Environmental+Toxicology+Tox+2000+notes (2)

Environmental Toxicology Tox 2000 notes(2)

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Unformatted text preview: see discussion of biomagnification in Chapter 1, Section 1.3.5). Time Concentration Effect Concentration Effect declines slowly after exposure Adaptation occurs before the end of the exposure period Time Figure 31 Responses as a function of duration of effects. Effect Effects are seen after exposure because of storage and release of substance. Effect declines rapidly after expsosure 3.1.4.6 TIME, CONCENTRATION AND EXPOSURE The response of organisms to substances present in the matrix is depen dent on the pharmacokinetics of uptake and equilibration. This may have a significant effect on the way a test should be conducted. This applies specifically to compounds with mid -range KOWs that will tend to be taken up from the water matrix by organism s such as fish ( Figure 3-10). In a container of limited size, such as is normally used for bioassay purposes, the concentration of the toxicant in the water may be reduced because of uptake by the organism. Toxicant Not taken up Metabolized and excreted Fish Concentration Tank Time Figure 3-102 Uptake of a toxic substance with a log KOW of 4 by fish If a large enough number of fish are present in the container ( Figure 3-11), they may reduce the concentration of the toxicant in the water to such an extent that the biological activity of the compound is affected and it appears to be less toxic that it should be. The reason for this is that there i s not enough toxicant in the tank for all the organisms and, to get a toxic response, more must be added. In a dose response bioassay, this introduces an artifact and a greater LC50 is recorded (indicative of lesser toxicity). This may be avoided through good experimental designs. If the toxicant under investigation has a mid-range KO W, it is better to use a large bioassay tank ( Figure 312) or, better still, use a flow-through tank () where fresh toxicant and water is continually added. This last option is the equivalent of an infinitely large tank 100 L Toxicant plus and any Static water toxicant lost F igure 3-11 to the 3 Bioassay tank organisms or 10 L 10 L the walls of the tank is replaced as flow-thru Static fresh toxicant is added. For this reason, bioassays conducted in flowthrough systems are considered the best predictors of environmental responses. From a more practical point of view, and also as a result of the kinetics of uptake and depuration, the LC50 is dependent on the time of exposure. Figure 34 Bioassay systems for fish This dependence may vary from one substance to another but the general relationship holds true (Figure 3-13) although the time required to reach the asymptote may be short or long. LC50 in g/L This relationship is important as it affects the interpretation of chronic exposures and this, in turn, affects the inter pretation of the NOEC. For instance, if the acut e exposure period ends at 96 h and both an LC50 and a NOEC are observed ( Figure 3-13), is this NOEC applicable to exposures beyond 96 h in duration? 3.1.4.7 CHRONIC TESTS 24 h 48 h 72 h 96 h Chronic tests are normally those of duration longer...
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This document was uploaded on 04/07/2014.

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