Hydrogen sulfide turns up in a surprisingly wide range of places, from deep-sea volcanic vents to your own digestive tract. It’s the gas responsible for the classic rotten-egg smell, and it forms wherever sulfur-containing materials break down in low-oxygen conditions. Here’s where you’re most likely to encounter it and why it appears in each setting.
Volcanic and Geothermal Areas
When magma releases sulfur gases underground, that sulfur reacts with groundwater on its way to the surface and forms hydrogen sulfide. The U.S. Geological Survey notes that the presence of hydrogen sulfide near a volcano actually signals relatively quiet activity, because the groundwater is filtering out much of the sulfur gas rising from below. You’ll smell it most strongly around hot springs, fumaroles, and geothermal vents in places like Yellowstone, Iceland, and New Zealand’s Rotorua region.
Deep-Sea Hydrothermal Vents
Miles below the ocean surface, superheated seawater circulates through volcanic rock along mid-ocean ridges. This process strips sulfur from minerals in the rock and dissolves it as hydrogen sulfide into the hydrothermal fluid. When that fluid erupts onto the seafloor through “black smoker” chimneys, it creates one of the most hydrogen sulfide-rich environments on Earth.
What makes these vents remarkable is that entire ecosystems run on this gas instead of sunlight. Specialized bacteria oxidize hydrogen sulfide to extract energy, forming the base of a food chain that supports giant tube worms, clams, and shrimp. Tube worms (Riftia) anchor themselves where warm, sulfide-rich fluid seeps out, and their blood contains a specialized hemoglobin that binds the sulfide and delivers it to symbiotic bacteria living inside their bodies. Certain clams take a different approach: they extend a foot down into sulfide-rich crevices while pumping oxygen-rich seawater over their gills, keeping the two chemical inputs physically separated.
Sewers and Wastewater Plants
Hydrogen sulfide is a constant companion in municipal sewer systems. Bacteria break down the proteins in sewage under low-oxygen conditions, producing the gas as a byproduct. For workers in wastewater treatment, average exposure over an eight-hour shift generally stays below 1 ppm. But brief spikes tell a different story. A study of 93 measurements at wastewater facilities found that 9% of readings peaked above 10 ppm, and handling sewage from cesspools produced the highest recorded spike at 260 ppm, well above the 100 ppm threshold considered immediately dangerous to life.
Beyond the health risk, hydrogen sulfide in sewers also corrodes concrete and metal infrastructure. Bacteria on sewer walls convert the gas into sulfuric acid, which eats away at pipe surfaces over time.
Farms and Manure Storage
Livestock operations, particularly hog farms with deep-pit manure storage, are one of the most dangerous settings for hydrogen sulfide exposure. Under normal conditions, the gas hovers in the hundreds of parts per billion range inside the barn, roughly 0.06 to 0.07 ppm. That’s well within safe limits.
The danger comes when stored manure is agitated or pumped out. Stirring the pit can spike concentrations by several thousand-fold in seconds. Simulations show that in barns with poor ventilation during agitation, room air can reach 131 ppm, exceeding the 100 ppm level considered immediately dangerous to life for anywhere from 6 to 44 minutes. These sudden bursts have caused fatalities among farm workers and animals, sometimes within minutes of exposure.
Oil, Gas, and Industrial Operations
The petroleum and natural gas industries are the largest non-natural source of hydrogen sulfide. Natural gas deposits often contain the gas, and it must be removed before the gas can be sold. Oil refineries also generate it during a process called hydrodesulfurization, which strips sulfur from crude oil to produce cleaner fuels. This single process converts roughly 80 to 90% of the sulfur compounds in crude oil into hydrogen sulfide.
Other industrial sources include paper and pulp mills (the Kraft process uses sulfur compounds that release the gas), textile manufacturing, and certain chemical plants. Landfills produce it as organic waste decomposes, and mining operations can release it when sulfide-bearing rock is disturbed.
Inside Your Own Body
Your cells produce hydrogen sulfide naturally. Three enzymes generate it in various tissues, and the gas acts as a signaling molecule that helps regulate blood vessel dilation, inflammation, and immune cell function. Your brain, cardiovascular system, and digestive tract all use it in small amounts for normal operations.
Your gut is a particularly active source. Sulfate-reducing bacteria in the colon, most commonly species from the Desulfovibrionaceae family, produce hydrogen sulfide as they metabolize dietary sulfur. Foods rich in sulfur-containing amino acids (meat, eggs, cruciferous vegetables like broccoli and cabbage) feed these bacteria. The result is a hydrogen sulfide-rich environment in the colon that your immune cells are specially adapted to tolerate. This is also why certain meals lead to particularly odorous gas.
In the Atmosphere
Hydrogen sulfide enters the atmosphere from all the sources above, including volcanic emissions, ocean vents, wetlands, and industrial activity. It doesn’t linger long. The gas has an atmospheric residence time of roughly 15 days before it reacts with naturally occurring hydroxyl radicals and other oxidants. The breakdown products are sulfur dioxide and eventually sulfate particles, which contribute to the global sulfur cycle and can influence cloud formation.
How Concentration Determines Risk
The gas becomes detectable by smell at very low concentrations, often below 1 ppm. At 10 ppm, prolonged exposure starts to pose health risks, and OSHA sets the workplace ceiling at 20 ppm for continuous exposure. The critical danger zone begins at 100 ppm, which NIOSH classifies as immediately dangerous to life or health. At that level and above, the gas paralyzes the olfactory nerve, so the rotten-egg smell disappears entirely. This olfactory fatigue is one of the most dangerous properties of hydrogen sulfide: the higher the concentration, the less you can smell it.
At concentrations above 500 ppm, a single breath can cause loss of consciousness. The gap between “I can smell something” and “this is lethal” is far narrower than most people realize, which is why confined-space work in sewers, manure pits, and industrial tanks requires continuous electronic gas monitoring rather than relying on your nose.