H2S: why “I can’t smell it any more” is the most dangerous moment

Hydrogen sulphide smells of rotten eggs at low levels — and then the smell disappears. Here is why.

Hydrogen sulphide (H2S) is found in oil and gas, sewage and wastewater, tanneries and anywhere organic matter breaks down without oxygen. At very low concentrations it smells strongly of rotten eggs. That smell is exactly what makes it treacherous.

Olfactory fatigue

As concentrations rise, H2S quickly deadens the sense of smell. A worker who notices the smell fading may assume the gas has gone, when in fact the concentration has increased. Never use your nose as a gas detector.

Heavier than air

H2S is heavier than air and collects in pits, trenches, tanks and low-lying areas — the places where people often need to work or carry out rescues.

Essential controls

• Wear a calibrated personal H2S monitor and bump-test it before every shift.
• Know the alarm levels and what to do at each one.
• Know the wind direction and the location of upwind muster points.
• Never enter a space or attempt a rescue without breathing apparatus and a rescue plan.

Our H2S courses cover detection, alarm response and escape in detail.