SSD species sensitivity distribution

Reliable chronic toxicity data were available for 21 species of plants (13 phytoplankton and 8 macrophytes) and 15 species of animals. The species sensitivity distributions (SSDs) for atrazine chronic toxicity (no observed effect concentrations [NOECs]) to plants and animals are shown in Figure 4.4. A log-normal distribution model was fitted to each SSD by least-squares regression. 

Confidence intervals nsing freqnentist and Bayesian approaches have been compared for the normal distribntion with mean p and standard deviation o (Aldenberg and Jaworska 2000). In particnlar, data on species sensitivity to a toxicant was fitted to a normal distribntion to form the species sensitivity distribution (SSD). Fraction affected (FA) and the hazardons concentration (HC), i.e., percentiles and their confidence intervals, were analyzed. Lower and npper confidence limits were developed from t statistics to form 90% 2-sided classical confidence intervals. Bayesian treatment of the uncertainty of p and a of a presupposed normal distribution followed the approach of Box and Tiao (1973, chapter 2, section 2.4). Noninformative prior distributions for the parameters p and o specify the initial state of knowledge. These were constant c and l/o, respectively. Bayes theorem transforms the prior into the posterior distribution by the multiplication of the classic likelihood fnnction of the data and the joint prior distribution of the parameters, in this case p and o (Fignre 5.4). 

The measured exposure coneentrations from the field survey are used as a base for a probabilistic risk assessment. In such an assessment the exposure is presented as an exposure distribution. In this study the exposure distribution presents the spatial variation in the eoneentra-tions measured in e.g. a specific harbour or for all harbours. The exposure distribution ean be eompared to the species sensitivity distribution (SSD) for TBT. The SSD represents the average sensitivity of species and the variation among speeies. As the effect data of TBT, on which the SSD is based, is derived from coneentrations in water it was necessary to reealeulate all exposure... 

The Species Sensitivity Distribution (SSD) applied in this study is derived from data published by Lepper (2002). The SSD describes the average sensitivity of species and the variation among species (Posthuma et al., 2002). The SSD for TBT is based on toxicity data for 29 marine... 

Figure 5 Species sensitivity distribution (SSD) for the NOEC of specific dioxin-induced effects invertebrate species (common and Foster s tern, cormorant, bald eagle, European otter, mink, harbour seal and sole) expressed as internal...

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). 

FIGURE 3.3 Species sensitivity distributions (SSDs) for dichloroaniline (DCA) and nonylphenol (NP) using fish estimates from ECOSAR and ICE (Asfaw et al. 2004) using fathead minnow (fhm) as the surrogate species. Note SSDs not including ECOSAR values for fish, daphnids, and algae are noted as -fhm, whereas SSDs including these values are noted as -fhme. Source Asfaw et al. (2004). 

FIGURE 4.4 Species sensitivity distribution (SSD) curves for different aquatic species groups for the insecticide chlorpyrifos. Note Shown are the toxicity data of arthropods, other invertebrates, fish, and algae. Results of the logistic regression on these data are represented by lines. Source Data were obtained from ECOTOX (USEPA 2001). 

Species Sensitivity Distributions (SSDs) and Mixture Extrapolation... 

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... 

At the species level, the TU approach — a point-estimate approach — has been used to express the toxicity of one compound as a fraction of another with the same mode of action. Transfer of the TU principle to species sensitivity distributions (SSDs), by scaling compounds in a similar way, results in hazard units (HUs). The scaling is done on point estimates taken from an SSD, such as the HC5 or the HC50, or on any other point estimate that is considered relevant for the assessment (such as legal quality criteria). 

An analysis of regularities observed in species sensitivity distributions (SSD) fitted on acute and chronic aquatic toxicity data for a large number of organic and inorganic toxicants is provided by De Zwart (2002). The log-logistic sensitivity model he used is characterized by the parameter a, which is the mean of the observed loglO-transformed L(E)C50 or NOEC values over a variety of test species, and /3, a scale parameter proportional to the standard deviation of the loglO-transformed... 

It is noteworthy that comparisons of existing assessment schemes reveal dissimilarities in the use of extrapolation methods and their input data between different jurisdictions and between prospective and retrospective assessment schemes. This is clearly apparent from, for example, a set of scientific comparisons of 5% hazardous concentration (HC5) values for different substances. Absolute HC5 values and their lower confidence values were different among the different statistical models that can be used to describe a species sensitivity distribution (SSD Wheeler et al. 2002a). As different countries have made different choices in the prescribed modeling by SSDs (regarding data quality, preferred model, etc.), it is clear that different jurisdictions may have different environmental quality criteria for the same substance. Considering the science, the absolute values could be the same in view of the fact that the assessment problem, the available extrapolation methods, and the possible set of input data are (scientifically) similar across jurisdictions. When it is possible, however, to look at the confidence intervals, the numerical differences resulting from different details in method choice become smaller because confidence intervals show overlap. 

A key element in decision making will be to understand the relationship between the level of chemical exposure and the consequent risks to health or the environment. There are two main ways in which we can understand this relationship through the species sensitivity distribution (SSD) or the dose-response curve. The SSD is perhaps the more useful for environmental assessment because it integrates all species, whereas the dose response describes the cause-effect relationship for only one species. Nevertheless, the dose-response relationship could be a valuable tool for environmental assessment when the species described is either particularly sensitive,... 

The toxic pressure of each of the compounds in a mixture is calculated using the species sensitivity distribution (SSD) concept. In this concept, laboratory toxicity data for various species are collected from a database, for example, the USEPA s Ecotox database (USEPA 2005) or the RIVM e-toxBase (Wintersen et al. 2004), and compiled for each compound. A statistical distribution of these data, called the SSD, is derived. Each SSD describes the relationship between exposure concentration (X) and toxic pressure (Y), whereby the latter is expressed as the probably affected fraction (PAF, %) per compound (Posthuma et al. 2002). Depending on the test endpoint chosen for deriving SSDs, there is the option to derive chronic and acute toxic pressures, based on SSDN0ECs and SSDEC50s, respectively. 

Figure 5.8 Interpretation of the concept of (acute) toxic pressure (multisubstance probably affected fraction of species (msPAF), based on species sensitivity distributions (SSDs) made from EC50s) using fish species census data from a large monitoring data set. The predicted msPAF (X-axis) is linearly associated to the observed impact of local mixtures on fish assemblages (species loss) in Ohio surface waters (approx. 700 sampling sites). (Study data from Posthuma and De Zwart [2006])...

Figure 5.10 Difference in the dose-effect models for humans and species assemblages (species sensitivity distribution [SSD], right). Threshold-type curves are used for many compounds it is assumed that below a certain daily intake there will be no effects. Nonthreshold chemicals (i.e. certain types of carcinogens) lead to increased probability of cancer, and for this a linear model is assumed in the relevant concentration range. Species sensitivities are assumed to follow a non-linear curve (the SSD), relating the exposure to the fraction of species affected, with a maximum of 100% of the species affected.

The main focus of the ecological risk assessment is to minimize undesired events caused by chemicals. Species sensitivity distribution (SSD) is an example of an ecotoxicological method which is based on such events at above the no-effect level/concentration. We can assume that within a community species differ in... 

Species sensitivity distributions (SSD) on the basis of NOEC values in the literature were used to calculate the toxic pressure (TP an effect value) from the presence of contaminants in the soil or in the pore water (Rutgers et al., 2001 Posthuma et al., 2002). The effect of the contaminants was corrected for the contaminant levels in the local reference samples (field A) ... 

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