What is Hydrogen Sulfide or H2S?

What is H2S?

H2S, short for hydrogen sulfide, is a colorless, flammable gas that is toxic at extremely low concentrations. H2S gas poses a severe and potentially lethal hazard to workers in several industries including oil and gas extraction, petroleum refining, and chemical manufacturing. 

H2S gas forms naturally when organic material breaks down in areas with little to no oxygen. H2S occurs in crude oil, natural gas, volcanic gases, well water, and hot springs. 

In oilfield environments, H2S formed underground can be released from well sites. Dangerous concentrations of H2S gas also may be present in coke ovens, paper mills, sewer systems, tanneries, mines, smelting facilities, and other workplaces.

H2S often accumulates in low-lying or enclosed areas like gas venting or mud systems, cellars, pits, and tanks. This occurs because H2S gas has a relative density of 1.19, meaning it is heavier than breathable air. This results in areas with very high, very dangerous concentrations of H2S.  

H2S Exposure and Health Effects 

At low concentrations, hydrogen sulfide smells like rotten eggs. Exposure to H2S rapidly degrades the sense of smell, however. This makes the odor a bad indicator for prolonged exposure, and for exposure at higher concentrations. At 30 ppm and up, the smell of H2S is described as “sweet” or “sickeningly sweet.” (OSHA.gov) 

Symptoms associated with hydrogen sulfide exposure include, but are not limited to: 

• Burning eyes
• Teary eyes
• Respiratory irritation 
• Coughing 
• Trouble breathing 
• Headaches
• Dizziness 
• Nausea
• Vomiting 
• Fatigue
• Memory issues
• Confusion 
• Skin irritation (dermal contact)  

Effects of H2S Exposure by Concentration

Exposure to H2S can be fatal within moments at sufficiently high concentrations. Immediate and short-term health effects of exposure range from respiratory irritation and headaches to trouble breathing, eye damage, and loss of consciousness, based on the concentration of H2S in the air.

• 2 to 5 parts per million (ppm). Prolonged exposure to H2S at low levels (2 to 5 ppm) can cause nausea, teary eyes, headaches, and sleep loss. For those suffering from asthma, low-level exposure to H2S can cause airway problems. 

• 20 ppm. Exposure can cause appetite loss, difficulty with memory, dizziness, fatigue, headaches, and irritability.

• 100 ppm. Olfactory fatigue (loss of smell) can occur in as little as 2 minutes. At 15 minutes, altered breathing and drowsiness may occur, followed by throat irritation after 1 hour of exposure. Exposure at this level can be fatal after 48 hours. 

• 500 to 700 ppm. Within 5 minutes, exposure causes staggering and collapse. Death can occur in as little as 30 minutes. 

• 700 ppm and Above. “Knockdown” (immediate collapse) and loss of consciousness within 1 to 2 breaths. Death within minutes. 

[Source: Hydrogen Sulfide. OSHA.gov]

H2S Regulations and Exposure Limits

OSHA enforces permissible exposure limits (PELs) for employees who work with or around hydrogen sulfide (H2S).  For general industry workplaces, the “ceiling limit” for H2S is 20 parts per million (ppm) and the “peak” limit is 50 ppm, for no more than 10 minutes, if the employee has no other exposure during their shift. [29 CFR 1910, Subpart Z—Table Z-2]

In the construction and shipyard industries, OSHA enforces an 8-hour PEL of 10 ppm [29 CFR 1926.55 and 29 CFR 1915.1000]. Hydrogen sulfide poses an immediate danger to life and health (IDLH) at a concentration of 100 parts per million (ppm), according to NIOSH. At this concentration level, exposure to H2S will interfere with a person’s ability to escape the hazard.

If you work in a workplace that has potential exposure to hydrogen sulfide, you must receive safety training to know how to work safely around it. This may include emergency response, what kind of personal protective equipment to wear, or just general awareness to areas of concern.

H2S Safety Training

The Hydrogen Sulfide Safety (H2S Safety) Online Course develops workers’ awareness of the hazards of hydrogen sulfide, detection and monitoring methods, and emergency planning and response requirements. Employees who complete this course are better prepared to recognize the symptoms and effects of H2S exposure and protect themselves and co-workers on the job.