Hazards are part of the environment in which we live. It is impossible to live in a totally hazard-free environment since each day one inevitably faces some degree of personal risk from road accidents, fire, theft, floods, etc. A hazard can therefore be defined as a threatening event.
Some authors classify hazards according to the triggering reason: voluntary hazards (e.g. smoking, paragliding) and involuntary hazards (e.g. fire, earthquake). Others classify hazards into three classes according to their nature:
  • Technological hazards are those accidental failures of design or management affecting large-scale structures and transport systems, or industrial activities that present lifethreatening risks to the local community (Smith 1996).
  • Natural hazards result from those elements of the physical environment harmful to Man and caused by forces extraneous to him (Burton et al. in Smith 1996).
  • Human-induced natural hazards are those that are caused by the human modification of the environment.
Threats posed by hazards are classified by the type of loss they cause :
direct (or primary) losses and indirect (or secondary) losses. They are also categorised according to their potential effects hazards a) social or human effects, b) with physical effects and c) with economic effects.
Natural hazards are dynamic and uncertain processes dynamic because they do not always happen in isolation (as one event could trigger another, e.g. an earthquake could trigger a landslide) and because they can reshape the environment; uncertain because their occurrence is generally difficult to forecast. Natural disasters can be defined as the impact of natural hazards upon a vulnerable community, resulting in disruption, damage and casualties that cannot be relieved by the unaided capacity of locally-mobilised resources (United Nations Disaster Relief Co-ordinator 1991). Smith (1996), however, points out that there is no universally agreed definition of the scale on which loss has to occur in order to qualify an event as a disaster, although most disasters do have a number
of common features:
·                                 The origin of the hazardous event is clear and produces characteristic threats to human life or well-being (e.g. a flood causes death by drowning).  The warning time is normally short; hazards are often rapid-onset events. This means that occurrences can be unexpected even though they occur within a known hazard zone, such as the floodplain of a small river basin. Most of the direct losses, whether of life or property, are suffered fairly shortly after the event, typically within minutes or hours.
·                        The exposure to hazard, or assumed risk, is largely involuntary, normally due to the location of people in a hazardous area, such as the unplanned expansion of cities onto unstable hill slopes.
·         The disaster occurs with an intensity that justifies an emergency response, such as the provision of specialist aid to victims. The scale of response can vary from local to international.

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