Overview

The third of the major hazards, after fire and explosion, is a release of a toxic chemical. The hazard presented by a toxic substance depends on the conditions of exposure and on the chemical itself. It ranges from a sudden brief exposure at high concentration to prolonged exposure at low concentrations over a working lifetime.

Both these situations present serious hazards. A very large release of a toxic chemical such as chlorine under the most unfavourable conditions is usually regarded as having a disaster potential greater than that of fire or explosion. However, a large toxic release is a much less frequent occurrence than a large fire or explosion.

The First Report of the ACMH (Harvey, 1976) states:

With toxic materials, the sudden release of very large quantities could conceivably cause even larger numbers of casualties than a flammable escape. In theory such a release could, in certain weather conditions, produce lethal concentrations in places 20 miles from the point of release but the actual number of casualties (if any) would depend on the population density in the path of the cloud and the effectiveness of the emergency arrangements that might include evacuation.

In a quite different way the long-term toxic effects of exposure at even low concentrations of certain chemicals over a working lifetime can also affect large numbers of people.

The aspects of the problem considered here are the general effects of toxic substances, the limit values set for these, the effects of particular toxic chemicals and...