Hydrogen sulfide (\(H_2S\)) is a colorless gas commonly recognized by its characteristic odor of rotten eggs, often found in industrial settings, sewers, and natural gas deposits. It is known by common names such as sewer gas or sour gas and is both highly flammable and extremely toxic to humans. \(H_2S\) is a rapidly acting chemical asphyxiant, meaning it interferes with the body’s ability to use oxygen at the cellular level. Understanding how much exposure is dangerous requires knowing the concentration in parts per million (ppm) and the duration of inhalation.
Effects of Exposure at Specific Concentrations
At very low concentrations, between 0.01 and 1.5 ppm, the rotten egg odor is first noticeable, serving as an initial warning sign. Prolonged exposure to levels as low as 20–50 ppm can cause irritation of the eyes, known as “gas eye,” and irritation of the upper respiratory tract.
Moving to moderate concentrations, exposure to 100–150 ppm can lead to coughing, significant eye irritation, and a rapid loss of the ability to smell the gas. Exposure to 170–300 ppm is considered the maximum that can be endured for one hour without serious, long-lasting consequences. At this range, symptoms like marked conjunctivitis and severe respiratory irritation are present, with the risk of developing fluid in the lungs (pulmonary edema) after prolonged exposure.
Concentrations exceeding 500 ppm rapidly become life-threatening, causing staggering and unconsciousness within minutes. Exposure to 700–1,000 ppm results in cessation of breathing and death, sometimes within minutes. The lethal concentration for 50% of humans (\(LC_{50}\)) is reported to be around 800 ppm for an exposure period of five minutes. Any concentration above 100 ppm is considered immediately dangerous to life and health (IDLH) by NIOSH, indicating a level that can interfere with the ability to escape.
How Hydrogen Sulfide Attacks the Body
Hydrogen sulfide is a cellular poison that acts by disrupting the process of cellular respiration. The primary mechanism of its toxicity involves interfering with the enzyme cytochrome c oxidase, located within the mitochondria of cells. This enzyme is the final step in the electron transport chain, a sequence of reactions that produces adenosine triphosphate (ATP), the main energy currency of the cell.
By inhibiting cytochrome c oxidase, \(H_2S\) effectively blocks the cell’s ability to use oxygen, causing a form of chemical or cellular asphyxiation. This is similar to the mechanism of cyanide poisoning. Tissues with high oxygen demand, such as the brain and the heart, are the most severely and rapidly affected by this blockage of energy production. The resulting lack of oxygen use can lead to immediate collapse, known as “knockdown,” and eventual respiratory and cardiac failure.
Olfactory Fatigue and Loss of Warning
While the rotten egg smell is noticeable at concentrations below 1.5 ppm, the ability to detect this odor quickly diminishes as the concentration increases. This effect is called olfactory fatigue, or “smell blindness,” where the olfactory nerves become rapidly desensitized or paralyzed.
Olfactory fatigue can occur at concentrations as low as 100 ppm, making the gas imperceptible to the exposed person shortly after they enter the area. At extremely high and lethal concentrations, the gas can overwhelm the olfactory nerves instantly, preventing any detection before unconsciousness occurs.
Immediate Action and Response
Suspected hydrogen sulfide exposure requires immediate evacuation to fresh air. Because of the gas’s rapid toxicity and the risk to others, rescuers must use positive-pressure, self-contained breathing apparatus (SCBA) before entering a contaminated zone. Fatalities have occurred when unprepared rescuers attempt to save a victim, highlighting the extreme danger of the environment.
Once a victim is safely removed, emergency medical services should be called immediately, and supportive care focused on breathing and circulation must be initiated. Supplemental oxygen is a standard first treatment, and in severe cases, advanced life support is necessary. Specific medical interventions, such as the use of nitrite therapies, may be considered by medical professionals; these treatments aim to convert \(H_2S\) into a less toxic form by binding it to methemoglobin in the blood.