Basic Level Pre-Course Reading

Chapter 5
Control the Situation
usually forms a pear-shaped plume as it travels downwind. High wind speeds will
break up the cloud and disperse it more rapidly.
• Air stability is another critical factor. During hot, sunny days, the ground-level air
tends to rise, taking the toxic cloud with it. These unstable conditions usually limit
the downwind distance over which the cloud will travel. During evenings and
overcast days, the ground air tends to stay low and so will the cloud.
• High ambient air temperature tends to volatilize a chemical agent more readily,
increasing the amount of vapour hazard near non protected persons.
• Low ambient air temperature tends to decrease the volatility of chemical agents.
Some agents such as RD mustard will freeze at 14 º C.
• Precipitation, particularly mist or heavy rain, will knock down agent vapours and
aerosols.
• Cloud cover and sunlight together have several effects on a toxic cloud. During
the day, clear skies and light cloud cover normally result in unstable conditions.
Increased cloud cover results in more stable conditions. At night, overall conditions
tend to be more stable, regardless of cloud cover.
Terrain is another important factor. The presence of buildings and features such as
hills, valleys, and trees, can have a considerable effect on the wind direction and the
degree of air turbulence. In a city with tall buildings, the wind can move in different
directions within a few blocks. Such prominent features may also break up a vapour
cloud.
Type and quantity of the agent are also factors. Some agents evaporate relatively
quickly and create a dense, short-lived vapour hazard. Other agents evaporate very
slowly and could create a low-density cloud of long duration.
Dissemination methods also affect cloud formation. Vapour from a liquid puddle
will form a less dense cloud than one from a pressurized spraying device. The use of
an explosive to disperse the agent will instantly create a vapour cloud. The
characteristics of the vapour cloud depend on the type and amount of agent, and the
skill of the terrorist in the construction of an explosive device.
5.2 Perimeters and Hazard Zones
Inner (safety) and Outer (security) perimeters should be quickly established by those
first on scene. It is critical to correctly cordon off the area. This is a primary safety
measure that will contribute to effectively controlling the situation.
Normally the Emergency Response Guidebook provides stand-off distances when
commercial Dangerous Goods are involved. In a terrorist incident, there will rarely be
Dangerous Goods symbols or placards to assist in applying these guidelines. In lieu of
this information, a “worst-case scenario” approach should be adopted. If the source of
contamination is enclosed, then the minimum distance for the inner (safety) perimeter
CBRN First Responder Training Program
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Basic Level Course