their Heat

Deep within the Earth, the extreme density and pressure mean its core can reach temperatures of over 5000 °C. Luckily for us, hot springs—loosely defined as a natural source of water with a higher temperature than its surroundings—don’t usually reach those kinds of temperatures, or we wouldn’t be able to enjoy a nice soak in them.

Most hot springs get their heat from the radioactive decay of certain elements in the ground. Unstable isotopes like potassium-40, thorium-232 and uranium-238 decay over time, releasing heat into the rocks around them. Underground sources of water come into contact with these hot rocks and make their way to the surface, producing warm pools that are perfect for soaking in.

radioactive rocks in museum
Some radioactive rocks on display at a museum (Flickr)

Other hot springs form near active volcanoes, where magma under the Earth’s crust can turn water to steam in an instant. This combination of water and stream can shoot out of the ground at extremely high pressures and temperatures! These high-pressure jets are known as geysers, such as Old Faithful at Yellowstone National Park.

They aren’t limited to being on land, though. Hydrothermal vents are found deep under the sea, where hot water emerges from cracks on the ocean floor. These vents support a large variety of life, contributing to the bustling ecosystems that thrive far beyond the reach of sunlight and humans.

Hydrogen Sulfide, The Rotten Molecule

When it comes to smell, hot springs range from having no detectable odor to being a steaming cauldron of rotten eggs! The reason for this is that minerals in the rocks are dissolved by the water as it comes to the surface. The heated water makes it a better solvent than cool water, which means large amounts of calcium, magnesium and even radioactive radium can be collected in hot spring water.

Rotten Eggs, Farts and Hot Springs

Many rocks underground contain sulfur mineral deposits, especially those previously exposed to volcanic gas. These sulfur minerals can produce a chemical called hydrogen sulfide (H2S) when they come into contact with water, through a process known as hydrolysis:

Metal-S + H2O → Metal-O + H2S

Hydrogen sulfide is better known as the gas that contributes to the smell of farts, rotten eggs and bad wine. It turns out that large amounts of it can also be found trapped in reservoir pockets underground, thanks to those sulfur-containing rocks. Hydrogen sulfide gas can dissolve well in water, and even better in the hot water.

The hot water rises to the surface in the form of a hot spring, where it loses some of this heat to its cooler surroundings. The dissolved H2S molecules become less soluble, releasing the gas around the hot spring as the water cools. To make matters worse, hydrogen sulfide is heavier than air, which means the molecule (and its smell) lingers instead of quickly dissipating into the atmosphere.

How Dangerous is Hydrogen Sulfide?

Other gases and minerals are also released from hot springs, but the odor of hydrogen sulfide is the most noticeable. We can detect H2S at just 0.00047 parts per million in air, that’s less than a part per billion! It’s a good thing that our noses can detect the rotten egg odor of hydrogen sulfide because, at higher concentrations, the gas can cause lasting eye and nerve damage, respiratory problems and even death.

Some health effects of hydrogen sulfide poisoning1.

Hydrogen sulfide gas is similar to carbon monoxide in that it binds to different proteins in our bloodstream, with cytochrome oxidase the most crucial of the lot. Cytochrome oxidase is a key enzyme in respiration—the cellular production of energy—and H2S blocks oxygen from participating in the process. This effectively causes asphyxiation of our cells, quickly killing them.

Our bodies also produce hydrogen sulfide as a signaling molecule, allowing cells to ‘talk’ to one another. This means that we possess the required machinery to break it down into less harmful products, so being exposed to low concentrations of the gas (such as someone farting in your face) doesn’t pose a health risk. While hot springs don’t usually contain H2S in sufficient quantities to cause any damage, its smell can ruin the experience of soaking in one!

The ‘Healing Powers’ of Hot Springs?

While soaking in a hot spring can be relaxing, there is currently no evidence that it can cure or treat any disease. ‘Hot spring therapy’, also called medical hydrology, is an example of alternative medicine, a category of pseudoscience that ‘treatments’ like aromatherapy also fall under. It is important that the public realizes this, as delaying or forgoing conventional treatment to soak in a hot spring can be dangerous.

It’s probably worth mentioning that drinking water from hot springs is not recommended. The high sulfur content in hot spring water can cause diarrhea, among other health risks2. There is also some evidence that hydrogen sulfide kills bacteria in our gut, which can disrupt our digestive system if ingested3. Some news and social media outlets once reported that hydrogen sulfide could cure cancer, though it was nothing but a sensationalization of scientific facts.

Radioactive isotopes like radon, found in many hot springs, are carcinogenic when inhaled, although its effects when ingested are currently unknown. As mentioned, hot water can dissolve a whole range of chemicals and minerals that are present in the environment. With no way to tell what’s in hot spring water, it’s probably best not to drink it, regardless of its smell.


  1. Lindenmann, J., Matzi, V., Neuboeck, N., Ratzenhofer-Komenda, B., Maier, A., & Smolle-Juettner, F. M. (2010). Severe hydrogen sulphide poisoning treated with 4-dimethylaminophenol and hyperbaric oxygen. Diving Hyperb Med40(4), 213-217.
  2. Esteban, E., Rubin, C. H., McGeehin, M. A., Flanders, W. D., Baker, M. J., & Sinks, T. H. (1997). Evaluation of Infant diarrhea associated with elevated levels of sulfate in drinking water: a case-control investigation in South Dakota. International Journal of Occupational and Environmental Health3(3), 171-176.
  3. Giampaoli, S., Valeriani, F., Gianfranceschi, G., Vitali, M., Delfini, M., Festa, M. R., … & Spica, V. R. (2013). Hydrogen sulfide in thermal spring waters and its action on bacteria of human origin. Microchemical Journal108, 210-214.

About the Author

sean author
Sean Lim

Sean is a consultant for clients in the pharmaceutical industry and is an associate lecturer at La Trobe University, where unfortunate undergrads are subject to his ramblings on chemistry and pharmacology.

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