Higher-Tiered Studies

A refined risk assessment and specific higher-tiered studies related to this assessment (e.g., field studies) should not only demonstrate that no unacceptable effects on birds/mammals occur under practical conditions of use, but also demonstrate why those effects will not occur. One possibility to describe the 

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A tiered approach is used for pesticide exposure assessment at the surface water level. Depending on the results of the initial risk assessment, more extensive testing relative to the environmental exposure or hazard may be required to define the full environmental risk. The data are generated from such increasingly comprehensive series of studies (higher tiered studies). At each tier a comparison has to take place between the estimated exposure and... 

Lepper P. Use of higher-tier studies conducted in the context of pesticide risk assessment for quality standard setting. Poster presented at the SETAC Europe 16th Annual Meeting 7-11 May 2006, The Hague, The Netherlands. 

Current procedures of higher tier risk assessment are often based on the extrapolation of responses observed in relatively simple and short-term (weeks) cosm tests to structurally more complex ecosystems in the field. The predictive value of studies in small cosms (microcosms), however, depends on factors such as fate and exposure of the stressor and the sensitivity and recovery potential of the populations present. The role of cosm studies in extrapolation is discussed in more detail in Chapter 4. 

Higher tier mixture extrapolation approaches such as this are used in practice, by calculating site-specific msPAF values, across a broad range of assessment questions. A practical example is provided by De Zwart (2005), who studied the impacts of pesticide use in The Netherlands. In this study, the specific mode of action of the pesticide was taken into account, as shown in Box 5.2. This analysis resulted in spatiotemporal indicators of relative toxic pressure across The Netherlands (Figure 5.3). 

This is an example exposure assessment that illustrates quantitative representations of uncertainty and variability at the higher tiers of an exposure assessment. This case-study is based on human exposures to a persistent, bioaccumulative and lipid-soluble compound through fish consumption. This compound is fictional and referred to here as PBLx, but it has properties that correspond to those of known persistent compounds. Specific goals of this case-study are to illustrate (1) the types of uncertainty and variability that arise in exposure assessments, (2) quantitative uncertainty assessment, (3) how distributions are established to represent variability and uncertainty, (4) differences among alternative variance propagation methods, (5) how to distinguish uncertainty from variability and (6) how to communicate the results of an uncertainty analysis. 

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. 

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