Tiered approach to risk assessment

Parkhurst, B.R., W. Warren-Hicks, R.D. Cardwell, J. Volison, T. Etchison, J.B. Butcher, and S.M. Covington (1996). Aquatic ecological risk assessment A multi-tiered approach to risk assessment, 91-AER-l. Alexandria, VA Water Environment Research Foundation. 

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. 

Because the whole idea of a tiered approach of the kind outlined above is in its initial stages, it will have to be validated and discussed further and will in all probability need to be refined afterwards. The aim here is to introduce the idea of a stepwise approach to the assessment of the risk to re-entry workers. The outlined procedure should be used to calculate the dermal re-entry exposure for real examples of rather dermally toxic compounds in order to gain experience with the recommended procedure. 

Assessors should base their selection of methods on clearly defined decision criteria, and they need to communicate the results using clear and transparent language. This includes statements on the extrapolation issues that were considered but not addressed, and the magnitude and direction of the bias that may have been introduced by the extrapolation or lack thereof. In lower tiers and prospective risk assessment, this should lead to setting more appropriate UFs and ensure that lower tier approaches are more conservative than higher tier approaches. All this helps assessors to make informed decisions, on one hand, but it also allows the identification of future research needs, on the other hand, especially when methods are not available. 

It is evident that some techniques do not have conceptually similar equivalents with various levels of complexity. Hence, tiering is not (yet) possible for all problem definitions. Moreover, it is clear that human risk assessment can sometimes operate on a higher conceptual tier than ecological risk assessment, for example, when BRN modeling and PBPK models are used. On the other hand, ecological risk assessment approaches may be sometimes more diverse, and can be better tailored to a risk assessment problem and its context. 

When route-to-route extrapolation is deemed to be appropriate for the risk assessment, a tiered approach to selection of dermal absorption values, as outlined below, is recommended. 

Tiered approaches to dermal exposure and risk assessment have been developed (OECD, 1997 de Heer et al, 1999 Harney, 2000 EC, 2002). Although the number of tiers differ depending on the specific approach, common to all approaches is the sequential refinement of the value used for dermal absorption in the risk assessment. For example, in a Tier 1 risk assessment, a conservative value of 100 % dermal absorption is often used. If required, a more refined default may be justifiable, based on a number of considerations such as the physico-chemical properties of the substance and the toxicological database. Use of dermal absorption data would be the third tier. Biological monitoring data would be a potential fourth tier, if required. 

An overview of a tiered approach for the use of dermal absorption data in dermal risk assessments is provided. Initial tiers utilize default assumptions, while higher tiers require results from in vivo and in vitro dermal absorption studies. For dermal absorption studies, challenges in data analysis, as well as in application of the data to risk assessment, are identified. 

First, individuals interpreting data must decide on whether to use a deterministic or a probabilistic approach to generate an exposure estimate for the analysis. The deterministic approach (point-estimate) is widespread and beginning with this approach is consistent with the tiered approach to exposure and risk assessment. 

Agreement on the role of toxicology triggers, if any, in the tiered approach to occnpational and residential exposure and risk assessment. 

ASTM s risk-based corrective action (RBCA) uses a tiered approach to data collection and analysis in supporting decisions on site assessment and response to petroleum. The RBCA procedure begins with the assessment of the site (see Figure 5-2 for RBCA process flow-chart). 

Interest has been expressed in the possibility of using biomarker assays as a part of risk assessment for regulatory purposes, and some workers have suggested tiered testing procedures that follow this approach (see, for example, Handy et al. 2003). It is to be hoped that regulatory schemes, such as that of REACH (see European Union 2003), will be sufficiently flexible to incorporate new assays and testing strategies as the science advances. 

Data used to describe variation are ideally representative of some population of risk assessment interest. Representativeness was a focus of an earlier workshop on selection of distributions (USEPA 1998). The role of problem formulation is emphasized. In case of representativeness issues, some adjustment of the data may be possible, perhaps based on a mechanistic or statistical model. Statistical random-effects models may be useful in situations where the model includes distributions among as well as within populations. However, simple approaches may be adequate, depending on the assessment tier, such as an attempt to characterize quantitatively the consequences of assuming the data to be representative. 

A tiered approach is also used for calculating estimated residues in animal commodities (meat, milk, and eggs), and higher-tier calculations can have a significant impact in decreasing estimates of dietary exposure and risk. The Tier III assessment for atrazine and simazine (Tables 27.3 and 27.4) is based on calculations of the estimated theoretical residue in animal commodities, whereas the Tier I assessments use tolerance values. These theoretical residues are often referred to as secondary residues. Calculations for estimating secondary residues in animal commodities are performed by constructing livestock (beef, dairy, and poultry) diets comprised of treated feed items to obtain a... 

This chapter has outlined the general principles of the use of extrapolation in criteria setting and risk assessment. Extrapolation is used in the setting of criteria, where only effects are considered, as well as in risk assessments, where both effects and exposures are considered. In formulating approaches to criteria setting and risk assessment, tiers are often used to simplify the process. In keeping with the... 

Tiering is often applied in risk assessment in order to reduce expenditures in time, money, and labor when the assessment requires only simple and possibly conservative output. Table 5.3 provides a suggested tiered approach in mixture extrapolation and is further described in the bulleted list below. The tiering is based on the way that mixture mechanisms are addressed in the approach. It is assumed that issues such as matrix and media extrapolation have been addressed according to the methods described in the pertinent chapters. 

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