Hydrogen sulfide

• 1. Introduction

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  Hydrogen sulfide is a colorless, flammable and toxic gas, and the human nose is astonishingly good at catching it. In odor tests with a human panel, half of the people could tell that something was in the air at just 0.47 parts per billion, even though they could not say what it was. At 4.7 parts per billion, half of them could name it. Most people simply call the smell rotten eggs. That early warning matters, because the gas is dangerous. Yet living bodies make it on purpose, bacteria use it in place of oxygen-based chemistry, and industry treats it as a raw material. Why would a poison also serve as a messenger inside cells? What happens when the nose stops reporting? And how did a foul-smelling gas end up in research on superconductivity?

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• 2. Early Warnings

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  In 1713, the Italian physician Bernardino Ramazzini published a book called De Morbis Artificum Diatriba. In it, he suggested that the diseases of sewer workers, and the blackening of coins in their clothes, might come from an unknown, invisible, volatile acid. He was circling a gas that chemistry had not yet identified. Later in the 18th century, toxic fumes rising from the sewers of Paris became a problem for citizens and authorities alike.

  In 1777, the Swedish chemist Carl Wilhelm Scheele was credited with discovering the chemical composition of the purified gas. Miners had their own blunt name for foul-smelling, hydrogen sulfide-rich gas mixtures underground. They called it stinkdamp.

  During World War I, the British Army used the gas as a chemical weapon. It was not considered an ideal war gas, partly because it burns and partly because its distinctive smell could be detected even from a small leak, alerting the enemy. It was nevertheless used on two occasions in 1916, when other gases were in short supply.

  Justus von Liebig described its toxic properties in detail in 1843.

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• 3. The Nose Gives Out

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  At 100-150 parts per million, the olfactory nerve is paralyzed after only a few breaths. The smell of rotten eggs disappears, a temporary loss of smell called anosmia, and awareness of danger often disappears with it. Victims may not realize the gas is present until it is too late. The poisoning is chemical. Hydrogen sulfide binds with iron in cytochrome enzymes inside the mitochondria, the cell's energy machinery, and that prevents cellular respiration. It can harm several body systems, though the nervous system is affected most. Its toxicity is comparable to that of carbon monoxide.

  Many personal gas detectors, worn by utility, sewage and petrochemical workers, are set to alarm at as low as 5 to 10 parts per million. They go into high alarm at 15. That caution has a basis in biology. Enzymes in the body oxidize the gas to sulfate, which is harmless, so low levels can be tolerated indefinitely. At a threshold believed to average around 300-350 parts per million, those enzymes are overwhelmed.

  United States agencies have set several ceilings. The NIOSH recommended ceiling is 10 parts per million, and the OSHA ceiling is 20. OSHA also sets a peak limit of 50 that must never be exceeded. NIOSH marks 100 parts per million as immediately dangerous to life and health, the level that interferes with the ability to escape.

  The effects climb steeply above those numbers. Eye irritation begins at a borderline of 10-20 parts per million, and eye damage comes at 50-100. At 320-530, fluid in the lungs becomes possible, with a risk of death. From 530 to 1000, the central nervous system is strongly stimulated and breathing speeds up, then stops. The concentration lethal to half of exposed humans in 5 minutes is 800. Above 1000, even a single breath can cause immediate collapse.

  Lower exposures cause eye irritation, a sore throat and cough, nausea, shortness of breath and fluid in the lungs. These effects are believed to arise when the gas combines with alkali in moist tissue to form sodium sulfide, a caustic. They usually fade within a few weeks. Long-term, low-level exposure may bring fatigue, loss of appetite, headaches, irritability, poor memory and dizziness. Reports from Russian and Finnish wood pulp workers tied chronic exposure around 2 parts per million to more miscarriages and reproductive health problems. As of 1995, those reports had not been replicated.

  In the United States, inhalation caused about 7 workplace deaths per year between 2011 and 2017. Only carbon monoxide, at 17 deaths per year, killed more workers among chemical inhalation cases. Short-term, high-level exposure can cause immediate collapse and loss of breathing, with a high probability of death. Survivors may be left with cortical pseudolaminar necrosis, degeneration of the basal ganglia and cerebral edema. Respiratory paralysis can strike at once, or it can be delayed for up to 72 hours.

  Treatment combines immediate inhalation of amyl nitrite, injections of sodium nitrite, or 4-dimethylaminophenol with pure oxygen. Doctors may add bronchodilators for bronchospasm, and in some cases hyperbaric oxygen therapy, which has both clinical and anecdotal support.

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• 4. When Containment Fails

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  On the 2nd of September 2005, a sewage line leaked in the propeller room of a Royal Caribbean cruise liner docked in Los Angeles. Three crewmen died. As a result, all such compartments are now required to have a ventilation system.

  The following year, a dump of toxic waste containing hydrogen sulfide is believed to have caused 17 deaths and thousands of illnesses in Abidjan, on the West African coast. That event is known as the 2006 Ivory Coast toxic waste dump.

  In September 2008, a clogged valve at a mushroom growing company in Langley, British Columbia, set up a different kind of trap. The pipe carried chicken manure, straw and gypsum to make compost fuel. Workers unclogged the valve in a confined space without proper ventilation, and gas built up by anaerobic decomposition was released. Three workers were killed, and two suffered serious injury, including long-term brain damage. An investigator said there could have been more deaths if the pipe had been fully cleared or if the wind had changed direction.

  In 2014, shop tenants at a recently built mall in Bangkok's Siam Square area, called Siam Square One, reported sinus inflammation, breathing difficulties and eye irritation. Levels as high as 83 parts per million were detected. Investigators traced the gas to imperfect treatment and disposal of wastewater in the building. That same year in North Scottsdale, Arizona, workers died at the Promenade shopping center after climbing into a chamber 15 feet deep without personal protective gear. Responders recorded "high levels of hydrogen cyanide and hydrogen sulfide coming out of the sewer." In November 2014, a substantial cloud of the gas shrouded central, eastern and southeastern Moscow, and the emergency ministry urged residents to stay indoors. The exact source was not known, though blame fell on a Moscow oil refinery.

  In June 2016, a mother and her daughter were found dead in their still-running 2006 Porsche Cayenne against a guardrail on Florida's Turnpike. Carbon monoxide poisoning was suspected at first. Urine tests later showed hydrogen sulfide was the cause, and the Orange-Osceola Medical Examiner's Office reported that the fumes came from the starter battery under the front passenger seat.

  In January 2017, three utility workers in Key Largo, Florida, died one by one within seconds of descending into a narrow space beneath a manhole cover. A firefighter who entered without his air tank, because he could not fit through the hole with it, collapsed within seconds. A colleague rescued him, and he recovered at Jackson Memorial Hospital after being airlifted there.

  On the 24th of May 2018, two workers died and 14 others were hospitalized at the Norske Skog paper mill in Albury, New South Wales. An investigation by SafeWork NSW found that a tank upstream had been left stagnant and untreated with a biocide during a 3-day maintenance period. Sulfate-reducing bacteria grew in it, feeding on small quantities of wood pulp and fiber in the water. When pumping resumed at the end of maintenance, gas escaped from openings around the top of the second tank and pooled in an area the company had not identified as a confined space. One worker collapsed while checking an apparent leak. The second man who died and a worker who was badly injured were trying to rescue him. Norske Skog pleaded guilty and was fined AU$1,012,500, and it was ordered to fund an anonymized educational video about the incident.

  In October 2019, a water pump failed in Odessa, Texas, and released produced water with a high concentration of hydrogen sulfide. An employee of Aghorn Operating Inc. died while responding to an automated phone call about the failure. His wife died after driving to the facility to check on him. A CSB investigation cited lax safety practices, including an informal lockout-tagout procedure and a nonfunctioning hydrogen sulfide alert system.

  The list has kept growing. On the 22nd of April 2026, a leak in Institute, West Virginia, killed two people and sent 27 to hospital.

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• 5. The Body's Own Gas

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  Three enzymes make hydrogen sulfide inside living cells: cystathionine gamma-lyase, known as CSE, cystathionine beta-synthetase, known as CBS, and 3-mercaptopyruvate sulfurtransferase, known as 3-MST. CBS and CSE are the main producers, working through the trans-sulfuration pathway, which moves a sulfur atom from methionine to serine to form cysteine. 3-MST contributes by way of cysteine breakdown. The dietary amino acids methionine and cysteine supply most of the raw material. Proteins such as ferredoxins and Rieske proteins can also yield the gas.

  The gut adds its own share. Bacterial breakdown of sulfur-containing proteins in the intestinal tract produces small quantities, which contribute to the characteristic odor of flatulence. The mouth produces some too, and that is part of halitosis.

  In the body, the gas works as a signaling molecule. It is involved in vasodilation, the widening of blood vessels, in animals. In plants, it increases seed germination and stress responses. It signals in three ways: by reacting with reactive oxygen species and reactive nitrogen species, by binding or reducing the metal centers of iron-heme proteins, and by sulfhydration of cysteine residues in proteins. It also interacts with the nitric oxide pathway, with effects that include inhibiting cGMP phosphodiesterases and forming another signal called nitrosothiol. It raises levels of glutathione, which reduces or disrupts reactive oxygen species in cells.

  Biology once studied the gas mainly as an environmental toxin. The field has since moved to its roles when produced inside the body, in healthy conditions and in disease. It has been implicated in cancer, Down syndrome and vascular disease.

  The dose decides the effect. At lower concentrations, it stimulates mitochondrial function in several ways, including direct electron donation. At higher concentrations, it inhibits Complex IV of the mitochondrial electron transport chain, which cuts ATP generation and biochemical activity in cells.

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• 6. Life Without Oxygen

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  In swamps, eutrophic lakes and the dead zones of oceans, decaying organic matter uses up the oxygen. Sulfate-reducing bacteria then use the sulfates in the water to oxidize that matter, and hydrogen sulfide comes out as their waste. Some of it reacts with metal ions in the water to form insoluble metal sulfides, such as ferrous sulfide. These are often black or brown, which gives sludge its dark color. Hydrogen sulfide is a central participant in the sulfur cycle, the biogeochemical cycle of sulfur on Earth.

  Other microbes treat the gas as fuel. Several groups of bacteria oxidize it to elemental sulfur or to sulfate, using dissolved oxygen, metal oxides or nitrate as electron acceptors. Purple sulfur bacteria and green sulfur bacteria go further. They use it as an electron donor in photosynthesis and produce elemental sulfur. That mode of photosynthesis is older than the one used by cyanobacteria, algae and plants, which takes electrons from water and releases oxygen. In the iron-sulfur world model of the origin of life on Earth, geologically produced hydrogen sulfide is postulated to drive the reduction of carbon dioxide.

  The gas is lethal to most animals, but a few specialized species, called extremophiles, thrive where it is abundant. In the deep sea, hydrothermal vents and cold seeps rich in the gas host life ranging from bacteria to fish such as the Sulphur molly. With no sunlight at those depths, these ecosystems run on chemosynthesis instead of photosynthesis. Freshwater springs rich in the gas are mostly home to invertebrates. A few fish live there too, including the pupfish Cyprinodon bobmilleri and the poeciliid Gambusia eurystoma from Mexico, and the poeciliid Limia sulphurophila from the Dominican Republic. Invertebrates and microorganisms in some caves, such as Movile Cave, are also adapted to high levels.

  Beyond Earth, hydrogen sulfide has often been detected in the interstellar medium. It also occurs in the clouds of planets in the Solar System.

  Back on Earth, it has been implicated in several mass extinctions. A buildup in the atmosphere may have caused, or at least contributed to, the Permian-Triassic extinction event 252 million years ago. Organic residues from the extinction boundaries indicate that the oceans were depleted of oxygen and held shallow plankton that metabolized the gas. Massive volcanic eruptions may have started the process by emitting carbon dioxide and methane, which warmed the oceans and lowered their capacity to absorb oxygen. The rising gas could have killed oxygen-generating plants and depleted the ozone layer, adding further stress. Small blooms have been detected in modern times in the Dead Sea and in the Atlantic Ocean off the coast of Namibia.

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• 7. Industrial Workhorse

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  Petroleum refineries are by far the largest industrial source of hydrogen sulfide. Hydrodesulfurization frees sulfur from petroleum by the action of hydrogen, and the resulting gas is converted to elemental sulfur by partial combustion in the Claus process. That process is a major source of the world's elemental sulfur. Coke ovens, paper mills using the Kraft process, tanneries and sewerage are other human-made sources.

  Making sulfur is also the gas's main use. In the Claus process, part of the gas is oxidized to sulfur dioxide, which then reacts with more hydrogen sulfide to give elemental sulfur, with alumina as the catalyst. Most of the gas is obtained by separating it from sour gas, which is natural gas with a high content of it. It can also be made by treating hydrogen with molten sulfur at about 450 °C, and hydrocarbons can serve as the hydrogen source.

  It is a starting point for many organosulfur compounds, including methanethiol, ethanethiol and thioglycolic acid. Hydrosulfides can be used to produce thiophenol. Combined with alkali metal bases, it forms sodium hydrosulfide and sodium sulfide. In the Kraft process of papermaking, sodium sulfide salts break the bonds between the lignin and cellulose components of pulp. Oxide ores are sometimes treated with the gas to turn them into sulfides, which are easier to purify by flotation. Metal parts are sometimes passivated with it, and hydrodesulfurization catalysts are routinely activated with it.

  Raw natural gas and biogas often contain it as an impurity. Amine gas treating usually removes it, leaving the natural gas unaffected, and the captured hydrogen sulfide is typically converted to sulfur through the Claus process. Drinking water needs its own treatments. Continuous chlorination, usually with sodium hypochlorite, handles levels up to 75 mg/L and yields insoluble solid sulfur. For concentrations below 2 mg/L, aeration is ideal. Air is injected into the water, and oxygen turns the gas into odorless sulfate.

  Not all of it is intended. In municipal landfills, buried organic matter quickly turns anaerobic, and warm, moist conditions follow. Where plasterboard or natural gypsum supplies sulfate, sulfate-reducing bacteria convert it to hydrogen sulfide. The bacteria cannot survive in air, but buried, high-carbon waste suits them well. In inert landfills, the paper and glue in plasterboard can supply the carbon. Ordinary citizens can also meet the gas near sewage treatment facilities, and on farms with manure storage.

  In 2011, oil from the Bakken formation was reported to carry increased concentrations, possibly due to oil field practices. The report listed "health and environmental risks, corrosion of wellbore, added expense with regard to materials handling and pipeline equipment, and additional refinement requirements."

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• 8. Chemistry Under Pressure

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  Silver coins left in the open slowly turn dark, and hydrogen sulfide in the air is the cause. It reacts with the silver on a coin's surface to form silver sulfide, the chemical behind black toning. The gas tarnishes other metals, including copper. On some coins, toning produces thin-film interference, which gives an attractive coloration and can add to numismatic value. Coins can also be treated with the gas on purpose, but collectors generally criticize artificial toning, and it can be told apart from the natural kind.

  The same reaction made the gas useful in laboratories. Hydrogen sulfide reacts with metal ions such as lead, copper, mercury and arsenic to form insoluble, often dark solids, so it became a reagent in qualitative inorganic analysis. Technicians precipitate the metals from a solution, then identify the components by their reactivity. Lead acetate paper detects the gas itself, because it turns into black lead sulfide.

  In the lab, a Kipp generator makes it by treating ferrous sulfide with a strong acid. For qualitative analysis, thioacetamide can generate it. Metal and nonmetal sulfides such as aluminium sulfide, phosphorus pentasulfide and silicon disulfide release it on contact with water.

  It is slightly denser than air, and its flammable range runs from 4.3 to 46 percent. It burns in oxygen with a blue flame to form sulfur dioxide and water. With excess oxygen, sulfur trioxide forms and quickly hydrates to sulfuric acid. Slightly soluble in water, it acts as a weak acid. Exposed to air, it slowly oxidizes to elemental sulfur. In the Girdler sulfide process, its easy exchange of protons and the easy distillation of the compounds involved are used to purify heavy water. Without a catalyst, at atmospheric pressure, it decomposes into hydrogen and sulfur around 1200 °C.

  The strangest behavior appears under crushing pressure. Above 90 gigapascals, which is 900,000 atmospheres, hydrogen sulfide becomes a metallic conductor of electricity. Cooled below a critical temperature, this high-pressure phase becomes a superconductor. The critical temperature rises with pressure, from 23 K at 100 gigapascals to 150 K at 200 gigapascals. Pressurize the gas at higher temperatures, then cool it, and the critical temperature reaches 203 K. That was the highest accepted superconducting critical temperature until the discovery of lanthanum decahydride in 2019. Predictions suggest that replacing a small part of the sulfur with phosphorus, at even higher pressures, might lift the critical temperature above 0 °C and achieve room-temperature superconductivity.

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