Ask a RIFM Scientist

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What is a NESIL, and how does it help ensure the safe use of fragrances?

Isabelle Lee, PhD

NESIL” is short for No Expected Sensitization Induction Level and is the amount of a fragrance ingredient one can be exposed to without causing skin sensitization.

That begs the question: What is skin sensitization? Skin sensitization refers to a development of an immune response to a substance that, after repeated contact, causes an inflammatory skin reaction, often marked by red, bumpy, swollen or itchy skin. At the Research Institute for Fragrance Materials (RIFM), we aim to ensure that never happens.

Most fragrance ingredients cannot cause skin sensitization, and of those that do, most are weak or very weak sensitizers. However, for those with any potential to do so, RIFM uses a weight-of-evidence approach—using all available data and internationally approved tools—to derive a NESIL. We call this a WoE NESIL, short for Weight-of-Evidence No Expected Sensitization Induction Level.

We recently detailed our WoE NESIL derivation process in a peer-reviewed paper published in the Elsevier science journal Food and Chemical Toxicology. The paper details the steps involved, including evaluating all historical animal and non-animal studies in addition to software predictions. RIFM has not performed any new animal studies for skin sensitization for more than a decade.

Like the other RIFM areas of evaluation, skin sensitization is analyzed based on real-world levels of an ingredient in fragranced consumer products. Once a WoE NESIL is derived, RIFM combines it with current exposure data to calculate Maximum Acceptable Concentrations, or “MAC values,” for each product type. MAC values, which provide the scientific foundation for the International Fragrance Association (IFRA) Standards, ensure that even the highest users can safely enjoy their favorite fragranced goods.

Deriving a WoE NESIL is dynamic and will change with evolving science as RIFM actively incorporates the latest new approach methodologies (NAMs).

Senior Scientist Isabelle Lee, PhD, leads RIFM’s Skin Sensitization safety assessment and research programs.