No observable adverse effect level

The no observable adverse effect level (NOAEL) denotes the level of exposure of an organism, found by experiment or observation, at which there is no biologically or statistically significant (e.g. alteration of morphology, functional capacity, growth, development or life span) increase in the frequency or severity of any adverse effects in the exposed population when compared to its appropriate control.[1][2]

In toxicology it is specifically the highest tested dose or concentration of a substance (i.e. a drug or chemical) or agent (e.g. radiation), at which no such adverse effect is found in exposed test organisms where higher doses or concentrations resulted in an adverse effect.[3]

This level may be used in the process of establishing a dose-response relationship, a fundamental step in most risk assessment methodologies.

In drug development, NOAEL of a new drug is assessed in laboratory animals drugs prior to initiation of clinical trials to establish a safe clinical starting dose in human trials.

The United States Environmental Protection Agency (US EPA) defines NOAEL as '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 precursors 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.'[4].

See also

  • lowest-observed-adverse-effect level


  1. ^ Seton Resource Center
  2. ^ Definition of NOAEL
  3. ^ Food Safety and Risk Assessment website at Glasgow Caledonian University
  4. ^