The terms “chemical accident” and “chemical emergency” are used to refer to an event or dangerous occurrence resulting in the release of a substance or substances hazardous to human health and/or the environment in the short or the long term. Such events or occurrences include fires, explosions, leakages or releases of toxic substances that can cause illness, injury, disability or death often to a large number of human beings.

While dispersed populations may be affected through contamination of water or the food chain resulting from a chemical, the exposed population is frequently either inside or outside an industrial site. In an urban area, the exposed population may be in the vicinity of a ruptured vehicle that has been transporting hazardous substances. Less frequently, the exposed population is at some distance from the accident site, including possibly areas across national borders. Potentially affected areas in neighbouring countries could include those with limited chemical emergency response plans or capabilities.

This definition needs to be set alongside the concept of a “chemical accident”, in which an exposure arising from releases of a substance or substances may result in illness or the possibility of illness. The number of people affected by a chemical accident may be very few (even just one), and illness, disability or death may occur a considerable time (for example, several years) after the exposure.

In addition to human health effects, chemical accidents may result in extensive or long-term damage to the environment, with considerable human and economic costs.