Cordon control: Unignited flammable vapours

Control Measure Knowledge

This control measure should be read in conjunction with Cordon controls: Hazardous materials


An unignited vapour cloud at concentrations above its ‘lower explosive limit’ (LEL) creates a dangerous atmosphere and the area should be evacuated. As flammable vapours are generally invisible, their exact presence can only be determined by monitoring equipment.

The presence of flammable vapour clouds can be predicted where there is a release of flammable gases or liquids. As flammable gases are invariably stored under pressure, if gas is escaping from its container, the temperature will drop. Using a thermal imaging camera can therefore assist in identifying leaking or open cylinders.

The extent of this hazard area will depend on:

  • The volume of release
  • Weather conditions, such as wind speed, direction, etc.
  • Local topography, proximity of storage tanks, buildings, pipelines, etc.
  • The explosive limits of the gas (N.B. For practical purposes explosive and flammable limits are the same)
  • Vapour density

Flammable vapour clouds can extend significant distances from the point of release. An effective cordon will need to be extended beyond the flammable vapour cloud to include the area to which radiant heat would spread if ignition occurred. Therefore, for large leaks of flammable liquids that produce significant vapour clouds, the initial cordons may be reduced or extended according to explosimeter monitoring, using a safe percentage of the lower explosive limit (LEL).

See National Operational Guidance: Hazardous materials – Contaminated members of the public

Strategic Actions

Tactical Actions