Effect Level Definitions
Adverse effect: A biochemical change, functional impairment, or pathologic lesion that affects the performance of the whole organism or reduces an organism's ability to respond to an additional environmental challenge.
Frank-effect-level (FEL): A level of exposure or dose which produces irreversible, adverse effects at a statistically or biologically significant increase in frequency or severity between those exposed and those not exposed.
Lowest-observed-adverse-effect level (LOAEL): The lowest exposure level at which there are statistically or biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control group. This is also referred to as lowest-effect level (LEL).
Lowest-observed-effect level (LOEL): In a study, the lowest dose or exposure level at which a statistically or biologically significant effect is observed in the exposed population compared with an appropriate control group.
No-observed-adverse-effect level (NOAEL): The highest 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 adverse nor precursors to adverse effects.
No-observed-effect level (NOEL): An exposure level at which there are no statistically or biologically significant increases in the frequency or severity of any effect between the exposed population and its appropriate control.
Adapted from: U.S. EPA. 2003. Integrated Risk Information System (IRIS). Online. National Center for Environmental Assessment, Washington, DC.