By their very nature as small, lipophilic chemicals capable of stimulating olfactory receptors, fragrance ingredients have an innate ability to penetrate the skin. Depending on their chemical reactivity, some of these ingredients have the potential to sensitize individuals on repeated application; whether they do so or not depends primarily on dose and exposure conditions. These factors have been extensively reviewed as discussed recently by Kimber et al. These researchers make the case, based on earlier work by Kligman, Friedmann and others, that dose per unit area is the relevant metric for assessing the risk of skin sensitization under most conditions. This thought process forms the basis for established risk assessment and management methods for fragrances and fragranced products—a process that is coordinated for the industry by the International Fragrance Association (IFRA) and its research arm, the Research Institute for Fragrance Materials (RIFM).
The necessary steps for acquisition of skin sensitization are well known and include: penetration of the sensitizing ingredient into the viable skin layers; incorporation of the hapten (the original ingredient or a reaction product thereof) into the native protein associated with Langerhans cells or other dendritic cells in the skin; migration of these cells through the afferent lymphatics to the draining lymph node; and activation and clonal expansion of T-lymphocytes located within these nodes. The activated T-lymphocytes are released into the bloodstream and migrate back into the skin, where they are able to mount an inflammatory attack against subsequent appearances of the hapten. The LLNA incorporates all of these steps except the dissemination of T-cells required for elicitation of a skin allergy response. Mice are dosed topically on the ears for three days, leading to skin penetration and (for sensitizers) reactions with dendritic cell surface proteins and migration of these cells to the draining lymph node. T-cell activation and expansion is inferred from the uptake of 3H-thymidine by the node approximately five days after the initial dose of the test compound.11 This methodology has been widely used in the cosmetic industry for the past 15 years.
In today’s environment, the methodology for assessing consumer product safety is undergoing rapid change. Public opinion, backed by regulatory action, has forced a sharp turn toward alternative testing strategies that do not employ vertebrate animals. In the case of skin sensitization, legislation passed by the European Union in 2006 (the 7th Amendment to the EU Cosmetics Directive) dictates that new cosmetic ingredients marketed in the EU after 2013 shall not have been tested on animals. The legislation has spurred a great deal of research.