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

Unessential a selfmaintaining or reproducing

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Unformatted text preview: tial. A selfmaintaining or reproducing population is persistent on a huma n time scale and can be easily conceptualized by humans as being in need of protection. Thus, most assessment endpoints in ecological risk characterization are defined at the population, rather than at the organismal level. Only in the case of the protection of rare, endangered, or long-lived species are organisms in the environment afforded similar protection to that enjoyed by humans. In general, ecological risk assessment is aimed at protection of the functions of populations, communities and ecosystems. This acknowledges the fact that populations are less sensitive than their most sensitive member and, likewise, that communities and ecosystems are less sensitive than their most sensitive components. Effects on a population are not necessarily of concern (to the ecosystem) as long as the functions of the population can be taken over by other organisms. The selection of these endpoints varies with the groups of organisms being consider ed such as the example given in Table 3-1. 3.1.4 Basic principles of ecotoxicology Substances likely to be of concern to human health and their environment have certain combinations of characteristics that increase the possibility of adverse effects. These characteristics include the inherent toxicity of the substance and the potential for exposure to the substance. Toxicity is the capability of a substance to produce injury in an organism. It is the inherent poisonous potency of the substance and usually can be measured under experimental conditions where the effects on organisms are observed. If these effects are adverse, they are deemed to be toxic effects. (1) MEASURING TOXICITY Measurement of effects involves the consideration of three variables: exposure concentration, exposure duration and response. Exposure concentration is a continuous variable and the proportion responding is a quantal variable. Duration of exposure may be a continuous variable but, in the real world, may be complicated by variation in exposure concentration during the exposure time. In the simplest case, the proportion of organisms responding is a function of concentration. This relationship is non linear but may be analyzed as a linear model where the concentration is expressed as a logar ithm and the percentage organisms responding as a probability ( probit scale, Section 1.4.2). The most commonly pub lished number with respect to toxicity is the LC50 o n the median lethal concentration. This the concentration at which half the organ isms are responding with the test endpoint or response, in this example, lethality. Commonly, the response is lethality but it may also be a specific effect such as immobil ity. In this case, the abbreviation EC50 is used. Several variations of this response are used in the literature. These are: LD50 ED50 EC50 LC50 Dose or amount of substance per unit weight that would kill 50% of the population. Dose or amount of substance per unit weight that would give the desired effect in 50% of the population. Concentra...
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