The dose of a chemical at which there were no statistically or biologically significant increases in frequency or severity of adverse effects seen between the exposed population and its appropriate control. Effects might be observed at this dose but were judged not to be "adverse."
Greatest concentration or amount of an agent, found by experiment or observation, that causes no detectable adverse alteration of morphology, functional capacity, growth, development or life span in an organism, system or (sub) population.
Compare? A level of exposure which does not cause observable harm.
The NOAEL is the highest dose from a study that did not find any adverse health effects. The NOAEL is often used to derive MRLs and RfDs.
The highest tested dose of a substance that has been reported to have no harmful (adverse) health effects on persons or animals.