Major Hazard: An imprecise term for a large scale chemical hazard, especially one which may be realised through an acute event. Or, a popular term for an installation, which has on its premises, a quantity of a dangerous substance which exceeds the amount prescribed by the above references. A term of recent origin is Major Accident Hazard. This term occurs in the title of the EEC Directive of 24th June 1982 and is synonymous with major hazard. A substance constitutes a hazard by virtue of its intrinsic chemical properties or of its temperature and pressure, or some combination of these. For example, air and water may pose a hazard if compressed and heated, but neither would be classed as a Hazardous Substance, as their chemical properties alone do not constitute a hazard. The term hazardous substance may be defined generally as follows: (1) Hazardous Substance: A substance which by virtue of its chemical properties constitutes a hazard. In the context of the NIHHS Regulations the term hazardous substance has a more specific meaning, applying only to those substances listed in the regulations or meeting specified indicative criteria. Similarly, the term Dangerous Substance has a specific meaning under the CIMAH regulations it is recommended that this term is used only with this specific meaning the CIMAH regulations. It is recommended that this term is used only with this specific meaning and that if a general term is required hazardous substance should be used. (2) Dangerous Substance: A specific term defined in the CIMAH Regulations referring to listed substances and others meeting given criteria. In assessing the threat posed by a hazard, the principal factors are the likelihood that it may be realised, and the likelihood and extent of the consequences, i.e. damage to people, property or the environment, in the event of its realisation. The term which expresses likelihood in the present context is risk. Little controversy surrounds this point; the controversy around the use of “risk” is whether it may also be used to mean other quite different things. For example, whether it should also have the meaning attributed to “hazard” as defined above or a combination of the meaning of “hazard” with the meaning of likelihood. This practice of giving a number of meanings to “risk” is common where meanings are commercial rather than scientific and where their meaning has to be deduced from the context. The Working Party recommends that the word “risk” should only be used to mean the likelihood of some specified undesired event. This would avoid the public confusion, which arises when an installation is described using one meaning, as “high risk”, and using another as “low risk”; it will encourage people to consider the “risk of something happening: (3) Risk: The likelihood of a specified undesired event occurring within a specified period or in specified circumstances. It may be either a frequency (the number of specified event occurring in unit time) or a probability (the probability of a specified event following a prior event), depending on the circumstances. When considering the risk of harm to populations exposed to hazards, it is helpful to consider two derivatives to risk. In cases where the potential is large, many factors dictate the severity, which might be realised, and there is a wide spectrum of possible harmful outcomes with associated likelihood’s. This is known as the Societal Risk. In this case, the undesired event in our definition of risk is an accident, which can affect a group of people. This is usually quantified as an F-N curve. Individuals amongst the population who could be affected by such an accident will not usually be exposed equally. This distribution of the risk is illustrated by considering the likelihood of particular individuals being affected, known as the INDIVIDUAL RISK. In this case, the undesired event in our definition of risk is harm to a specific individual (or person living at a particular location). Individual risk can, of course, be used in the limiting case where only one individual could be affected in an accident. (4) Societal Risk The relationship between frequency and the number of people suffering from a specified level of harm in a given population from the realisation of specified hazards. (5) Individual Risk The frequency at which an individual may be expected to sustain a given level of harm from the realisation of specified hazards. The usefulness and limitations of some of the above concepts can be illustrated by an example. One might assume that 10 tonnes of a toxic material poses a greater threat than 1 tonne. However, whether any quantity actually poses a hazard depends upon the circumstances under which it is held. In the case of bulk storage, the larger quantity evidently has more potential for causing harm since, where released rapidly under the same conditions, harmful concentrations would extend to a greater distance. Although the larger quantity, in this case, could be said to pose a greater hazard, the magnitude of the effects resulting from an actual release depends on many other factors. There may be no people within this range of harmful effects and therefore no risk of injury. If there are people present, the number affected may vary depending on the wind direction, weather conditions and other factors as well as the quantity released. The likelihood of various sizes of detrimental effects to a population has been defined as the societal risk. The maximum number of people that could be harmed in an accident and the associated probability of occurrence is part of this concept. This maximum possible number affected in any one accident will usually be less than the total number of people who are within the range of possible harmful effects. Societal risk tells one nothing about its geographical distribution. People in the prevailing downwind direction from the storage would be more likely to be affected than those in the opposite direction. This will be reflected in the level of individual risk at such locations.