TUN/General/TBT-TUN-007

Tunnel Ventilation Requirements

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Tunnel Ventilation Requirements

Toolbox Talk Record

Ref: TBT-TUN-007  |  Issue: 1  |  Date: March 2026
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What?

  • Tunnel ventilation provides fresh air to workers and removes hazardous gases, dust, and diesel fumes.
  • Without adequate ventilation, oxygen levels deplete and toxic gases accumulate in the tunnel heading.
  • Diesel plant operating underground generates carbon monoxide, nitrogen dioxide, and particulate matter.
  • Ventilation systems typically use large-diameter ducting with axial fans forcing fresh air to the face.
  • The minimum fresh air supply is generally 6 cubic metres per minute per person underground.
  • Continuous atmospheric monitoring for oxygen, CO, NO2, and methane is required in all tunnels.
  • Ventilation duct must be maintained intact; tears, disconnections, or blockages reduce airflow critically.
  • The Confined Spaces Regulations 1997 apply to tunnels where the atmosphere is or may become hazardous.
  • Ventilation must be maintained during all breaks and shift changes to prevent gas build-up.
  • Emergency ventilation procedures must be in place for fire, gas release, or fan failure events.

Why?

Prevent asphyxiationWorkers have died in tunnels from oxygen depletion and toxic gas accumulation when ventilation systems failed or were inadequate.
Diesel fume exposureDiesel exhaust emissions in enclosed tunnels cause serious respiratory disease and cancer with prolonged exposure.
Legal requirementThe Confined Spaces Regulations 1997 and COSHH 2002 require adequate ventilation where hazardous atmospheres may develop.
Fire safetyVentilation systems are critical for smoke management during tunnel fires and for maintaining tenable escape routes.
Do Don't
  • Ensure the ventilation system is running before anyone enters the tunnel.
  • Monitor oxygen, CO, NO2, and methane levels continuously at the heading.
  • Inspect ventilation ducting daily for tears, disconnections, or blockages.
  • Maintain ventilation during all breaks, shift changes, and overnight periods.
  • Know the emergency procedure for ventilation fan failure or gas alarm.
  • Report any damage to ventilation ducting or equipment immediately.
  • Position atmospheric monitors at the face and at regular intervals behind.
  • Use electric plant in preference to diesel where possible underground.
  • Ensure backup ventilation or emergency procedures exist for fan failure.
  • Brief all tunnel workers on gas alarm thresholds and evacuation routes.
  • DON'T enter the tunnel until the ventilation system is confirmed running.
  • DON'T switch off or reduce ventilation during breaks or shift changes.
  • DON'T ignore tears, holes, or disconnections in ventilation ducting.
  • DON'T bypass or disable atmospheric monitoring alarms for any reason.
  • DON'T operate diesel plant underground without adequate exhaust extraction.
  • DON'T block or restrict airflow through the tunnel with stored materials.
  • DON'T continue working if gas alarm thresholds are reached; evacuate immediately.
  • DON'T assume the atmosphere is safe because it was tested hours ago.
  • DON'T re-enter the tunnel after a gas alarm without authorisation.
  • DON'T obstruct access to emergency ventilation controls or escape routes.

See also: Tunnelling Safety Awareness | Emergency Evacuation From Tunnels
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