Hydrogen sulfide (H2S) is a colorless, flammable gas commonly known as sewer gas or sour gas. At very low concentrations, H2S is recognizable by its pungent odor of rotten eggs. This gas is highly toxic and presents a significant, life-threatening hazard upon exposure.
Where Hydrogen Sulfide is Encountered
H2S occurs naturally where organic matter undergoes anaerobic bacterial decomposition. This makes it a concern in municipal sewage systems, stagnant bodies of water like swamps, and agricultural settings involving manure pits. Because the gas is heavier than air, it collects and concentrates in low-lying, enclosed spaces such as manholes, basements, and underground vaults.
Industrial activity is a major source, particularly in the oil and gas sector where H2S is extracted as a component of natural gas and crude petroleum. Refineries, mining operations, and facilities involved in pulp and paper processing or tanneries produce the gas as a byproduct. Workers in these industries face an elevated risk of exposure.
How H2S Affects the Body at a Cellular Level
The toxicity of hydrogen sulfide stems from its action as a metabolic poison within the body’s cells. When inhaled, H2S rapidly enters the bloodstream and travels to the mitochondria. Here, it acts by directly inhibiting the enzyme cytochrome c oxidase, the final component of the electron transport chain.
By binding to this enzyme, H2S prevents the cell from using oxygen to generate adenosine triphosphate (ATP), the energy currency. This disruption of cellular respiration induces a state of chemical asphyxiation at the cellular level. Tissues with high oxygen demand, such as the nervous system and the heart, are the most susceptible to energy deprivation.
Symptoms Based on Exposure Intensity
The physical effects of hydrogen sulfide exposure relate directly to the gas concentration and the duration of contact. At very low concentrations (2 to 5 parts per million, or ppm), prolonged exposure may cause mild, reversible symptoms. These include eye irritation, often called “gas eye,” headaches, mild nausea, and a cough.
Olfactory fatigue is a danger sign that occurs when the gas concentration reaches moderate levels, around 100 ppm. At this concentration, the gas paralyzes the olfactory nerve, causing the sense of smell to be lost after a few minutes. This loss of the rotten egg odor gives the false impression that the gas has dissipated, preventing the exposed person from recognizing the ongoing danger.
Exposure to moderate-to-high concentrations (150 to 300 ppm) results in more severe systemic effects. Symptoms escalate to respiratory tract irritation, difficulty breathing, and pulmonary edema (accumulation of fluid in the lungs). Neurological symptoms such as staggering gait, dizziness, confusion, and excitability also become apparent.
High or lethal concentrations, exceeding 700 ppm, cause immediate health consequences. Exposure at this level can lead to rapid unconsciousness, sometimes described as “knockdown,” within one or two breaths. This is followed by respiratory failure, coma, and cardiac arrest, resulting in death within minutes without intervention.
Immediate Steps After Exposure
The priority following suspected H2S exposure is the immediate evacuation of the affected person to fresh air. If the individual is non-responsive, rescuers must wear appropriate self-contained breathing apparatus before entering the contaminated zone. Emergency medical services and Poison Control should be contacted immediately, describing the gas exposure.
If there is skin or eye irritation, the victim requires decontamination, including removing all clothing and flushing the affected areas with water. Treatment focuses on supportive care, with supplemental oxygen administration to combat cellular hypoxia. In severe cases, specialized medical treatments, such as hydroxocobalamin or sodium nitrite, may be considered to reverse the poisoning effects.