In toxicology, the margin of exposure (or MOE) of a substance is the ratio of its no-observed-adverse-effect level to its theoretical, predicted, or estimated dose or concentration of human intake. It is used in risk assessment to determine the dangerousness of substances for which a conventional health based/guidance value (HBGV) (such as the Tolerable Weekly Intake) cannot be established. The MOE approach was formalized for European regulatory use by the European Food Safety Authority (EFSA) in 2005, specifically for substances that are both genotoxic and carcinogenic, as an alternative to mathematical low-dose extrapolation models and to the principle of reducing exposure as low as reasonably achievable (ALARA). Since then, the MOE approach has been extended to other substance classes, with the terminology being further standardized in a 2025 EFSA statement. The MOE is calculated as the ratio between a toxicological reference point derived from a dose-response relationship and the estimated human exposure to the substance. Therefore, a larger MOE indicates a greater distance between the dose at which adverse effects are observed and the dose to which humans are exposed, being associated with lower concern.
Background and motivation
Limitations of the ALARA principle Before the adoption of MOE in 2005, the standard regulatory advice in Europe for genotoxic and carcinogenic substances was the ALARA ("As Low As Reasonably Achievable"). While the ALARA principle is based on the assumption that there is not safe dose for direct-acting genotoxic carcinogens, it was recognized as unpractical and inadequate for practical risk management. This is because it does not provide any basis for comparison of relative risk of different substances and therefore, it does not allow setting priorities for regulatory action based on the magnitude of the concern. ALARA is purely based on hazard identification, ignoring carcinogenic potency and actual human exposure, not being able to distinguish between high and low-concern substances. This is particularly ineffective when considering unavoidable contaminants such as acrylamide, aromatic amines, or other process‑related genotoxicants.
Limitations of low-dose extrapolation Another alternative approach used by some regulatory agencies, involves mathematically extrapolating animal carcigenicity data to the low doses relevant for human exposure. However, EFSA concluded that this type of extrapolation is fundamentally problematic because:
It is rarely known whether any given animal or mathematical model can reflect the underlying biological processed existent at lower doses of exposure. This is because many "low-dose" risk models are based on DNA damage and mutations measured at high doses, which may not reflect the biological response at low doses, where repair, apoptosis, immune stimulation and other defenses are more active. Moreover, evidence for thresholds or sublinear responses at low doses (including adaptive and protective effects) means simple linear scaling from high to low doses can misrepresent true risk. Results can vary by several orders of magnitude depending on the models used, with the actual obtained experimental data having little influence on the result. In the case of Endocrine Disruptors and other mixtures, these can show non-monotonic curves, from which extrapolating linearity from high doses can substantially underestimate low-dose effects or miss them entirely.
Definition and calculation
Basic formula As defined by EFSA, the MOE is the ratio between the Reference Point (RP) and the estimated human exposure:
M O E = R e f e r e n c e P o i n t ( R P ) E s t i m a t e d H u m a n E x p o s u r e {\displaystyle MOE={\operatorname {Reference\,Point\,(RP)} \! \over \operatorname {Estimated\,Human\,Exposure} \!}}
Both the RP and the exposure must be expressed in the same units (typically micrograms or miligrams per kilogram of body weight per day (µg/kg bw/day or mg/kg bw/day). The Reference Point is a dose derived from an experimental or observational dose-response relationship that reflects the critical toxicological effect. It is important to note that, unlike the NOAEL-based approach, the RP in the MOE framework does not imply the existence of a safe threshold, it is simply a defined point on the dose-response curve used for comparative purposes.
Reference points (RP) EFSA's preferred RP for genotoxic carcinogens is the BMDL10 - the benchmark dose lower confidence limit corresponding to a 10% increase in tumor incidence compared to control. This is because, using the lower confidence limit incorporates the statistical uncertainty of the study and ensures with a 95% confidence that the chosen BMDL is not exceeded at that dose. Moreover the Benchmark Dose approach is preferred as it uses all data points on the dose response curve, and is less sensitive to study design choices such as dose spacing. However, when the data is insufficient to derive a BMDL10, EFSA recommends the usage of the T25 - the dose producing a 25% tumor incidence in animals. In other situations, one can use as a reference point the NOAEL (no-observed-adverse-effect-level) and the LOAEL (lowest-observed-adverse-effect-level), particularly for non-genotoxic substances where a BMDL cannot be derived, or where a HBGV (Health-based Guidance Value) is not appropriate.
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