Step 2 - Risk Estimation

Information produced from the hazard identification phase will be processed to estimate risk. In the risk estimation phase, the likelihood and possible consequences of each System Failure Event (SFE) will be estimated either on a qualitative basis or a quantitative basis (if the events are readily quantified). The risk estimation phase can be further broken down into several steps as follows  

Structuring and quantification of influence diagrams - The purpose of influence diagrams is to identify the influences, which determine the likelihood of an accident, and to enable those influences to be quantified. It also provides information for use in Step 3 of the FSA process. An example of an influence diagram for a fire accident in given in Appendix 3. An influence diagram takes into account three different types of influence, which are due to  

Additionally, each influence diagram incorporates dimensions of design, operation and recovery.  

Quantification of RCT - The quantification of the RCT is accomplished by using available historical data finm the incident database and where such data is absent, expert judgement is used to complement the quantification. The level of potential consequences of a SEE may be quantified in economic terms with regard to loss of lives/caigo/property and the degradation of the environment caused by the occurrence of the SFE. Rnally, the calculation of FN (i.e. frequency (F) - fatality (N)) curves and Potential Loss of Life (PLL) through the RCT is carried out. 

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Whenever possible, relative comparisons of risk should be made (Step 8). Comparing relative risk estimates for alternative strategies avoids many of the problems associated with interpreting and defending absolute estimates. Table 9 contains examples of typical conclusions you can reach using relative risk estimates. In some cases, however, absolute estimates may be required to satisfy your needs. Table 10 contains a list of examples of typical conclusions possible using absolute risk estimates. 

Dose-Response Evaluation The process of quantitatively evaluating toxicity information and characterizing the relationship between the dose a contaminant administered or received, and the incidence of adverse health effects in the exposed population. From a quantitative dose-respoiise relationship, toxicity values can be derived that are used in the risk characterization step to estimate the likelihood of adverse effects occurring in humans at different exposure levels. 

Risk characterization is the last step in the risk assessment procedure. It is the quantitative or semi-quantitative estimation, including uncertainties, of frequency and severity of known or potential adverse health effects in a given population based on the previous steps. Risk characterization is the step that integrates information on hazard and exposure to estimate the magnitude of a risk. Comparison of the numerical output of hazard characterization with the estimated intake will give an indication of whether the estimated intake is a health concern. ... 

The third method used to interpret the level of risk associated with chlorpy-rifos use is Monte Carlo simulation. This method provides a range of exposure estimates for the evaluation of the uncertainty in a risk estimate based on ranges of input variables. The first step in performing a Monte Carlo simulation is determination of a model to describe the dose. This model describes the relationship between the input parameters and dose, and a specific model is presented here for one group of workers. 

First, we investigate some of the regulatory motivations for chronic risk analysis. Next, it is necessary to point up the similarities and differences between acute and chronic risk and delineate the steps in estimating health risks posed by environmental chemicals. Following some illustrations of model structure, we conclude by discussing specific factors in fate analysis that suggest choices of model components. 

In this symposium a comprehensive overview of the risk estimation step and its relationship to the output of multimedia fate models is given in the paper by Fiksel (5). Examples of the application of and linkage among the various techniques are also presented in that paper. 

An important input to the Risk Estimation step, as shown in Figure 1, is the analysis of health effects associated with the pollutant in question. Since environmental toxicology is itself a complex and difficult field, we have confined this paper to a discussion of how dose-response estimates can be utilized within a risk assessment, with emphasis on human carcinogenesis. Thus, the scope of this paper corresponds to the four steps surrounded by a dashed line in Figure 1. 

Because estimates of health risk are based on the levels of radionuclides in or near the vicinity properties, the quality of the potential health risk estimates depends upon the availability of appropriate measurement data. Hence, the first steps involved the determination of the appropriate environmental pathways of exposure and developing the source term for the exposure of persons potentially at risk. For our work, the radiological source-term data was based on measurements made principally by the Oak Ridge National Laboratory and the Mound Laboratory. 

When applied to a particular site and/or project, RA procedures include several generic steps such as hazard identification, hazard assessment, risk estimation and risk evaluation. 

It is most important that the whole life cycle of a process plant can be evaluated on safety. Safety and risk analyses evaluate the probability of a risk to appear, and the decisions of necessary preventative actions are made after results of an analysis. The aim of the risk estimation is to support the decision making on plant localization, alternative processes and plant layout. Suokas and Kakko (1993) have introduced steps of a safety and risk analysis in Figure 2. The safety and risk analysis can be done on several levels. The level on which the analysis is stopped depends on the complexity of the object for analysis and the risk potential. 

If yes, further evaluation of additivity and interactions is necessary for the components of concern (having risk estimates equal to or greater than 1 X 10 ). Go to step 4. 

The Monographs represent the first step in carcinogenic risk assessment, which involves examination of all relevant information in order to assess the strength of the available evidence that certain exposures could alter the incidence of cancer in humans. The second step is quantitative risk estimation. Detailed, quantitative evaluations of epidemiological data may be made in the Monographs, but without extrapolation beyond the range of the data available. Quantitative extrapolation from experimental data to the human situation is not undertaken. 

Integration of the results of the first three steps in a risk assessment typically results in a quantitative estimate of risk. Estimated... 

Risk Estimation. In this step, the effect per unit of exposure is estimated. For genetic effects, there are two components estimation of damage to germ cells, and estimation of impact on future generations. To estimate genetic risk, results from germinal tests in the mouse can be used to extrapolate to possible human effects. 

If carcinogenicity data are unavailable or inconclusive, the next step (Level 4) is to test for mutagenicity in the mouse. In many cases, the presence or absence of mutagenicity in the short-term tests and in the mouse may be sufficient evidence for a decision. In the most difficult cases, a quantitative risk estimate may be needed. 

Constraints, uncertainties and assumptions having an impact on the risk assessment should be explicitly considered at each step in the risk assessment and documented in a transparent manner. Expression of uncertainty or variability in risk estimates may be qualitative or quantitative, but should be quantified to the extent that is scientifically achievable. 

The final step, risk characterization, involves the integration and analysis of the existing database to provide a numerical estimate of the incidence of the adverse effect in a given population, assuming specific conditions of exposure. 

Risk characterization is the most important and final part of a risk assessment. It summarizes and interprets the information from hazard identification, dose-response, and exposure steps, identifies the limitations and uncertainties in risk estimates, and communicates the actual likelihood of risk to exposed populations. The uncertainties identified in each step in the risk assessment process are analyzed and the overall impact on the risk estimate(s) is evaluated quantitatively and/or qualitatively. 

The need for certain analyses may not be identified until after the risk estimation step. For example, the need to analyze the risks to a fish population (an assessment endpoint) due to an indirect effect such as zooplankton mortality (a measurement endpoint) may not be established until after the risk to zooplankton has been characterized. In such cases, another iteration through analysis or even problem formulation may be necessary. 

The interpretation of ecological significance places risk estimates in the context of the types and extent of anticipated effects. It provides a critical link between the estimation of risks and the communication of assessment results. The interpretation step relies on professional judgment and may emphasize different aspects depending on the assessment. Several aspects of ecological significance that may be considered include the nature and magnitude of the effects, the spatial and temporal patterns of the effects, and the potential for recovery once a stressor is removed. 

Risk estimate Risk characterization Integrates the results of the previous steps into a risk statement that includes one or more quantitative estimates of of risk. y Risk characterization Estimating the incidence of an effect under the conditions of exposure described in the exposure assessment. 

Two additional steps that may be inclnded as part of a QRA inclnde sensitivity analysis and the evaluation of risk reduction options. Sensitivity analyses are often used to evaluate the influence of various data components and assumptions of the QRA. One should always remember that the risk analyst is dealing with risk estimates, and, in order to use these estimates properly, it is essential that the potential extent of uncertainty or key assumptions that are a major influence on the risk results be known and understood. 

When applied to teehnological risks, in particular oil and gas exploration, the phase of interdisciplinary risk estimation includes four major steps ... 

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