First, U/mL values were converted to specific activity (U/mg protein) values, and the latter were used to depict LineweaverBurk and secondary plots (Segel 1975)
That said, for some people it simply comes down to their skin not responding well to certain ingredients, which may result in an unwanted reaction to skincare products (known as irritant contact dermatitis)
References Coassolo, L., B
For the Bronze, Silver, and Gold tiers, you have flexibility
Local rectal delivery in a gastrointestinal condition cannot be assumed to predict systemic exposure, tissue concentrations, effectiveness, or safety from an injection or oral product
Genetic abnormalities associated with increased gut permeability or leakage, include the human leukocyte antigen (HLA) gene involved in the presentation of foreign antigens to T-cells