Dose response is the relationship between the effect on living things and a stimulus from a physical, chemical, or biological source. Examples of stimuli include therapeutic drugs, pesticides, pathogens, and radiation. A quantal response occurs when the living thing either responds or does not respond to a stimulus of a given dose. Graded responses proportional to the size of the stimulus are also found in environmental applications. Some types of responses have a significant time effect. The Effective Dose for affecting 50% of a population of test subjects, ED50, and the Lethal Dose for killing 50% of a population of test subjects, LD 50 , are commonly used parameters for reporting the toxicity of an environmental pollutant.