Soldiers facing potential hazards and civilian rescue services both monitor conditions closely. Complex computer models can be used to predict the way an agent will spread and for how long it will be a threat.

1. Wind direction

Once released, the agent spreads downwind, fanning outwards as it disperses. The red area on the diagram is the danger zone, the area outside it is safe.

The edges of the danger zone have a lower concentration of agent than the centre.

2. Wind turbulence

The stronger the wind, the more turbulence is created as it passes over the land.

Turbulence causes the cloud of agent to disperse and become diluted faster - although this may mean it spreads over a larger area.

The risk of high casualties tends to be greatest if an attack is carried out at dawn or dusk, when conditions are more likely to be still.

3. Heat and light

Biological agents degrade when exposed to ultraviolet light, so biological attacks are most effective at night.

High temperatures make it easier for a liquid chemical agent to evaporate into a vapour which can be inhaled, but also shorten the time the agent persists on the ground.

Low temperatures make evaporation slower, meaning the agent remains in liquid form and persists longer after the attack. High humidity also slows evaporation.

4. Air Pressure

Biological and chemical attacks have a greater impact on low-pressure, overcast days.

On high-pressure days, when the air is heavier and there is less cloud, agents in gas form rise faster out of the lower levels of the atmosphere where they can cause damage.