Hydrogen sulfide forms whenever sulfur-containing materials break down in low-oxygen environments. The sources range from volcanic vents and deep groundwater to the bacteria in your own gut. Recognizable by its rotten egg smell at concentrations as low as 0.0005 parts per million (ppm), this gas is produced through both biological and industrial processes that are surprisingly common.
Bacteria Are the Primary Biological Source
The most widespread cause of hydrogen sulfide is bacterial activity. Sulfate-reducing bacteria are among the oldest life forms on Earth, and they produce hydrogen sulfide as the end product of anaerobic respiration, essentially “breathing” sulfur compounds instead of oxygen. These bacteria thrive anywhere oxygen is scarce and sulfur is available: deep soils, wetlands, ocean sediments, sewers, and the human digestive tract.
The process works in two main ways. Bacteria either reduce inorganic sulfur compounds (sulfate and sulfite) or ferment sulfur-containing amino acids found in proteins, like cysteine and methionine. Both pathways release hydrogen sulfide as a byproduct. This is why stagnant, oxygen-poor water tends to develop that characteristic rotten egg smell over time.
Your Diet Directly Affects How Much Your Gut Produces
Sulfate-reducing bacteria make up a significant portion of normal gut flora, and they’re the major contributors of hydrogen sulfide produced inside the human body. What you eat has a direct effect on how much they produce.
Dietary protein is the biggest driver. Fecal sulfide concentrations increase proportionately with the amount of meat intake, because animal protein delivers more sulfur-containing amino acids to the lower gut. A cross-over study comparing animal-based and plant-based diets found that protein, both as a percentage of calories and in total grams, was a positive contributor to hydrogen sulfide production. Animal protein in a highly processed, low-fiber Western diet resulted in the highest levels.
Fiber works in the opposite direction. Total fiber, insoluble fiber, and overall carbohydrate content all reduce hydrogen sulfide production. Plant-based protein within a minimally processed diet (where natural fiber structures remain intact) led to lower production. When the diet lacks fiber, gut bacteria turn to degrading the protective mucus lining of the intestine, which itself contains sulfur and feeds even more substrate to hydrogen sulfide-producing bacteria. Certain food additives, including sulfites, sulfates, and carrageenan, also contribute to sulfide levels in the gut.
Mouth Bacteria and Bad Breath
Hydrogen sulfide is one of the main compounds responsible for halitosis. Anaerobic bacteria living in oral biofilm, particularly in the spaces between teeth and along the gum line, break down sulfur-containing amino acids from food debris and produce hydrogen sulfide in the process. People with halitosis have both higher concentrations of these sulfur gases and greater bacterial diversity in their mouths compared to people without the condition. Bacterial genera like Peptostreptococcus and Alloprevotella, along with the species Eubacterium nodatum, are especially abundant in halitosis patients and correlate directly with sulfide levels.
Gum disease amplifies the problem. Gingivitis, caused by microbial plaque buildup near the gum line, is associated with elevated sulfide levels in gum tissue. Inflammation and bleeding during dental probing both correlate with higher sulfide concentrations.
Volcanoes, Hot Springs, and Geothermal Areas
Hydrogen sulfide is a natural volcanic gas, released when magma interacts with sulfur deposits deep underground. It escapes through fumaroles, volcanic craters, and geothermal springs, sometimes reaching dangerous concentrations. Near volcanic vents, levels typically range from 0.1 to 0.5 ppm, but during active eruptions the numbers climb dramatically. During the 1976-1977 eruption of Soufrière in Guadeloupe, summit concentrations hit roughly 74 ppm, while the town of St. Claude, 3 to 4 kilometers away, recorded 0.2 to 0.37 ppm.
Geothermal regions pose ongoing risks even without eruptions. In Rotorua, New Zealand, which sits on an active geothermal field, ambient outdoor concentrations regularly exceed 1 ppm. Indoor levels in certain buildings have been recorded above 200 ppm in poorly ventilated or enclosed areas. In the Alban Hills volcanic region near Rome, residential measurements frequently exceeded 10 to 15 ppm, with spikes up to 40 ppm.
Groundwater and Well Water
If your well water smells like rotten eggs, sulfur-reducing bacteria in the groundwater are almost certainly the cause. These bacteria feed on naturally occurring sulfur in rock and thrive in the low-oxygen conditions found in wells and household plumbing. The problem is most common in wells drilled into acidic bedrock like shale and sandstone, where sulfur minerals are more abundant and conditions favor bacterial growth.
Sewer Systems and Wastewater
Sewers are ideal incubators for hydrogen sulfide. Wastewater contains sulfate, and the interior of sewer pipes is often anaerobic, giving sulfate-reducing bacteria exactly what they need. Sulfide accumulates to particularly high levels in networks with long transport times, where sewage sits in pipes for extended periods before reaching a treatment plant.
The consequences go beyond odor. At dissolved sulfide concentrations of just 0.1 to 0.5 milligrams per liter, concrete sewer pipes begin to corrode. Above 2 milligrams per liter, corrosion becomes severe, eating through concrete at rates of 1 to 10 millimeters per year. This is why hydrogen sulfide management is a major concern for municipal infrastructure.
Industrial Processes
Two major industries produce hydrogen sulfide as a routine byproduct. In petroleum refining, a process called hydrodesulfurization removes sulfur from crude oil by reacting it with hydrogen gas. The sulfur in the oil is stripped away and converted directly into hydrogen sulfide, which is then captured and typically converted into elemental sulfur for sale or disposal.
Paper manufacturing through the Kraft pulping process is another significant source. The chemical reactions involved in breaking down wood fibers use sodium sulfide, which reacts with water vapor and carbon dioxide at various stages to release hydrogen sulfide. This happens in the recovery boiler, the smelt dissolving tank, and in direct-contact evaporators where flue gas meets the cooking chemicals. The distinctive smell near paper mills comes largely from these sulfur emissions.
Why Concentration Matters
Hydrogen sulfide is detectable by smell at extremely low levels, with the odor threshold starting around 0.0005 ppm. But the gas becomes more dangerous as concentrations rise, and at high levels it actually paralyzes the olfactory nerve, eliminating the warning smell entirely. OSHA sets a workplace ceiling of 20 ppm for general industry, with a maximum peak of 50 ppm allowed for no more than 10 minutes. Construction and shipyard workers face a stricter limit of 10 ppm as a time-weighted average. The National Institute for Occupational Safety and Health recommends a ceiling of 10 ppm over any 10-minute period.
For context, the background concentration of hydrogen sulfide in normal outdoor air is just 0.00005 to 0.024 ppm. People living near volcanic areas, industrial facilities, or wastewater treatment plants can be exposed to levels tens or hundreds of times higher than that baseline.