Scenario-Based Risk Assessment

The relative importance of metabolic transformation of precursor compounds in exposure to PEGS and PFGA has been scarcely evaluated and, to our knowledge, the only study that afforded the problem by a Scenario-Based Risk Assessment (SceBRA) approach estimated the relative importance of precursor-based doses of PEGS and PFGA of 2-5% and 2-8% in an intermediate scenario and 60-80% and 28-55% in a high-exposure scenario. This indicates that these precursors are of low importance for the general population. 

Scenario-based safety risk assessment, where the calculation estimates the frequency with which the hazardous scenario will lead to the calculated consequence (a certain number of fatalities within the total exposed population). The distinction between this calculation and an Individual Risk calculation is that this calculation does not focus on any specific individual but instead considers and aggregates the impact on the whole population. A single scenario-based risk assessment does not account for all the sources of harm to which an individual may be exposed in a given establishment. When scenario-based LOPA is carried out. Individual Risk should also be considered to ensure that Individual Risk limits are not exceeded. 

Table 8 Is based on HSE s Guidance on ALARP decisions in control of major accident hazards (COMAH) SPC/Permissioning/12. Note that a scenario-based risk assessment with a single fatality is not the same as an Individual Risk calculation. 

Risk Assessment. This model successfully described the disposition of chloroform in rats, mice and humans following various exposure scenarios and developed dose surrogates more closely related to toxicity response. With regard to target tissue dosimetry, the Corley model predicts the relative order of susceptibility to chloroform toxicity consequent to binding to macromolecules (MMB) to be mouse > rat > human. Linking the pharmacokinetic parameters of this model to the pharmacodynamic cancer model of Reitz et al. (1990) provides a biologically based risk assessment model for chloroform. 

SCCP concentrations in river water were approximately half of those in effluents from sewage treatment plants, which indicates that the measured concentrations of river water were extremely high, considering that usual concentrations are about 10% of those in effluents. Thirdly, based on the assumption that the measured concentrations have distribution with the mean of measured concentrations and the standard deviation of SS, the 95th percentile is calculated extremely high. Therefore, it is considered relevant to assume this as a worst case scenario for risk assessment. 

The distinction between an individual Risk assessment and a scenario-based safety assessment is important for how the consequence is calculated and for how this Is presented in the LOPA. It Is of particular relevance to how some protection layers (In particular evacuation, see paragraphs 118-122) and conditional modifiers (probability of presence and probability of fatality, see paragraphs 142-145) are applied. 

Table 8 Risk matrix for scenario-based safety assessments...

Maqsood, L, Jianbing, L., Guohe, H. Uefei, H., 2005. Simulation-based risk assessment of contaminated sites under remediation scenarios, planning periods, and land-use patterns a Canadian case study. Stochastic Environ Research and Risk Asses, 19 146 157. 

The next part of the procedure involves risk assessment. This includes a deterrnination of the accident probabiUty and the consequence of the accident and is done for each of the scenarios identified in the previous step. The probabiUty is deterrnined using a number of statistical models generally used to represent failures. The consequence is deterrnined using mostiy fundamentally based models, called source models, to describe how material is ejected from process equipment. These source models are coupled with a suitable dispersion model and/or an explosion model to estimate the area affected and predict the damage. The consequence is thus determined. 

The degree of confidence in the final estimation of risk depends on variability, uncertainty, and assumptions identified in all previous steps. The nature of the information available for risk characterization and the associated uncertainties can vary widely, and no single approach is suitable for all hazard and exposure scenarios. In cases in which risk characterization is concluded before human exposure occurs, for example, with food additives that require prior approval, both hazard identification and hazard characterization are largely dependent on animal experiments. And exposure is a theoretical estimate based on predicted uses or residue levels. In contrast, in cases of prior human exposure, hazard identification and hazard characterization may be based on studies in humans and exposure assessment can be based on real-life, actual intake measurements. The influence of estimates and assumptions can be evaluated by using sensitivity and uncertainty analyses. - Risk assessment procedures differ in a range of possible options from relatively unso-... 

It is advisable, then, in a tiered approach to concentrate first on crops and activities (scenarios) that are considered to be relevant with respect to the expected level of exposure and to exclude those not relevant. Second, whether or not the toxicological properties of the product may lead to general restrictions on re-entry should be investigated. If both the likelihood of reentry and the hazard due to the toxicity of the compound cannot generally be neglected, a risk assessment over several steps should be carried out. The assessment may be based on surrogate data and "worst-case" assumptions at first and then refined, if necessary. One possible approach to a tiered evaluation procedure is presented in Figure 1. 

Based on these failure modes, a detailed risk assessment of most probable or most severe hydrogen accident scenarios has been conducted in some studies [66] including fuel tank... 

Transfer waivers are justifiable, in the very specific situations where an inter-laboratory trial is not justified based on an appropriate risk assessment (ICH Q9, 2005), the following examples reflecting scenarios that are typically encountered ... 

In order to avoid cross-contamination, another concern with respect to equipment is the cleaning process, which must comprise cleaning SOPs to ensure adequate cleanliness. Cleaning validation must be performed based on risk assessment and worst-case scenarios. 

The dominance of the risk to inadvertent intruders at near-surface waste disposal sites allows the use of this type of scenario to develop a risk-based waste classification system. However, NCRP recognizes that exposures of the public and protection of the environment also are of concern in determining acceptable disposal practices at specific sites. The potential for off-site releases of hazardous substances is the primary reason that classification of waste based on risks to hypothetical inadvertent intruders does not obviate the need for site-specific risk assessments to determine waste acceptance criteria in the form of limits on disposal of particular hazardous substances. 

In addition, the reliance on generic scenarios for inadvertent intrusion in classifying waste cannot, by definition, represent site-specific risks. However, this is not a serious shortcoming because such scenarios have been used in establishing subclasses of low-level radioactive waste for disposal in near-surface facilities (NRC, 1982a). Furthermore, as emphasized in this Report, establishment of a risk-based waste classification system using particular exposure scenarios does not obviate the need to perform site-specific risk assessments for the purpose of establishing waste acceptance criteria at each disposal site. 

In implementing the risk-based waste classification system developed in this Report, the selection of exposure scenarios appropriate to waste disposal is an important technical issue that must be addressed. NCRP believes that scenarios for inadvertent intrusion into near-surface disposal facilities are appropriate in classifying waste for purposes of disposal and, further, that scenarios involving permanent occupancy of disposal sites after loss of institutional control would be appropriate (see Section 6.1.3) such scenarios are commonly used in regulating near-surface disposal of low-level radioactive waste and in risk assessments at hazardous waste sites subject to remediation under CERCLA. 

The scenarios were based on what we saw as important tensions in regulatory regimes. First, we saw a tension between universalising risk assessment and a... 

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