Margin of Safety (MoS)

The Margin of Safety (MoS) is the central quantitative metric in cosmetic ingredient safety assessment. It compares the highest dose at which no adverse effects are observed (NOAEL) to the actual systemic exposure a consumer receives from using the product.

The MoS is calculated as: MoS = NOAEL / SED, where NOAEL is the No Observed Adverse Effect Level from the most relevant toxicological study, and SED is the Systemic Exposure Dose calculated from the product's concentration, amount applied, dermal absorption rate, body surface area, and body weight.

The SCCS Notes of Guidance (SCCS/1647/22) establishes that a MoS of 100 or greater is generally considered to indicate adequate safety. This 100-fold safety factor accounts for inter-species variation (factor of 10, when extrapolating from animal data to humans) and inter-individual variation (factor of 10, accounting for differences within the human population including sensitive subgroups).

If the calculated MoS falls below 100, the safety assessor may still conclude the ingredient is safe if additional evidence supports a lower safety factor — for example, if human data are available (reducing the inter-species factor) or if the exposed population is well-defined. Conversely, additional uncertainty factors may be applied for ingredients with genotoxicity concerns, insufficient data, or exposure to vulnerable populations such as children.