The Silent Killer   

In 1975, a gas release from an oil drilling operation in Denver City, Texas killed nine people. On September 2, 2005, a leak in the propeller room of a Royal Caribbean Cruise Liner docked in Los Angeles resulted in the deaths of 3 crewmen due to a sewage line leak.  In 2003, fumes pouring from a burst gas well in a town in Kaixian county near the city of Chongqing China’s southwest killed at least 191 people, injured hundreds more and forced 31,000 residents to flee. A dump of toxic waste is believed to have caused 17 deaths and thousands of illnesses in Abidjan, on the West African coast, in the 2006 Côte d’Ivoire toxic waste dump. In September 2008, three workers were killed and two suffered serious injury, including long term brain damage, at a mushroom growing company in Langley, British Columbia. In 2009 at ADCO Shah field, UAE, three workers died whilst draining a transfer line inside a pit.

What do all of these incidents have in common? All of these incidents were caused by the release of Hydrogen Sulphide(Sulfide) (H2S) The Silent Killer.

What do we know about H2S?

H2S  is one of the leading causes of workplace gas inhalation deaths in the oil and gas industry, water treatment, and other industries. It’s also known as  sewer gas, swamp gas, stink damp, and sour damp) is a colourless gas known for its pungent “rotten egg” odour at low concentrations. It is extremely flammable and highly toxic.

H2S is used or produced in a number of industries, such as

  • Oil and gas refining
  • Mining
  • Tanning
  • Pulp and paper processing
  • Rayon manufacturing

H2S also occurs naturally in sewers, manure pits, well water, oil and gas wells, and volcanoes. Because it is heavier than air, H2S can collect in low-lying and enclosed spaces, such as manholes, sewers, and underground telephone vaults. Its presence makes work in confined spaces potentially very dangerous.

H2S gas causes a wide range of health effects. Workers are primarily exposed to H2S by breathing it. The effects depend on how much H2S inhaled and for how long. Exposure to very high concentrations can quickly lead to death.

Short Term Exposure to H2 

Concentration (ppm) Symptoms/Effects


Typical background concentrations
0.01-1.5 Door threshold (when rotten egg smell is first noticeable to some). Door becomes more offensive at 3-5 ppm. Above 30 ppm, odour described as sweet or sickeningly sweet.
2-5 Prolonged exposure may cause nausea, tearing of the eyes, headaches or loss of sleep. Airway problems (bronchial constriction) in some asthma patients.
20 Possible fatigue, loss of appetite, headache, irritability, poor memory, dizziness.
50-100 Slight conjunctivitis (“gas eye”) and respiratory tract irritation after 1 hour. May cause digestive upset and loss of appetite.
100 Coughing, eye irritation, loss of smell after 2-15 minutes (olfactory fatigue). Altered breathing, drowsiness after 15-30 minutes. Throat irritation after 1 hour. A gradual increase in the severity of symptoms over several hours. Death may occur after 48 hours.
100-150 Loss of smell (olfactory fatigue or paralysis).
200-300 Marked conjunctivitis and respiratory tract irritation after 1 hour. Pulmonary oedema may occur from prolonged exposure.
500-700 Staggering, collapse in 5 minutes. Serious damage to the eyes in 30 minutes. Death after 30-60 minutes.
700-1000 Rapid unconsciousness, “knockdown” or immediate collapse within 1 to 2 breaths, breathing stops, death within minutes.
1000-2000 Nearly instant death

Long term exposure to H2S:   Some people who breathed in levels of H2S high enough to become unconscious continue to have headaches and poor attention span, memory, and motor function after waking up. Problems with the cardiovascular system have also been reported at exposures above permissible exposure limits. People who have asthma may be more sensitive to H2S exposure. That is, they may have difficulty breathing at levels lower than people without asthma.


To protect workers from H2S  exposures:

  • Evaluate exposure to know whether H2S gas is present and at what levels. Identify processes that could release or produce H2S. This includes identifying known sources of H2S and evaluating possible fire and explosion hazards. Use risk assessment methodology for identifying and controlling the risks. Test (monitor) the air for H2S by a competent person
  • Eliminate the source of H2S whenever possible.
  • If the source cannot be eliminated, control exposures by:

o   Using engineering controls as the next best line of defence.

o   Developing administrative controls and safe work practices to reduce exposures to safe levels.

  • Use personal protective equipment if engineering controls and work practices alone cannot reduce H2S to safe levels.

DO NOT rely on your sense of smell to indicate the continuing presence of H2S or to warn of harmful levels. You can smell the “rotten egg” odour of H2S at low concentrations in air. But after a while, you lose the ability to smell the gas even though it is still present (olfactory fatigue). This loss of smell can happen very rapidly, and at high concentrations and the ability to smell the gas can be lost instantly (olfactory paralysis).

Control Exposures
  • Use adequate exhaust and ventilation systems to reduce H2S  levels. Make sure that the system is:

o   Non-sparking

o   Grounded

o   Corrosion-resistant

o   Separate from other exhaust ventilation systems

o   Explosion-proof

  • Train and educate workers about hazards and controls. Training topics may include:

o   Characteristics, sources, and health hazards of H2S

o   Symptoms of H2S exposure

o   Types of H2S detection methods and applicable exposure limits

o   Workplace practices and procedures to protect against H2S exposure

o   Emergency plans, locations of safety equipment, rescue techniques, first-aid

o   Confined space procedures

  • Establish proper rescue procedures to rescue someone from an H2S  exposure safely.
  • Adopt safe practices
  • All employees, including contractual workers who are required to enter the H2S zone must be equipped with a personal gas monitor individually.
  • Check individual H2S monitor (by pressing test button) and EEBA  before entering into H2S Zones.
  • Always keep track of wind direction with the help of wind sox.
  • Rely on a personal H2S  detector for immediate action than fixed detector.
  • Get familiarization of Emergency PPEs (EEBA / SCBA) cabinet in the field. Also, be familiar with the routes from the work location to the nearest escape mask store.
  • Establish a regime for the periodic donning exercise of SCBA with time marking.


  • H2S is an Extremely Toxic, Flammable, Fatal Gas.
  • It cannot be replaced or substituted during refinery processes.
  • H2S is heavier than air; Accumulates in Low / poorly ventilated areas, Smells like Rotten Eggs but Only at very low concentrations
  • Best available option to safeguard from H2S is by….
    • Using given tools like Personal H2S Detectors.
    • Using proper Personal Protective Equipment (PPE
    • By following H2S Emergency Management System
  • The goal is for everyone to get home safe and sound, and not become the victim of the silent killer.

If you need further guidance, assistance or other information, please contact us

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