Witch hazel (Hamamelis virginiana), classified as a member of a witch hazel family (Hamamelidaceae), is recognized for its use in traditional medicine as well as modern dermatology and cosmetology. Its adstringent, antibacterial, antioxidant, anti-inflammatory, and soothing properties help relieve inflammatory skin conditions and skin damage. In cosmetics products it is most often used in skin care, for skin moisturizing and soothing irritated skin and scalp. In this thesis, we assessed the safety of witch hazel extract as a cosmetic ingredient using the threshold of toxicological concern approach (TTC), which classifies compounds into Cramer toxicity classes based on their chemical structure. A TTC value is assigned to each of the three classes. Using the literature data, we examined the qualitative and quantitative composition of the leaves and bark of the extract, and based on the physico-chemical properties of the compounds using the ChemOffice software, we calculated the potential absorption range and daily systemic exposure. We found out that most of the extract's components do not pose a health risk, as their systemic exposure does not exceed the TTC limit values, with the exception of hamamelitannin in witch hazel bark extract, which exceeds the TTC limit value. For compounds with unknown concentrations, we determined the maximum permissible concentration in which they can be present in the extract based on the TTC value and the potential absorption range. For some compounds, we were able to obtain experimental NOAEL values from in vivo toxicological studies, which were used to calculate safe daily doses. We found out that the safe daily doses are higher compared to the TTC values and systemic exposure values. Especially, the large differences between the NOAEL values and the Cramer toxicity class values indicate the conservatism of the TTC approach, which is otherwise useful when the experimental data on the toxicity of a particular compound is insufficient. However, based on our calculations and results obtained, we were able to assess that the witch hazel extract is safe for use in cosmetic products.
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