A risk index (Risk Severity Index, S) has been devised to allow the assessment of the risk level originated by a given installation or site over the affected zone. A set of threshold levels for thermal radiation, toxic concentration and overpressure, together with the probabilities and frequencies associated to critical events and their effects have been the basis for calculating the values of S. A computer tool has been designed to perform a quick calculation of the diverse Risk Severity Indexes (for a critical event, for a dangerous phenomenon, for a type of effect and for the whole installation) and to plot a map of the risk severity levels around the site. The methodology has been applied to diverse test cases and it has proved to be useful for risk assessment, for comparative studies and for land use planning.