Probability assessment factors

In the new EU chemicals regulation REACH, which entered into force on 1 June 2007, detailed guidance documents on different REACH elements, including risk characterization and the use of assessment factors, are currently in preparation (spring 2007). These documents will probably be available on the EU DG Environment REACH Web site (EU 2006) when published. 

One of the main conclusions drawn from the evaluation of the available data on default assessment factors was that the conventionally used factor of 100 (10 for animal-to-human and 10 for human-to-human variations) is probably an underestimate. It is stated that it is likely that the animal-to-human extrapolation is greatly underestimated, and in the case of human-to-human variability, an assessment factor of 10-16 is considered as a minimum. 

Gronlund (1992) has investigated methods used for quantitative risk assessment of non-genotoxic substances, with special regard to the selection of assessment factors. Gronlund found that humans, in most cases, seem to be more sensitive to the toxic effects of chemicals than experimental animals, and that the traditional 10-fold factor for interspecies differences apparently is too small in order to cover the real variation. It was also noted that a general interspecies factor to cover all types of chemicals and all types of experimental animals cannot be expected. It was concluded that a 10-fold factor for interspecies variability probably protects a majority, but not all of the population, provided that the dose correction for differences in body size between experimental animals and humans is performed by the body surface area approach (Section 5.3.2.2). If the dose correction is based on the body weight approach (Section 5.3.2.1), the 10-fold factor was considered to be too small in most cases. 

Gronlund (1992) has investigated methods used for quantitative risk assessment of non-genotoxic substances, with special regard to the selection of assessment factors. Gronlund found that the 10-fold factor suggested for interindividual variability probably protects a majority but not all of the population. 

Vermeire et al. (1999) stated that in the standard procedure for deriving acceptable limit values, various assessment factors are multiplied to obtain an overall assessment factor. However, multiplication of assessment factors implies a piling up of worst-case assumptions the probability of simultaneous occurrence of worst-case situations for the same chemical is smaller than the occurrence of a single worst-case situation. Therefore, the higher the number of extrapolation steps, the higher the level of conservatism. The piling up of worst-case assumptions can be avoided by using probability distributions (Section 5.11). 

WHO/IPCS (1994, 1996) stated that the total UF applied when setting a TDI should not exceed 10,000. A higher UF than 10,000 indicates that the resulting TDI would be so imprecise as to lack meaning. US-EPA (2002) noted that the application of their five standard UFs with a default value of 10 for the chronic reference value (yielding a total UF of 100,000) is inappropriate, and it was recommended to limit the total UF for any particular chemical to no more than 3,000, and to avoid the derivation of a reference value that involves application of the full 10-fold UF in four or more areas of extrapolation. KEMI (2003) recommended that probability distributions of the assessment factors should be used in the calculation of the overall assessment factor, if available. Kalberlah and... 

The precision of the tolerable intake depends therefore largely on the magnimde of the overall assessment factor. The precision is probably to one significant figure at best, and more usually to one order of magnitude for assessment factors of 1000 or more, the precision becomes even less. 

Because of the variation in assessment factors and their uncertainty (lightning characteristics, attachment locations, damage models, etc.), a probabilistic or statistical approach can be used to express a relationship for the probability of failure of a critical chemical equipment item. For a single equipment item, this relationship can be expressed as ... 

There is most probably another factor influencing risk assessment. In general, the costs of an accident are relatively easy to assess. The assessment of the probability of the accident occurring is, however, more problematic. As will be seen, biases, caused by the way in which we process information, may mislead us into over-estimating or under-estimating probabilities. These biases are produced by what are known as heuristics. 

Kala, Z. 2007. Influence of partial safety factors on design reliability of steel structures Probability and fuzzy probability assessments. Journal of Civil Engineering and Management 13(4) 291 296. 

Risk assessment factor = Consequence x exposure x probability... 

Multiply the values that you have selected for consequence, exposure, and probability together to determine this risk assessment factor and find the value on the following scale. 

Quantitative assessment Determine probabilities of human errors Identify factors and interactions affecting human performance... 

THERP (NUREG/CR-1278), is used to estimate HEPs for a risk assessment. It provides error probabilities for generic tasks and describes the process used to modify these rates depending on the specific performance shaping factors (PSFs) involved in the task... 

Confidence limits for the conclusions cannot be expressed simply because of the complexity of hazard assessments. The estimation of uncertainties is itself a process subject to professional judgment. However, the team s estimates of probability are believed to be realistic, but may be pessimistic by a factor of perhaps two or three, but less than a factor of ten. Uncertainties also exist... 

Performance-influencing factors analysis is an important part of the human reliability aspects of risk assessment. It can be applied in two areas. The first of these is the qualitative prediction of possible errors that could have a major impact on plant or personnel safety. The second is the evaluation of the operational conditions under which tasks are performed. These conditions will have a major impact in determining the probability that a particular error will be committed, and hence need to be systematically assessed as part of the quantification process. This application of PIFs will be described in Chapters 4 and 5. 

The answer to this question will depend on two factors the frequency with which the CT occur, and the likelihood of errors arising when performing these tasks. The frequency of the interactions can usually be specified relatively easily by reference to plant procedures, production plans, and maintenance schedules. The probability of error will be a fimction of the PIFs discussed extensively in Chapter 3 and other chapters in this book. In order to obtain a measure of error potential, it is necessary to make an assessment of the most important PIFs for each of the CT. 

Generally, the slope factor is a plausible upper bound estimate of the probability of a response per unit intake of a ehemieal over a lifetime. The slope factor is used in risk assessments to estimate an upper-bound lifetime probability of an individual developing cancer as a result of e.xposure to a particular level of a potential carcinogen. Slope factors should always be accompanied by the weight-of-evidence classification to indicate the strength of the evidence that the agent is a human carcinogen. Calculational details are presented below. 

Neutralizing capacity is not the only measure of a required amine feed rate. Once all acidic characteristics have been neutralized, amine basicity becomes the important issue because this raises the pH above the neutralization point, to a more stable and sustainable level. Consequently, in practice we are concerned with the level of amine necessary to raise the condensate pH to a noncorrosive level. This practical amine requirement is difficult to obtain from theoretical calculations because it must take account of the amine volatility, DR, and the boiler system amine recycling factor (as well as temperature). As noted earlier, the basicity of an amine has little or no relationship to its volatility or DR, so that reliable field results are probably a more important guide in assessing the suitability of an amine product than suppliers tables. 

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