The Assumption of Risk

During the past two decades there have been many reports of liver failure resulting in death or transplantation in patients being treated for ADHD with pemoline. However, a descriptive meta-analysis of the existing scientific literature and drug reporting databases showed that current assumptions of the risk of acute hepatic failure posed by pemoline alone are overestimates. 

Another way to evaluate risks is to calculate the sensitivity of the total risk estimates to changes in assumptions, frequencies, or consequences. Risk analysts tend to be conservative in their assumptions and calculations, and the cumulative effect of this conservatism may be a substantial overestimation of risk. For example, always assuming that short-term exposure to chemical concentrations above some threshold limit value will cause serious injury may severely skew the calculated risks of health effects. If you do not understand the sensitivity of the risk results to this conservative assumption, you may misallocate your loss prevention resources or misinform your company or the public about the actual risk. 

The relationships between the importance measures is based on the assumption that the systems are not reconfigured in response to a component outage. If this is done, the basic definition of the importance measure is still valid but there is not such a simple relationship. Disregarding this complication, some interpretations of the importances may be made. The Bimbaum Importance is the risk that results when the i-th system has failed (i.e., it is the A, term in Equation 2.8-9). Inspection Importance and RRWI are the risk due to accident sequences containing the i-th system. Fussell- Vesely Importance is similar except it is divided by the risk so may be interpreted as the fraction of the total risk that is in the sequences contains the Q-th system. The Risk Achievement Worth Ratio (RAWR) is the ratio of the risk with system 1 failed to the total risk and is necessarily greater than one. The Risk Achievement Worth Increment (RAWI) is the incremental risk increase if system 1 fails and the Risk Reduction Worth Ratio (RRWR) is the fraction by which the risk is reduced if system 1 were infallible. 

The next level of presentation is a technical summary that gives details of the risks including the system s importance measures systems, effects of data changes, and assumptions that are critical to the conclusions. It details the conduct of the analysis - especially the treatment of controversial points. The last level of presentation includes all of the details including a roadmap to the analysis so a peer can trace the calculations and repeat them for verification. 

Much of the attention focused on e.xposure assessment has come recently. This is because many of the risk assessments done in tlie past used too many conseix ative assumptions, wliich caused an ovcrcstimation of the actual exposure. Without exposures there are no risks. To experience adverse effects, one must first come into contact with the toxic agent(s). Exposures to chemicals can be via inlialation of air (brcatliing), ingestion of water and food (eating and drinking), or absorption Uu ough the skin. These arc all pathways to the human body. 

More attention lias been recently focused on exposure assessment. Tliis is because many of the risk assessments performed in tlie past used too many and overly conservative assumptions. This in turn, caused an overestimation of the actual exposure. 

Decision-making may be undertaken by an individual under assumptions of certainty, risk, or uncertainty (partial or complete ignorance) on the states of Nature. 

In any risk assessment, the analysts will need to make some assumptions. These are important as the validity of the risk analysis is dependent on the validity of the assumptions. The assumptions can be in the development of a rule set, use... 

It may often be useful to initially conduct an exposure assessment based on worst-case assumptions, and to use default values when model calculations are applied. Such an approach can also be used in the absence of sufficiently detailed data. If the outcome of the risk characterization based on worst-case exposure assumptions is that the substance is not of concern, the risk assessment for that substance can be stopped with regard to the effect/population considered. If, in contrast, the outcome is that a substance is of concern, the assessment must, if possible, be refined using a more realistic exposure prediction in order to come to a definitive conclusion. 

The Handbook also pointed out that risk characterization does not stand alone, as it is one of the four steps in risk assessment. There is only a single technical characterization of risk as a final product of the risk assessment. This technical characterization must be written with enough detailed technical information to allow another expert (e.g., other risk assessors, peer reviewers) to reasonably reconstruct what was done in the risk assessment, including to be able to identify the assumptions made during the risk assessment. Since the risk characterization is a part of the risk assessment itself, it should be kept in mind that the goal of the risk characterization is not to repeat the entire risk assessment, but just to identify the key elements from the risk assessment that really make a difference in its outcome. 

The major elements to be considered in the risk characterization part include key information, context, sensitive subpopulations, scientific assumptions, policy choices, variability, uncertainty, bias and perspective, strengths and weaknesses, key conclusions, alternatives considered, and research needs. Whether every element is actually written into the risk characterization or not, depends upon the purpose of the risk assessment and the detail necessary to adequately characterize it. By the time the risk assessment is completed, the universe of policy choices, management decisions, and uncertainties should have been identified, as well as the conclusions of the risk assessment. Because key findings differ for each risk assessment, it is not possible to define exactly what they are genericaUy. Professional judgment is necessary to define them. 

The Risk Characterization Handbook (US-EPA 2000) is thus a practical guide in how to perform the risk characterization. However, the Handbook does not include any detailed information on the practices employed in the risk assessment itself, including use of uncertainty factors and use of default and extrapolation assumptions in the risk characterization step. This information and practices are provided in the US-EPA staff paper from 2004 titled An Examination of EPA Risk Assessment Principles and Practices (US-EPA 2004). 

Identifying the assumptions and uncertainty factors or safety factors at each level of refinement of the risk assessment... 

Irrespective of the risk, assumptions and decisions will have to be made because of uncertainty. Implications of attempting to characterize all variability and uncertainty in the risk assessment need to be considered. For example, exaggerating uncertainties can obscure the scientific basis of risk management decisions, leaving the impression that the decision has been arbitrary in nature (NRC 1989). The purpose of the uncertainty factor together with the type of assessment (e.g., deterministic or probabilistic, protective or best estimate) must be clearly communicated. Uncertainty factors can be described in 3 categories ... 

Risk assessment starts with risk identification, a systematic use of available information to identify hazards (i.e., events or other conditions that have the potential to cause harm). Information can be from a variety of sources including stakeholders, historical data, information from the literature, and mathematical or scientific analyses. Risk analysis is then conducted to estimate the degree of risk associated with the identified hazards. This is estimated based on the likelihood of occurrence and resultant severity of harm. In some risk management tools, the ability to detect the hazard may also be considered. If the hazard is readily detectable, this may be considered a factor in the overall risk assessment. Risk evaluation determines if the risk is acceptable based on specified criteria. In a quality system environment, criteria would include impact on the overall performance of the quality system and the quality attributes of the finished product. The value of the risk assessment depends on how robust the data used in the assessment process is judged to be. The risk assessment process should take into account assumptions and reasonable sources of uncertainty. Risk assessment activities should be documented. 

The basic assumption of the International Commission on Radiological Protection (ICRP) is that for stochastic effects, a linear relationship without threshold is found between dose and the probability of an effect within the range of exposure conditions usually encountered in radiation work. However, ICRP cautions that if the dose is highly sigmoid, the risk from low doses could be overestimated by linear extrapolation from data obtained at high doses. Furthermore, ICRP... 

Evaluation of the risk index (RI) in Equation 1.1 or 1.2 requires assumptions about allowable risks or doses from waste disposal to be used in defining the different waste classes (see Section 1.4.1). These assumptions should be based on an understanding of differences in the approaches to risk management for radionuclides and hazardous chemicals embodied in current laws and regulations, including the different meanings that have been attached to the terms acceptable and unacceptable commonly used to describe the significance of health risks. 

Exempt Waste. Waste classified as exempt would be regulated as if it were nonhazardous, and would be generally acceptable for disposition as nonhazardous material (e.g., disposal in a municipal/ industrial landfill). As noted in Section 1.4.1, disposal is the only disposition of exempt materials considered in this Report. Limits on concentrations of hazardous substances in exempt waste would be derived based on an assumption that the risk or dose to a hypothetical inadvertent intruder at a disposal site should not exceed negligible levels. The use of a negligible risk or dose to determine exempt waste is based on an assumption that a disposal facility for nonhazardous waste could be released for unrestricted use by the public soon after the facility is closed. 

Low-Hazard Waste. Waste classified as low-hazard would be generally acceptable for disposal in a dedicated near-surface facility for hazardous wastes. Limits on concentrations of hazardous substances in low-hazard waste would be derived based on an assumption that the risk or dose to a hypothetical inadvertent intruder at a disposal site should not exceed acceptable (barely tolerable) levels. 

In contrast, risk management for substances that cause deterministic effects must consider unavoidable exposures to the background of naturally occurring substances that cause such effects. Based on the assumption of a threshold dose-response relationship, the risk from man-made sources is not independent of the risk from undisturbed natural sources, and the total dose from all sources must be considered in evaluating deterministic risks. In the case of ionizing radiation, thresholds for deterministic responses are well above average doses from natural background radiation (see Section 3.2.2.1)... 

In general, calculation of the risk or dose from waste disposal in the numerator of the risk index in Equation 6.2 or 6.3 involves the risk assessment process discussed in Section 3.1.5.1. As summarized in Section 6.1.3, NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites should be used in calculating risk or dose for purposes of waste classification. Implementation of models describing exposure scenarios for inadvertent intruders at waste disposal sites and their associated exposure pathways generally results in estimates of risk or dose per unit concentration of hazardous substances in waste. These results then are combined with the assumptions about allowable risk discussed in the previous section to obtain limits on concentrations of hazardous substances in exempt or low-hazard waste. 

If the risk index for all substances that cause deterministic responses in the waste (RId) in Equation 6.5 is zero (i.e., the doses of all substances that cause deterministic responses are less than the allowable values), classification is determined solely by the risk index for all substances that cause stochastic responses (RP) in Equation 6.4 the latter must be nonzero based on the assumption of a linear, nonthreshold dose-response relationship. On the other hand, if the risk index for all substances that cause deterministic responses is unity or greater, the calculated risk exceeds the allowable risk for the waste class of concern without the need to consider the risk posed by substances that cause stochastic effects. The only advantage of the form of the composite risk index in Equation 6.6 is that it indicates more explicitly that the total risk posed by a given waste is the sum of the risks posed by the two types of hazardous constituents, however approximate that representation may be. 

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