The consequence assessment of a major accident involving a release of toxic gas by a pipeline is usually performed through the evaluation of the associated hazard area, which is an area where the concentration of the toxic substance exceeds a fi xed threshold level and induces harmful effects in people and the environment; its extension represents a signifi cant source of information required for the development of both safety and security strategies associated with dangerous pipelines. Since

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... e threshold level adopted in the calculation strongly affects the extent of this hazard area, the purpose of this paper is to analyse such infl uence and the potential implications on the decisional process concerning prevention, preparedness and response actions in the case of major accidents. The paper describes the methodological approach adopted for this purpose, as well as the main results obtained using the threshold levels most commonly applied in the industrialized countries. Although the estimate of the hazard areas involves a high level of uncertainty, this study aims at supporting the development of pipeline safety and security strategies, thus increasing the overall safety level in this vital sector.