Hydrogen sulphide is a colourless gas that is present at dangerous levels in wastewater treatment works, sewers, and pumping stations. At low concentrations it smells of rotten eggs, but at higher concentrations it paralyses the sense of smell, giving a false sense of safety. H2S is immediately dangerous to life at 100 parts per million and has caused rapid fatalities on UK water industry sites. Personal gas monitors and continuous area monitoring are essential wherever H2S may be present.
Key Hazards
Rapid loss of consciousness and death from H2S exposure above 100 ppm
Olfactory fatigue at higher concentrations making the gas undetectable by smell
Multiple fatalities from rescuers entering H2S-contaminated spaces without protection
Chronic health effects from repeated low-level H2S exposure over time
Control Measures
Carry a personal H2S gas monitor at all times when working on any wastewater treatment site.
Ensure the gas monitor is bump-tested at the start of every shift before entering any work area.
Know the alarm levels: low alarm at 5 ppm, high alarm at 10 ppm, and IDLH at 100 ppm.
Move upwind immediately if your personal H2S monitor alarms and raise the alarm to others.
Never enter below-ground chambers, tanks, or channels without atmospheric testing and a permit.
Ensure forced ventilation is operating in any enclosed area where H2S accumulation is possible.
Never rely on your sense of smell to detect H2S because it paralyses the olfactory nerve at high levels.
Ensure breathing apparatus and a trained rescue team are available for all H2S risk areas.
Attend H2S awareness training before starting work on any wastewater or sewer-related site.
Emergency / Rescue
If someone collapses in a suspected H2S atmosphere, do not enter without breathing apparatus. Raise the alarm, call 999, and activate the rescue plan. Rescuers entering without BA have been killed attempting to save H2S casualties.
Remember
H2S is immediately dangerous to life at 100 ppm and can kill within a single breath at high concentrations.
You cannot rely on smell to detect H2S because it paralyses your sense of smell above 100 ppm.
Personal H2S monitors must be worn and bump-tested every shift on all wastewater sites.
Never attempt to rescue someone from an H2S atmosphere without breathing apparatus.
Below-ground chambers and tanks on WwTW sites must always be treated as H2S risk areas.
H2S is heavier than air and accumulates in low-lying areas, pits, and below-ground structures.
Applicable Legislation: Confined Spaces Regulations 1997 · COSHH Regulations 2002 · EH40 Workplace Exposure Limits · Health and Safety at Work etc. Act 1974