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

Uptake of the substance may be slow thus reducing the

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Unformatted text preview: ing the biologically available concentration in the short exposures or the long -term exposures may interfere with pharmacological processes that would normally result in repair or metabolisms of the substance. As a result, the product of exposure concentration and exposure time is not a constant, however, it may be modeled and used to predict responses with 350 exposure times that are Equation r² different from the normal ones Linear 0.897 Y = -37.9x + 342.5 used in toxicity testing, for Logarithmic Y = -85.18ln(x) + 314.4 0.984 example, predicting 48 -h 300 Exponential Y = 353.7 e 0.932 LC50s from 96 -h LC50s. Y = 317.06 x Power -0.1485x LC50 ( g/L) -0.3297 250 0.996 3.1.4.4 MEASURING EFFECTS Although quantal responses such as mortality or morbidity 200 are widely used as an endpoint for toxicity assessment, some organisms do not lend themselves to assessment of 150 quantal effects. In this case, 1 2 3 4 toxicity may also be judged by Time (days) the use of severity scales. Figure 3-74 These are often assigned when Example of the relationship between time of exposure and toxicity of it is difficult to determine if chlorpyrifos in fathead minnows the organisms have been affected (as in the case of plants which may not show symptoms of effect rapidly). A plot of these scales or scores may or may not give a linear relationship with concentration. In either case it is inappropriate to extrapolate beyond the range of the data as the model is untested in those regions. The same comments apply to other measurements of effect, such as % yield ( for example, of a bean crop exposed to sulfur dioxide; Suter et al 2007) . In these cases, any suitable transformation may be used to describe the concentration -response relationship, however, the nature of the equation is of little biological importance, except that it may be used for interpolation of data (). Extrapolation of the log -dose – probit-percent-mortality for some distance outside the data range is acceptable, at l east within a limited range, although threshold and hormetic phenomena (stimulatory effects at small exposures) may come into play (Calabrese et al. 2006). Quantal responses Non-quantal responses 99 NOEC? 110 90 90 70 50 30 70 LC50 10 50 IC50 30 1 10 1 100 1 100 Concentration Concentration Figure 3-1 Illustration of quantal and non-quantal responses 3.1.4.5 DURATION OF EFFECTS Duration of effects must also be considered in the assessment of toxicity. Traditionally, the intensity of effect at the end of the exposure period has been the only concern. Thus, after, or during a period of exposure, the effects may still be present or may have ceased ( Figure 3-9). Persistence of effects is mostly driven by the pharmacokinetics of the substance ( Chapter 2). Rapid excretion and/or metabolism will result in a short duration of effect. Storage of a hi ghly lipophilic compound such as DDT in fats may result in delayed poisoning when these fats are mobilized and the DDT released. Examples of this are seen in hibernating bats and in migrating birds (...
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