Most chemical accidents occur within the workplace, and may have no direct, large-scale or long-term effects. On such a limited scale, a rapid assessment is a relatively simple undertaking.

However, when a large number of people and a wider area are exposed to a chemical hazard, the assessment becomes a major exercise.

Most accidents occur at the interfaces between transport, storage, processing, use and disposal. This is where there are the fewest controls and the greatest probability of poor practices.

The risks of accidental chemical releases are greater with the number of new hazardous substances produced. First, production, storage, transport, and use of flammable, explosive or toxic chemicals have grown significantly in both developing and developed countries. Second, greater and more centralized productions have increased the quantities of chemicals manufactured and the distances they are transported. Third, population growth close to chemical plants and along transportation routes has meant that there are larger communities in greater number at high risk following a chemical accident.

The health impact of a chemical exposure is determined by the chemical itself, the exposure routes, and the amount of exposure. Exposure pathways vary, depending on the stage of the release. During the release, health effects from dermal exposure and inhalation can be expected. In the post-impact phase, the greater risks are dermal exposure, through contact with contaminated objects, and ingestion of contaminated food or water.

In many countries the ministry of health may not be responsible for managing chemical emergencies. However, the considerable health impact of a major chemical emergency calls for the active involvement of the health sector in the emergency preparedness process and in the assessment. The health sector should work closely with government agencies responsible for fire and rescue, paramedical services, security, environment, transport and dangerous goods.

Chemical incidents can cause an emergency:

- by acute release (e.g. exposure to corrosive effects of ammonia and gas used as refrigerant); and

- by chronic release (e.g. pyrrolizidine alkaloids found in plants that contaminate staple food crops and produce liver disease).

Also food contamination with chemicals or toxins can produce acute or slow onset emergencies that, either way, have long-term effects.

This protocol will focus on assessing an acute chemical release which requires an immediate response.

As discussed in Chapter 1, a chemical emergency should be first assessed within 24 hours following the incident at the latest. A more comprehensive assessment should be carried out later. Box 4 contains a sample checklist for rapid health assessment in chemical emergencies.