Toxicity testing concentration methods

In the case of surface water, the LOQ must not exceed a concentration which has an impact on nontarget organisms deemed to be unacceptable according to the requirements of Annex VI. At present, no harmonized limits for surface water exist. Therefore, provisions in Annex VI of Directive 91/414/EEC will be used to calculate guidance limits for analytical methods for surface water. In SANCO/825/00 the limits given in Table 6 are established [the relevant concentrations (the lowest will always be taken into consideration) depend on the species as indicated and can be taken from toxicity tests]. 

Concentration Methods Used To Prepare for Toxicity Testing... 

Before beginning a toxicity testing program, each of the procedures that are recommended must first be tested at the water-treatment site (sample-collection site) to ensure the adequacy of the concentration method (e.g., solubility of the components, minimization of artifacts, development of a quality assurance program). A mass balance based upon total organic carbon (TOC) is desired during this initial testing phase. 

Risk is generally considered as a product of the probability of an adverse effect and the magnitude of that effect. In ecotoxicology, risk depends on the probability and intensity of exposure and the sensitivity of the exposed organisms, whereby the interpretation of risk can involve aspects of space and time and value judgments (e.g., believing that 1 species is more important than another). The sensitivity is often determined in laboratory toxicity tests, in which dose- or concentration-effect curves are established. Extrapolation methods exist for both components of risk and the additional aspects. 

The set of questions leads to a logical approach for all potential mixture assessment situations. For example, mixtures of which the composition is unknown or partially known and which have an unknown frequency of occurrence and unknown or unique concentration ratios should be assessed on a case-by-case basis using toxicity tests with whole mixture samples in the field or the laboratory. If this is not feasible, the risk assessor can opt to gather additional data about the mixture composition or its occurrence and subsequently reassess the mixture. It also leads to a logical approach for mixtures of which the composition is known, namely, the application of component-based methods or the use of appropriate data from similar cases. 

A battery of aquatic biological tests has been developed to evaluate the toxic effects elicited from effluents. For all the types of toxicity tests, selection of exposure concentration and duration, test species and strains, and monitored water quality parameters are critical. The short- and long-term methods for... 

The results of acute toxicity tests are reported as the LC50 and EC50 (concentration that reduces growth 50%) values and their 95% confidence intervals. Probit analysis is the most commonly used statistical method to determine LC50 values. Graphical interpolation can be used to estimate the LC50 value where the proportion of deaths versus the test concentration is plotted for each observation time. 

The probit method is perhaps the most widely used method for calculating toxicity vs. concentration or dose. As its name implies, the method used a probit transformation of the data. A probit is a unit of divergence from the mean of a normal distribution equal to one standard deviation. The central value of a probit would be 5.0, representing the median effect of the toxicity test. A disadvantage of the method is that it requires two sets of partial kills. However, a confidence interval is easily calculated and can then be used to compare toxicity results. There are several programs available for the calculation, and as discussed below, they provide comparable results. 

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