Hydrogen sulfide (H₂S) is a colourless gas with a characteristic odor of rotten eggs that is a product of the decomposition of sulfur-containing organic materials.  It occurs naturally in fossil fuel deposits, sulfurous rocks, and is also released from hot tar and asphalt.  Accidents involving H₂S have occurred in mines, caves, oil fields, petroleum refineries, sewers, liquid manure storage tanks, agricultural facilities, and the cargo holds of fishing boats.

Chemical suicides using common household products to create H₂S gas are becoming more common. 

The toxicity of H₂S depends on the concentration and the duration of exposure.  Most deaths occur at the scene as a result of respiratory paralysis, also known as “knockdown.”  Trauma may also occur as a result of falls following a loss of consciousness.

• CliniCall consultation required when attending cases of suspected H2S exposure.
• Prompt rescue and treatment can save lives.  Rescue of unconscious victims must only be undertaken by trained personnel equipped with self-contained breathing apparatuses and appropriate protective clothing.  Atmospheric gas monitoring is mandatory.
• Decontamination is required.  Remove and dispose of clothing.
• Inhalation of high concentrations of H₂S causes immediate respiratory paralysis and a rapid loss of consciousness, followed shortly after by death from asphyxia.
• Patients who are ventilated immediately following rescue often recover completely.  Those who remain unconscious for longer periods of time are at risk for permanent hypoxic brain injuries. 

• Early endotracheal intubation and mechanical ventilation with high concentrations of oxygen is recommended in patients with central nervous system depression or respiratory distress.
• Patients with respiratory paralysis may not begin breathing spontaneously for hours.
• Aspiration and pulmonary edema may develop in severe cases.

Patients who are asymptomatic should be observed for at least several hours following exposure. 

• H₂S is highly toxic.  The characteristic odour of the gas is an unreliable predictor of danger; prolonged exposure to low concentrations of H₂S, or brief exposures to higher concentrations, results in olfactory fatigue and renders individuals insensitive to the smell.
• At concentrations between 50-100 ppm, H₂S is irritating to lungs, mucosal membranes, and eyes.  Prolonged exposure at this level may cause pulmonary edema.
• Concentrations > 500 ppm may produce severe toxicity within minutes.  A single breath at concentrations between 800-1,000 ppm may be rapidly fatal.
• The toxicity of H₂S is due to its ability to paralyze respiratory muscles and produce profound hypoxia.