††††††Probably the most common and simplest toxicity measurement is the dose-response relationship (see Content section).† Toxicity is expressed as the plot (normal or logarithmic) of the amount of chemical (dose or concentration, on the x-axis) that causes a biological response (y-axis). Figure 1 shows an illustration of a log dose response curve.† In this graph point A represents the intersection of the horizontal line drawn from the point of 50% biological effect with the toxicity curve. †From point A, a vertical straight line is drawn to its intersection with the x-axis.† This intersection is point B, and it represents the concentration of toxicant that causes the studied biological response in exactly half of the exposed organisms.††† If the studied biological effect is lethality, then this is the LC50 or LD50.
Figure1. Plot of the cumulative percent of organisms giving a biologicalresponse against the log of the concentration of toxicant.
††††††††There are several factors that influence an individualís response to toxic insult as compared to other individuals of the same species.† This is what is called tolerance and it happens at different intensities.†††††††† When the distribution of tolerance in a population of individuals of the same species is plotted against the logarithm of the dose or concentration of the toxicant, a normal (Gaussian) distribution is observed (Figure 2).† Individuals that are on the extremes of the curve are said to be intolerant or susceptible, if they show effects at lower levels than the rest of the population and tolerant or no susceptible, if they are affected at higher levels that the majority of individuals.† A common example would be the differences in tolerance to alcohol consumption shown by different individuals when the same effects are observed.
Figure2. Tolerance distribution is a population of individuals exposed toincreasing concentrations of a toxicant.
†††††††Exposure to a toxicant is the basis for any risk assessment.† In other words, exposure estimation is necessary in order to evaluate the likelihood of a toxic event.† How to estimate exposure?† Exposure is estimated by using quantitative toxicity measurements.†     When quantifying toxicity all the measurements refer to a dose or concentration that causes a toxic effect in a target.† It is important to differentiate between dose and concentration.† Concentration-based measurements are those in which the amount of chemical causing the toxic response is expressed as a concentration in the environment.† For example, concentration of chemical X in soil, air, water, etc.† It is measured in concentration units like mg/L, mol/L, etc.† Dose-based measurements are those expressed as mass of toxicant per mass of organism tested.† For example, mass of toxicant X (mg) per Kg of tissue.† These tests are used when a known amount of a toxicant is administered directly to a test organism, like by injection, cutaneous application, etc.† However if the toxicant is dissolved in a medium and then the test organism is exposed to that medium, like in the case of aquatic and inhalation studies, then concentration-based measurements apply.†††††††The most common measurement of toxic effects is the 50% level.† This refers to the amount (dose or concentration) of a chemical that causes the expected effect in 50% of the population exposed.† In other words, at this value 50% of all the organisms exposed are tolerant and the other half are intolerant.† When the effect studied is not lethality then the concentration is referred to as the effective concentration (EC) for the respective percent affected, for example, EC50, EC90 and EC10.† Lethality is expressed as the lethal dose (LD) or lethal concentration (LC) and the most common parameter is the lethal dose or concentration at 50% (LD50 or LC50).††††††††An endpoint refers to the biological effect measured in a population during a toxicity study.† Endpoints can be lethal or sublethal.† Lethality is measured as the total number of animals that died at the respective concentration or dose.† Sublethal endpoints are all the other and they include a wide variety of responses like mobility, growth and developmental effects, respiration changes, bioluminescence, teratogenic effects and many others.
††††††††In environmental studies due to the low concentrations found, another set of parameters are measured based on statistically significant differences between controls and treatments. These are the No Observed Effect Level (NOEL) or No Observable Adverse Effect Level (NOAEL) and the Lowest Observable Effect Level (LOEL) or Lowest Observable Adverse Effect Level (LOAEL).† These concepts are illustrated in Figure 3.
Figure3. Diagram illustrating the no observed adverse effect level (NOAEL)and the lowest observed adverse effect level (LOAEL)
Summary of Terms
1- LC50: Concentration of a chemical in the environment that kills 50% of the test population.
2- EC50: Concentration of a chemical in the environment that causes an expected effect in 50% of the test organisms when compared to a control population under standard conditions and over an specified exposure period.
3- NOEL: The maximum concentration or highest dose used in a toxicity test that produce no response (no statistical significant difference) when compared to a control.
4- LOEL: The lowest concentration of a chemical used in a toxicity test that causes an effect (statistically significant difference) when compared to a control.
The term bioassay refers to experiments designed to evaluate the toxic effect of a compound under controlled conditions. The conditions of the experiments can be changed according to the goals of the test in order to ensure that the test system is a good representation of the real scenario.† Variables include different species of test animals, duration of exposure, toxicant concentration and frequency of exposure, among others.     Bioassays are commonly classified according to the duration of exposure into acute, subacute, chronic and multigenerational. Acute toxicity tests are conducted for short periods of time, for example 24 or 96 hours maximum.† Subacute tests are carried out for a time period that cannot exceed 10% of the lifespan of the organism being tested. Chronic measurements include exposure to the toxicant for a significant portion of the life of the species.† When the goal of the assay is to determine if the toxicant may cause reproductive or teratogenic effects then it should be conducted throughout several generations and therefore it is named as multigenerational.Life-cycle tests are those that require that the animals involved be exposed to the toxicant from the stage when they are embryos and through a whole reproductive cycle, normally meaning more than12 months. They are most common for aquatic animals like fish and invertebrates.† Freshwater species normally used in this test are the fathead minnow (Pimephales promelas), bluegill (Lepomis macrochirus), brook trout (Salvelinus fontinales), the water flea (Daphnia magna), amphipod and snail (Physa integra) and some other marine species.† These tests however, are very complicated and shorter versions are now being used. They include the use early life stages of these species.†     Mutagenicity tests are also widely used and they include tests like the Ames test in which the potential mutagenicity of a toxicant is evaluated by its capacity to revert a mutation already present in a strain of Salmonella typhimurium bacteria.     In plants, toxicity is studied by exposing the plants to the chemical via spraying or brushing or by root uptake.† Possible endpoints are relative growth rate, deformities, loss of chlorophyll, etc.
††††††Due to increasing public awareness and concern for the number of animals involved in toxicity studies, several nations have passed legislation that regulates the use of animals and the application of certain tests.† Other alternative methodologies have developed as a result and they include in vitro bioassays, bioassays using microorganisms and quantitative structure-activity relationships (QSARs) in which the toxicity of a compound is predicted based on its physicochemical properties and other parameters.