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Concentration of substance per unit volume that would

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Unformatted text preview: tion of substance per unit volume that would give the desired effect in 50% of the population. Concentration of substance per unit vo lume required to kill 50% of the population. The general use of the 50% response is because of the greater precision of the estimate and the narrower confidence intervals about this point. In practice, risk assessors are more interested in small or large percentage responses and would need the slope of the line to better be able to extrapolate the data to useful numbers. More useful criteria of small levels of effect under conditions of chronic exposure are: LOAEC Lowest Observed Adverse Effect Concentr ation, the smallest concentration at which adverse effects are observed (this depends on the range of doses tested) NOAEC No Observed Adverse Effect Concentration, that concentration at which no effects are observed. MATC The geometric mean of the LOAEC and the NOAEC. This is an approximation of the LC0 or EC0 Also see the discussion of benchmark dose in Section 4.3.2 in Chapter 4 These criteria are often obtained from graphical interpolation but there are many computer programs designed to estimate them (Mayer 1996, USEPA 1992b). The concentration-response model is a subset of a three -dimensional model that also includes the time dimension (Figure 3-4). The important point here is that, for most organisms, exposures from the surrounding matrix is not instantaneous and uptake kinetics may be important. Depending of the size of the organism and the nature of the chemical, it may take some time for the organism to come into equilibrium with the exposure matrix and to fully ex press maximal response. The concentration-response model is a subset of a three -dimensional model that also includes the time dimension (). The important point here is that, for most organisms, exposures from the surrounding matrix is not instantaneous and uptake kinetics may be important. Depending of the size of the organism and the nature of the chemical, it may take some time for the organism to come into equilibrium with the exposure matrix and to fully express maximal respons e. If duration of exposure is brief and the organisms recover between exposures, then exposure duration can be factored out and exposure is a function of concentration (Figure 3-5 A). In this case, the most suitable laboratory model for exposure is a single exposure of duration similar to that in the field. If the concentration is essentially constant (such as in the effluent from a factory operating 24 h per d and 7 d per week) then exposure become a function of Proportional mortality 3.1.4.2 Duration of exposure e Concentration Tim Figure 3-41 Proportion of organisms responding as a function of concentration and duration of exposure duration (Figure 3-5 B). In this case, the most suitable laboratory model for exposure is a continuous exposure of duration similar to that in the field. This is the equivalent of a chronic toxicity test. Concentration A B Concentration If duration of exposure is short and there is no recovery between exposures (no reciprocity) then the organisms respond to the total cumulative exposure (Figure 3-5 C). In this case, the most...
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This document was uploaded on 04/07/2014.

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