Definition and characteristics

Toxic hazard is a difficult notion to understand. It relates to certain targets’ exposure to a hazard which corresponds with a cloud generated following accidental discharge of a chemical substance into the atmosphere (leak, tank failure etc.).
Toxic gas discharge can be continuous (jet) or short-lived (puff). It is dispersed into the atmosphere under the effect of its initial velocity, the gravity effect due to gas density, wind, atmospheric conditions (atmospheric stability, i.e. vertical temperature profile), hygrometry and relief.

Atmospheric dispersion corresponds with the evolution of a cloud of hazardous products over time and space. Two simultaneous phenomena occur when a gas is dispersed into the atmosphere. These phenomena are gas transport, notably under the effect of the wind, and gas dissemination. During its dissemination, the cloud absorbs air and therefore dilutes. Its toxicity thus decreases as the distance from the discharge point increases.

The parameters most influential on distant toxic effects are as follows:

• Gas density: the higher the density, the harder the dissemination of the cloud into the atmosphere. A heavy gas stays on the ground without diluting much with air; it therefore remains rather concentrated over vast transport distances
• Orographic conditions (wind velocity and atmospheric stability): for example, wind will carry the gas further as its speed increases. Similarly, a turbulent atmosphere increases gas dissemination, consequently reducing cloud toxicity as the distance increases. Conversely, a stable atmosphere (i.e. with a vertical temperature decrease higher than that due to thermodynamics alone) tends to prevent gas molecules from rising; this atmosphere is therefore adverse as it prevents vertical dissemination.

How toxic gas dispersion occurs

There are numerous possible causes, mostly including:

• Capacity or pipe rupture (for example as a result of a hazardous chemical reaction)
• Ignition of certain products.

Toxic dispersion effects

Toxic gases are characterised by their effects on man by the notion of dose, taking into account gas concentration in the air and time of exposure to this atmosphere.

Three thresholds are distinguished:

• The irreversible effects threshold
• The first lethal effects threshold (corresponding with a lethal concentration for 1% of the people exposed)
• The significant lethal effects threshold (corresponding with a lethal concentration for 5% of the people exposed).

Gases with high toxicity and high density, such as chlorine, are particularly dangerous: significant effects can reach distances of up to ten kilometres for the most serious types of discharge (example: total failure of a large capacity).

How to prevent and protect against toxic dispersion

Protection resources against the causes of toxic leakage will not be examined in great detail.
There are several ways to ensure protection against toxic effects, for example:

• By measures designed to limit discharge as near as possible to the source of this discharge: for example, emergency services can put up water curtains to enhance gas dissemination (dilution) or even absorb part of these gases by contact with the water (ammonia responds very well to this)
• By measures designed to shelter (referred to as containment) potential targets,
• By personal protection measures prescribed in case of accident (for example gas masks for the employees of an industrial site etc.)

For further information:

How are risks studied?
Atmospheric dispersion: data sheet 2 of the Circular of 28 December 2006