Hydrogen sulphide (H2S) is a colourless, toxic and flammable gas that can kill within minutes at high concentrations. It is found in sour oil and gas, sewers, wastewater plants, manure storage and many industrial processes. Because it deadens the sense of smell and collects in low areas, workers must rely on gas detection, training and planned response, never on their noses.
Properties that make H2S dangerous
- Heavier than air. H2S tends to settle and collect in low spots: pits, trenches, sumps, tank bottoms, valve boxes and the lower parts of buildings.
- Colourless. There is nothing to see.
- Misleading smell. At low levels it smells of rotten eggs. At higher levels it quickly paralyses the sense of smell, so a worker may believe the gas has gone just as the concentration becomes lethal.
- Flammable. H2S can burn or explode within its flammable range, and burning produces sulphur dioxide, which is also toxic.
- Soluble in water and liquids. H2S dissolves in water, crude oil and sludge, and is released when the liquid is agitated, heated, pumped or depressurized.
- Corrosive. It attacks some metals and can weaken equipment over time, which can lead to leaks.
Where H2S is found
H2S forms naturally wherever organic material breaks down without oxygen, and it occurs in some geological formations. Common sources include:
- Sour oil and gas wells, pipelines, batteries, gas plants and tank farms
- Sewers, manholes, lift stations and wastewater treatment plants
- Manure pits, lagoons and livestock barns
- Pulp and paper mills, tanneries and some food processing
- Landfills and composting sites
- Plumbing work on drains and vents, sumps and septic systems
- Tanks and vessels holding sour product, even after they are drained
Many H2S incidents happen during non-routine work: cleaning a tank, opening a line, entering a vault or agitating a pit. These are moments when a dissolved or trapped gas is suddenly released.
How H2S affects the body
H2S interferes with the body's ability to use oxygen at the cellular level. The effects depend on concentration and length of exposure. At low concentrations it irritates the eyes, nose and throat and can cause headache, nausea and dizziness. As the concentration rises, it causes loss of smell, breathing difficulty and fluid in the lungs. At high concentrations it can cause sudden collapse, sometimes called knockdown, followed by breathing failure and death within minutes. Survivors of serious exposure can have lasting effects on the brain and nervous system.
Provinces set occupational exposure limits for H2S, and the limits are not the same in every jurisdiction. Check the current limits in your province's legislation and use them for your alarm set points and respiratory protection decisions.
Do not become the second victim. Many H2S deaths are rescuers who rushed in without breathing apparatus to help a co-worker. If someone collapses, move upwind or uphill to safety, raise the alarm and follow your emergency plan. Only trained rescuers wearing suitable breathing apparatus should go in.
Controlling H2S exposure
Control starts with a hazard assessment that identifies where H2S may be present and what tasks could release it. Then apply the hierarchy of controls:
Eliminate or reduce the source
Isolate and purge lines and vessels before opening them. Treat, flush or ventilate tanks and pits before work. Plan tasks so that agitating or pumping sour material is done with nobody in the exposure zone.
Engineering controls
Use fixed H2S monitors with audible and visual alarms in buildings and areas where gas can accumulate. Provide mechanical ventilation, closed sampling systems and properly maintained seals and vents. Put wind socks on outdoor sites so workers know which way is upwind.
Administrative controls
Use work permits, confined space procedures, restricted access zones and buddy systems. Train every worker who may be exposed and practise emergency response. Make sure contractors and visitors know the hazard and the alarm signals.
Personal protection
Personal H2S monitors should be worn in the breathing zone by every worker in an area where H2S may be present. Where concentrations may exceed safe limits, supplied-air respiratory protection is required: a self-contained breathing apparatus (SCBA) or supplied-air breathing apparatus (SABA) with an escape cylinder. Air-purifying respirators are generally not suitable for H2S emergency or high-concentration work. Respirators require fit testing and a respiratory protection program.
Detection you can trust
A gas detector only protects a worker if its sensor responds and its alarm works. Bump test personal monitors before use as the manufacturer recommends, calibrate them on schedule and keep records. Do not ignore or silence alarms. See Gas detector bump testing and calibration for the details.
How ehZee can help
We help employers identify where H2S may be present, review detection, respiratory protection and emergency plans, and arrange H2S Alive training for crews. Our H2S Alive training page explains how we work.
Common questions
If I can smell rotten eggs, am I safe?
No. Smelling it means H2S is present. And not smelling it does not mean it is absent, because higher concentrations deaden the sense of smell. Rely on your detector.
Who needs H2S training?
Anyone who may be exposed to H2S at work. Many oil and gas operators require H2S Alive before workers can come on site. See What H2S Alive training covers.