When to use TTC as a Risk Assessment Tool for Impurities in a Cosmetic Formulation?

In vitro toxicology and Dermato-Cosmetology (IVTD), Vrije Universiteit Brussel, BE

The Treshold of Toxicological Concern (TTC) is a pragmatic risk assessment tool establishing human threshold values for chemical substances below which there is a very low probability of adverse effects to human health.  It is used for very small amounts of substances for which only the chemical structure and exposure are known. For cosmetic products, quality exposure data are available in the Notes of Guidance of the Scientific Committee on Consumer Safety (SCCS/1564/15). TTC is based on the analysis of historical in vivo toxicity studies and chemical structures of a broad range of chemicals present in separate databases covering either carcinogenic or non-cancer toxicity endpoints. Before using TTC, one should first check that the substance under consideration belongs to a structural class that is well represented in the databases. As such UV-filters and oxidative hair dyes are excluded since these complex structures are not present in the databases. The TTC approach should not be used for substances for which EU legislation requires the submission of toxicity data or when data are available that allow for a standard risk assessment or if the substance falls into one of the exclusion categories.
TTC values for non-cancer toxicological endpoints are: for compounds belonging to Cramer class I 30µg/kgbw/d and Cramer class III 1.5µg/kgbw/d; Cramer class II substances are considered as belonging to class III. 
TTC values for substances with genotoxic alerts: 2.5ng /kgbw/day.                                                            
Recently, higher threshold values have been proposed, but these have not yet been discussed by the SCCS and are not yet practically used.