<P> A dose--response curve is a simple X--Y graph relating the magnitude of a stressor (e.g. concentration of a pollutant, amount of a drug, temperature, intensity of radiation) to the response of the receptor (e.g. organism or population under study). The response may be a physiological or biochemical response, or even death (mortality), and thus can be counts (or proportion, e.g., mortality rate), ordered descriptive categories (e.g., severity of a lesion), or continuous measurements (e.g., blood pressure). A number of effects (or endpoints) can be studied, often at different organizational levels (e.g., population, whole animal, tissue, cell). </P> <P> The measured dose (usually in milligrams, micrograms, or grams per kilogram of body - weight for oral exposures or milligrams per cubic meter of ambient air for inhalation exposures) is generally plotted on the X axis and the response is plotted on the Y axis . Other dose units include moles per body - weight, moles per animal, and for dermal exposure, moles per square centimeter . In some cases, it is the logarithm of the dose that is plotted on the X axis, and in such cases the curve is typically sigmoidal, with the steepest portion in the middle . Biologically based models using dose are preferred over the use of log (dose) because the latter can visually imply a threshold dose when in fact there is none . </P> <P> The first point along the graph where a response above zero (or above the control response) is reached is usually referred to as a threshold - dose . For most beneficial or recreational drugs, the desired effects are found at doses slightly greater than the threshold dose . At higher doses, undesired side effects appear and grow stronger as the dose increases . The more potent a particular substance is, the steeper this curve will be . In quantitative situations, the Y - axis often is designated by percentages, which refer to the percentage of exposed individuals registering a standard response (which may be death, as in LD). Such a curve is referred to as a quantal dose - response curve, distinguishing it from a graded dose - response curve, where response is continuous (either measured, or by judgment). </P> <P> A commonly used dose - response curve is the EC curve, the half maximal effective concentration, where the EC point is defined as the inflection point of the curve . </P>

The threshold on a dose-response curve is the