Hydrogen Sulphide (H2S) in Sewers

Hydrogen sulphide (H2S) is a highly toxic gas produced by bacterial decomposition of organic matter in sewers.
At low concentrations it smells of rotten eggs, but at higher levels it paralyses the sense of smell.
The workplace exposure limit is just 5 ppm for short-term exposure and 1 ppm for long-term exposure.
Concentrations above 100 ppm can cause loss of consciousness within minutes; above 500 ppm is rapidly fatal.
H2S is heavier than air and accumulates at the bottom of manholes, wet wells, and sewer chambers.
Disturbance of sewer sediment, warm weather, and low-flow conditions all increase H2S generation.
Continuous personal gas monitors with audible alarms are mandatory for all sewer entry operations.
H2S is also flammable at concentrations between 4.3% and 46% by volume in air.
Rescue of an H2S casualty in a sewer requires breathing apparatus — entry without BA is suicidal.
The Confined Spaces Regulations 1997 and COSHH 2002 both apply to work in sewers with H2S risk.

Rapid lethality	H2S kills faster than almost any other industrial gas — a single breath at high concentration causes immediate collapse.
Smell deception	Workers cannot rely on smell to detect dangerous levels — H2S paralyses the olfactory nerve above 100 ppm.
Rescue danger	Attempting rescue without BA in an H2S atmosphere turns rescuers into additional casualties within seconds.

Do

Don't

Wear a continuous personal H2S gas monitor with audible and vibrating alarms
Monitor the atmosphere at manhole or chamber openings before and during entry
Ventilate the sewer space with forced fresh air supply before and throughout entry
Ensure a rescue team with breathing apparatus is on standby before anyone enters
Set the gas monitor alarm at 5 ppm and evacuate immediately if it activates
Follow confined space entry procedures for all sewer access operations
Brief all workers on the properties of H2S including the smell paralysis effect
Keep escape routes clear and know the fastest way out at all times
Avoid disturbing sediment deposits that release trapped H2S gas
Carry out a COSHH assessment specific to H2S for every sewer entry task

DON'T rely on your sense of smell to detect H2S — it stops working at dangerous levels
DON'T enter a sewer space without a calibrated personal H2S gas monitor
DON'T attempt rescue of an H2S casualty without self-contained breathing apparatus
DON'T enter a sewer without forced ventilation operating and confirmed as effective
DON'T disturb sewer sludge or sediment without reassessing the H2S levels
DON'T ignore the gas monitor alarm — leave the space immediately every time
DON'T work in sewers during hot weather without increasing ventilation and monitoring
DON'T assume low levels at the opening mean low levels at the working depth
DON'T re-enter after an alarm without retesting the atmosphere and getting clearance
DON'T underestimate H2S — it is the single biggest killer in confined space incidents

See also: Atmospheric Monitoring and Gas Testing | Confined Space Entry Procedures

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