## 2 Theoretical Basis about Risk
### ALARP Risk Evaluation Criteria  
Risk assessment is widely adopted in the safety field where it is necessary to safeguard human life from the consequences of dangerous activity performance. In this sector, risk management was traditionally based on a prescriptive regulation regime, in which the requirements for the design and operation of plants were specified in detail. Over time, there has been a gradual shift towards goal-oriented regimes that place more emphasis on the target to be achieved rather than the means to do so. This has led to integrated risk governance approaches that generate higher levels of performance in terms of both productivity and risk reduction [7; 8; 9; 10]. In these approaches, the Quantitative Risk Assessment (QRA) is increasingly used, so much so that many regulatory agencies impose quantitative methods that can improve the economic efficiency of the decision-making related to risk management [20; 21]. Consider, for instance, the guidelines of ERM-Hong Kong Ltd. [22] for the management of landslide risk and the regulation of the Association of Professional Engineers and Geoscientists of British Columbia (APEGBC) [23] who also considers the application of quantitative assessments for land use planning. However, when riskiness is assessed in terms of the probability of loss of human life, it becomes complex to establish acceptable levels of risk as it is necessary to include in the assessment not only ethical, social, and legal issues but also political and economic ones [24; 25; 26]. For this reason, decision makers and political actors often resort to quantitative methods together with useful criteria to evaluate the tolerability/acceptability of the risk. In 1992, the British agency of the Health and Safety Executive (HSE), to manage the inherent hazard of industrial processes, defined some general principles of risk acceptance [27]:  
* _The accountability principle_. The definition of risk acceptance criteria must be clear and transparent, based on quantitative definitions and objective assessments;
* _The principle of equivalency_. The risk must be compared with limit thresholds that derive from the analysis of similar activities or similar systems to that to be assessed and who are widely recognized as acceptable or tolerable;
* _The holistic principle_. Security decisions must be based on a holistic view of all risks. Only when the overall risk of the activity itself has been examined, can mitigation measures be proposed;  
Figure 1: Risk Management Process (RMP) phases. * _The ALARP Principle._ Risks must be reduced to be _As Low As Reasonably Practicable._ In other words, all mitigation measures must be implemented until the costs appear disproportionate to the achievable benefits. In the ALARP context, threshold values are defined for \"acceptable\" risks and \"tolerable\" risks. According to ALARP principle, risks that fall below the tolerance threshold must be necessarily reduced because they are unacceptable. Risks included between the tolerability and acceptability thresholds are in the ALARP area, so it is necessary to mitigate them until they are reasonably practicable. Finally, risks above the acceptability threshold are \"widely acceptable\", so there is no need to mitigate them. The ALARP criteria, generally applied in the high-risk areas of industrial engineering, requires those responsible for the activity to reduce risks to levels that are 'As Low as Reasonably Practicable' [28]. As such, the principle implies an effective recognition that while in most circumstances risk can be reduced, beyond a certain point further risk reduction is always more costly to implement. Figure 2 shows the ALARP principle. The reference literature also distinguishes individual risk from social risk. The former is defined as the \"frequency at which an individual is expected to be subjected to a certain level of harm as a result of an accident in a major risk industry\".