REM/General/TBT-REM-004

Ground Gas Monitoring

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Ground Gas Monitoring

Toolbox Talk Record

Ref: TBT-REM-004  |  Issue: 1  |  Date: March 2026
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What?

  • Ground gases include methane, carbon dioxide, hydrogen sulphide, and volatile organic compounds.
  • These gases migrate through soil from landfills, natural deposits, contaminated land, or sewers.
  • Ground gas can accumulate in excavations, confined spaces, and enclosed structures to dangerous levels.
  • Methane is explosive at concentrations between 5% and 15% in air and is odourless.
  • Carbon dioxide displaces oxygen and can cause asphyxiation above 1.5% concentration by volume.
  • Hydrogen sulphide is toxic and deadens the sense of smell at high concentrations without warning.
  • Monitoring must be carried out before entry, continuously during work, and at regular intervals.
  • Instruments must be calibrated to manufacturer guidelines with records maintained on site.
  • The Confined Spaces Regulations 1997 and CIRIA C665 set out ground gas monitoring requirements.
  • All operatives in at-risk areas must understand alarm thresholds and required response actions.

Why?

Prevent explosionsMethane in enclosed spaces has caused fatal explosions on UK construction sites.
Avoid asphyxiationOxygen-depleted atmospheres from CO2 displacement can kill without warning.
Toxic exposureH2S and VOCs cause serious illness or death at low concentrations.
Legal requirementEmployers must assess and control ground gas risks under health and safety regulations.
Do Don't
  • Carry out a desk study to identify ground gas sources before work starts.
  • Monitor for methane, CO2, O2, and H2S as a minimum at all excavations.
  • Calibrate gas detection instruments to the manufacturer's schedule.
  • Record all monitoring results and compare against trigger and action levels.
  • Ventilate excavations and enclosed spaces before and during work.
  • Ensure all operatives know alarm thresholds and the evacuation procedure.
  • Use intrinsically safe equipment where flammable gases may be present.
  • Re-monitor after breaks, shift changes, or any change in conditions.
  • Include ground gas risk in the site-specific risk assessment.
  • Report any unexpected odours, readings, or symptoms immediately.
  • DON'T enter any excavation without checking the atmosphere first.
  • DON'T rely on smell to detect gases — some are odourless or deaden senses.
  • DON'T use uncalibrated or out-of-date gas detection equipment.
  • DON'T ignore low-level readings — they can increase rapidly.
  • DON'T smoke, use naked flames, or create sparks near gas risk areas.
  • DON'T assume gas levels are safe because they were clear earlier today.
  • DON'T work in oxygen-depleted atmospheres without breathing apparatus and rescue.
  • DON'T store monitoring equipment near contaminants affecting sensor accuracy.
  • DON'T dismiss headaches or dizziness — these may indicate gas exposure.
  • DON'T skip monitoring because the site looks or smells normal.

See also: Contaminated Land Safety Awareness | Atmospheric Monitoring and Gas Testing
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