Potentially affected fraction

Figure 3 Correspondence between ecological risk expressed as the Potentially Affected Fraction of species (PAF) and the calculated ISI level corresponding to the 95th percentile exposure concentration in Dutch harbours.

Table 5 Example application of process in Box B to evaluate the risk of TBT in Dutch sediments and biota. Ecotoxi-cologjcal risk determined as potentially affected fraction (PAF %) based on the sediment levels interpolated in the SSD for TBT (from Schipper et al., 2008a), in the field observed intersex index (ISI) values in periwinkle L. littorea and Vas Deferens Sequence Index (VDSI) in dogwheUc N. lapillus as biomarkers for TBT-exposure (unpublished own results). Based on a combination of the criteria ISI<0.3, VDSI>1 and TBT-PAF<10%, the risk status of the sediments is classified (last column).

Table 8 Calculated annual input of TEQ in the Dutch coastal zone from the rivers, atmospheric emission and disposal of dredged material (period 1999-2005). For the TEQ-lev-els in sediment, the ecotoxicological risk is estimated as % Potentially Affected Fraction of species (PAF) and calculated based on the SSD for chronic dioxin-toxicity (Figure 6). 

SimpleBox was created as a research tool in environmental risk assessment. Simple-Box (Brandes et al. 1996) is implemented in the regulatory European Union System for the Evaluation of Substances (EUSES) models (Vermeire et al. 1997) that are used for risk assessment of new and existing chemicals. Dedicated SimpleBox 1.0 applications have been used for integrating environmental quality criteria for air, water, and soil in The Netherlands. Spreadsheet versions of SimpleBox 2.0 are used for multi-media chemical fate modeling by scientists at universities and research institutes in various countries. SimpleBox models exposure concentrations in the environmental media. In addition to exposure concentrations, SimpleBox provides output at the level of toxic pressure on ecosystems by calculating potentially affected fractions (PAF) on the basis of species sensitivity distribution (SSD) calculus (see Chapter 4). 

Species sensitivity distributions (SSDs) are used for both prospective and retrospective risk assessments (Posthuma et al. 2002b). In prospective risk assessments, the concept is used to derive hazardous concentrations (e.g., HC5), which are used to derive environmental quality criteria. In retrospective risk assessments, the SSD approach is used to determine the local toxic pressure in terms of the potentially affected fraction (PAF) of species for each compound separately. Subsequently the multisubstance (ms)PAF, or optionally the combi-PAF, for the local mixture can be calculated. Originally, the combi-PAF concept was developed by Hamers et al. (1996) and assumes that only compounds exerting narcotic effects... 

Multiple-species risk for independent combined effects in terms of the potentially affected fraction of species can be assessed using models that are essentially the same as for the prediction of response-additive effects in single species. The underlying assumption in the application of a response addition model for compounds or groups of compounds with different modes of action is that correlation of species sensitivities to the different constituents of the mixture is again considered absent. The calculation... 

This chapter proposes the use of SSD and mixture toxicity models in ecological risk assessment of species assemblages by calculating the multisubstance potentially affected fraction of species on the basis of measured or predicted (biologically active) concentrations of toxic compounds in the environment. The msPAF method has been scrutinized for its conceptual basis. To address this scrutiny, we cite the human toxicology work of Ashford (1981) as a cross-link. 

Klepper O, van De Meent D. 1997. Mapping the potentially affected fraction (PAF) of species as an indicator of toxic stress. No. 607504001. Bilthoven (The Netherlands) National Institute of Public Health and the Environment (RIVM). 

Traas TP, van de Meent D, Posthuma L, Hamers T, Kater BJ, De Zwart D, Aldenberg T. 2002. The potentially affected fraction as a measure of ecological risk. In Posthuma L, Suter GW, Traas TP, editors. The use of species sensitivity distributions in ecotoxi-cology. Boca Raton (FL) CRC Press. 

Refined effect assessment. The results from four acute or chronic bioassays of different taxonomic groups are expressed in a concentration factor where 50% effect is measured (EC50). These data then are used to construct a sensitivity distribution (De Zwart and Sterkenburg, 2002). The potentially affected fraction (PAF) is determined for the 100% sample (the as is sample). The negligible effect level is at a value for PAF = 5%. 

The development of key indicators aims at a single score indicator to measure effects of land use, which-as pointed out for single-score indicators in general-requires the use of weighting factors. Lindeijer presents several sets of weighting factors for land use classes as well as other approaches for aggregation [20]. One of these is the so-called PAF concept applied in ecoindicator 99. It measures the potentially affected fraction of species, that is, the part of the total munber of species in an area which is potentially affected in terms of laboratory test effects. 

Now, what is the physics behind these equations We note that the potential difference A(j) across the interphase affects the free energy of the reaction. A fraction of this potential affects the free energy of formation of the activated complex (i.e., the free energy of activation of the reaction). This fraction is defined as p. When we conduct an experiment, we can control the changes in A(j), but we have no control over the changes in A(]), which amounts to a fraction P of the former. 

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