Assessment Factors General Aspects

In the context of assessment factors, it is important to distinguish between the two terms variability and uncertainty. Variability refers to observed differences attributable to true heterogeneity or diversity, i.e., inherent biological differences between species, strains, and individuals. Variability is the result of natural random processes and is usually not reducible by further measurement or study although it can be better characterized. Uncertainty relates to lack of knowledge about, e.g., models, parameters, constants, data, etc., and can sometimes be minimized, reduced, or eliminated if additional information is obtained (US-EPA 2003). 

It should be recognized that a lack of knowledge of variability is a source of uncertainty. 

The terminology within this area is not standardized. Other terms include safety factor, uncertainty factor, extrapolation factor, adjustment factor, and conversion factor. None 

Toxicological Risk Assessments of Chemicals A Practical Guide 

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As mentioned previously, the assessment of hazard and risk to humans from exposure to chemical substances is generally based on the extrapolation from data obtained in smdies with experimental animals. In the absence of comparative data in humans, a basic assumption for toxicological risk assessment is that effects observed in laboratory animals are relevant for humans, i.e., would also be expressed in humans. In assessing the risk to humans, an assessment factor is applied to take account of uncertainties in the differences in sensitivity to the test substance between the species, i.e., to account for interspecies variability (Section 5.3). If data are available from more than one species or strain, the hazard and risk assessment is generally based on the most susceptible of these except where data strongly indicate that a particular species is more similar to man than the others with respect to toxicokinetics and/or toxicodynamics. Two main aspects of toxicity, toxicokinetics and toxicodynamics, account for the namre and extent of differences between species in their sensitivity to xenobiotics this is addressed in detail in Chapter 5. 

In rivers and streams heavy metals are distributed between the water, colloidal material, suspended matter, and the sedimented phases. The assessment of the mechanisms of deposition and remobilization of heavy metals into and from the sediment is one task for research on the behavior of metals in river systems [IRGOLIC and MARTELL, 1985]. It was hitherto, usual to calculate enrichment factors, for instance the geoaccumulation index for sediments [MULLER, 1979 1981], to compare the properties of elements. Distribution coefficients of the metal in water and in sediment fractions were calculated for some rivers to find general aspects of the enrichment behavior of metals [FOR-STNER and MULLER, 1974]. In-situ analyses or laboratory experiments with natural material in combination with speciation techniques are another means of investigation [LANDNER, 1987 CALMANO et al., 1992], Such experiments manifest univariate dependencies for the metals and other components, for instance between different metals and nitrilotriacetic acid [FORSTNER and SALOMONS, 1991], but the interactions in natural systems are often more complex. 

With regard to the standardized multiaxial assessment, of the score of schemas that have been published (and reviewed by Mezzich, 1979), the ones that are currently most frequently seen are those included in the CIE-10 (World Health Organization, 1997) and the DSM-IV (APA, 1994). Although these systems vary in the number of axes they have, their contents do largely match up axis I (Clinical diagnoses) of the CIE-10 covers the contents of the first three axes of the DSM-IV axis II of the CIE-10 assesses the area of disabilities divided into four aspects (personal care, occupational, with the family, and social in general), and corresponds to axis V of the DSM-IV finally, axis III (Contextual factors) of the CIE-10 essentially matches axis IV of the DSM-IV. 

Dietary Reference Intakes (DRIs) are used quite a lot and refer to a set of four nutrient-based reference values that represent the approach to provide quantitative estimates of nutrient intakes. The DRIs replace and expand on the Recommended Dietary Allowances (RDAs) for the United States and the Recommended Nutrient Intakes (RNIs) for Canada. The DRIs consist of the RDAs, the Tolerable Upper Intake Level (UL), the Estimated Average Requirement (EAR), and the Adequate Intake (AI). Generally, each of these values represents average daily nutrient intake of individuals in the diet (Goldhaber, 2003 Murphy and Poos, 2002 Parr et ah, 2006 Trumbo et ah, 2001 Yates et ah, 1998). In addition, dietary intake data for minerals could be assessed within the context of the bioavailability and other factors affecting the utilization of elements by the human body, such as age, sex, and health aspects (Dokkum, 1995). 

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