Hydrogen Sulfide (H2S)

Hydrogen sulfide (H2S) is a severely hazardous gas that is colorless, highly flammable, and has a trademark “rotten egg” smell. Like many other gases, it naturally exists within the air in small and unharmful concentrations. While limited exposure and its side effects can be treated, there are no antidotes to H2S gas poisoning and high concentrations can be near-instantly fatal.

H2S is produced naturally in hot springs, natural gas, crude petroleum, and in the bacterial breakdown of animal/human waste and organic materials. Industries that may produce this hazardous gas include wastewater or sewage treatment, coke ovens, paper mills, rayon textile factories, agricultural farms with manure landfills or storage pits, tanneries, and natural gas/petroleum refining and drilling. Workers in these industries are at risk for hydrogen sulfide exposure and should take the necessary precautions for protection.

The gas tends to collect in enclosed, low-lying areas that are poorly ventilated, such as manure pits and sewer lines. The primary route of exposure is inhalation, as the lungs rapidly absorb H2S. The gas is both a chemical asphyxiant and irritant, and impacts the central nervous system as well as a person’s ability to use oxygen. The impact on a person’s health depends on the level of exposure and duration.

A low concentration is enough to cause a shortness of breath and irritate the throat, nose, and dyes. Prolonged exposure to low concentrations can cause digestive disturbances, irritability, fatigue, insomnia, headaches, and eye inflammation. A moderate concentration causes vomiting, nausea, dizziness, and severe respiratory/eye inflammation. High concentrations lead to an inability to breathe, rapid unconsciousness, convulsions, shock, comas, and death. 

H2S gas at a level of 100ppm or above is classified to be Immediately Dangerous to Life and Health (IDLH). This high concentration can cause fatal side effects within just a few breaths. OSHA’s established permissible exposure limits for H2S gas are:

  • 10 ppm, 8-hour limit for shipyards and construction.
  • 20 ppm, general industry ceiling limit.
  • 50 ppm up to 10 minutes if no other exposure occurs during the shift. This is the general industry peak limit.
  • 100 ppm, the IDLH threshold. In these environments, people may only enter if they have PPE such as a full facepiece and supplied-air respirator. 

There have been many fatal accidents on worksites due to hydrogen sulfide gas. Unfortunately, 39% of these deaths occurred to coworkers who attempted to rescue the person who was initially affected by the gas. Rapid unconsciousness also often leads to serious or fatal falls. The following safety practices are an effective foundation to protecting against H2S exposure and related accidents:

  • Ensure that manure pits and other confined spaces are properly ventilated to prevent the buildup of gas. This typically involves a continuous, powered ventilation system.
  • Continually test air for the presence and concentration of hydrogen sulfide.
  • People entering the space/area must use respiratory protection and other necessary personal protective equipment, as well as communication and rescue equipment.
  • Do not enter a manure pit or other enclosed space that may contain H2S gas by yourself. A standby person should be nearby and remain in constant contact visually or auditorily. While the space is occupied, this standby person must remain at the opening and have mechanical lifting equipment on hand to be able to lift a victim out if necessary.
  • Workers who enter the space should wear a safety belt or harness with a lifeline secured to the mechanical lifting equipment. 

Due to the dangerous nature of hydrogen sulfide gas, communication about the presence or potential presence of H2S is extremely important. Signage is a helpful tool for this communication, as it alerts workers and reminds them to use detection equipment. The use of visual communication can protect workers’ health and save lives in environments where H2S may be present.


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