No Observable Adverse Effect Level

An exposure level at which there are no statistically or biologically significant increases in the frequency or severity of adverse effects between the exposed population and its appropriate control; some effects may be produced at this level, but they are not considered as adverse, or as precurors to adverse effects. In an experiment with several NOAELs, the regulatory focus is primarily on the highest one, leading to the common usage of the term NOAEL as the highest exposure without adverse effects.

From long-term toxicological studies of agricultural chemical active ingredients, levels which indicate a safe, lifetime exposure level. Used in setting pesticide residue tolerances.