In toxicity assays, we attempt to determine the dose of a chemical substance required to cause acute effects on living organisms.\u00a0 In performing toxicity assays, there are many considerations in experimental design.<\/p>\n
When selecting a species for toxicity testing, usually \u201cindicator species\u201d are chosen.\u00a0 An indicator species is a species of organism that is used in the laboratory to act as an indicator for what may be happening in a more complex ecosystem.\u00a0 The choice of indicator organisms varies depending on the problem.\u00a0 For example, if we were concerned with water pollution, we would use aquatic organisms.\u00a0 If we were concerned about a chemical leaking into the soil, we might use a plant.\u00a0 For human toxicity, ideally we\u2019d use humans, but since this is obviously not ethical, we use \u201cmodel species.\u201d\u00a0 A model species is used as a surrogate for the species of interest.\u00a0 Often rats and mice are used as model species for humans.<\/p>\n
Once a species is chosen, the dosing method must be determined.\u00a0 Chemicals can be applied dermally (through the skin), given in food or water, injected, or supplied in the air or water in which the organisms are living.\u00a0 When using living organisms, there are also different endpoints that can be considered.\u00a0 Usually, the easiest is death.\u00a0 We can also use other endpoints such as growth rate, birth rate, birth defects, a biochemical change (e.g<\/em>. hormone levels) or cell function.<\/p>\n When examining the results of a toxicity assay, we can use simple statistical methods to evaluate data.\u00a0 For example, the average (mean) is calculated.\u00a0 This average can be the number of surviving organisms, average shoot length, average life span, average egg laying rate, etc.\u00a0 In addition, we can calculate a standard deviation and the coefficient of variation (CV) for each average.\u00a0 The CV is the standard deviation divided by the mean and is a relative measure of data dispersion compared to the mean.\u00a0 The smaller the CV, the more reliable the data.\u00a0 In toxicology, we generally like the CV to be less than 5%.\u00a0 It is then common to use graphical methods to display the data.\u00a0 The most common way to display data for toxicity assays is a dose-response curve, where dose is plotted on the x-axis and response is plotted on the y-axis.\u00a0 From this type of graph we can estimate a number of variables.\u00a0 The LD50<\/strong> is the dose of chemical that causes death in 50% of the population.\u00a0 Similarly, the ED50<\/strong> is the dose of chemical that causes an effect in 50% of the population.\u00a0 In addition, the NOEL<\/strong> is the concentration of chemical that causes no effect (no observed effect level).<\/p>\n Please take a look at the lab handout before coming to lab.\u00a0 There will be a couple of pre-lab questions directly from it and it will be helpful to have read the procedure at least once before executing the steps.\u00a0 In addition\u00a0read the following handout from a different textbook – \u00a0Environmental Science: Principals, Connections and Solutions<\/em> by G. Tyler Miller Jr. \u00a0The supplement focuses on the LD50 dose response curve.<\/p>\n Handout:\u00a0Toxic Seeds Part 1_2015<\/a><\/p>\n