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

In this case the most suitable laboratory model for

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Unformatted text preview: suitable laboratory model for exposure is a continuous exposure of duration similar to total duration of all the pulses observed in the field. Exposure Concentration If exposure is variable with time, then the exposure should be integrated with time to mea sure the area under the curve (Figure 3-5 D). The time unit used for this integration can be chosen from appropriate durations of laboratory tests, i.e. 96-h acute LC50 or 21 -d chronic test. Effect C Concentration When refining the characterization of toxicity, it is appropriate to consider the actual use pattern and physical and chemical parameters of the substance i n relation to data Time weighted mean concentration generation. Exposure is a function of how much of the D compound is present in the environment and how much is available to the organism. In the case of pesticides, these factors are related to the use rate and application method, physical and chemical properties of the substance, and Time runoff and drift rates. An example of customizing a standard acute toxicity test would be to add additional Figure 3-52 exposure concentrations, time periods, or measures of effect. Examples of relationships between exposure duration and concentration If the substance in question is used once per year and has a half-life in water of 1 h, assessing mortality on a shorter time scale (e.g., 4 h, 8 h, etc) in a 48 - or 96-h study would allow data to be created which compares short-term exposure to short -term response. RECIPROCITY Direct reciprocity, the LC50 on day 2 is half that on day 1 etc. 4 Toxicity (LC50) The degree of response to a substance is a function of duration and intensity (concentration) of exposure, especially in situations where organisms are expose d through their surrounding medium. At greater concentrations of toxicant, shorter times of exposure are required to generate a specific response, and vice versa, and this is usually a reciprocal relationship. The reciprocity between concentra tion of toxicant and time to an effect can be used to describe toxicity curves. Reciprocity is the result of the non -instantaneous kinetics of uptake and distribution of the sub stance in 5 3 2 1 0 1 2 3 4 Time in days Figure 33 Illustration of direct reciprocity 5 organisms ( see Chapter 2). This ideal relationship of linear reciprocity is shown in Figure 3-6. Some substances, such as organo phosphorus insecticides, show linear reciprocity. This is, in part, due to the slow reversibility of the inhibition of the target enzyme, acetylcholinesterase ( see Chapter 1, Section 1.7.5 ), when it is inhibited by organophosphorus inhibitors. In most other cases, it is known that there is not linear reciprocity between the duration of exposure for pulsed exposures (Giesy and Graney 1989). For instance, pulsed exposures to greater concentrations may be less toxic than longer -term exposures to lesser concentrations (). This may be the result of several processes. Uptake of the substance may be slow, thus reduc...
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