Yeast

In skincare, “yeast” typically refers to yeast extract/ferment used at low concentrations as a humectant/skin-conditioning ingredient, and it is generally well tolerated with low irritancy in routine patch testing. However, as a biologically derived mixture containing proteins/peptides and other metabolites, it carries a small but real risk of irritation or sensitization in highly reactive or eczema-prone individuals, so it cannot be scored as very gentle/inert. Considering compromised-skin patients and cumulative routine exposure, a conservative gentle score is most consistent with safety. Safety Notes: In commercial skincare, “yeast” most commonly appears as yeast extract/ferment (e.g., Saccharomyces ferment filtrate/lysate), where it is frequently used at very low levels (~0.01–0.5%) as a supporting antioxidant/soothing or conditioning component in leave-on serums/creams and rinse-off cleansers. Many mainstream leave-on products center around ~0.5–5% yeast ferment/extract blends, while the highest consumer-available formulations (e.g., ferment-forward essences/ampoules and some yeast-based masks) can reach ~10% of yeast-derived material, typically as a filtrate/lysate rather than live yeast. Levels above this are uncommon due to odor, color, stability, and potential sensitization considerations, and there is no specific FDA/EU maximum limit for yeast/yeast extract itself beyond general cosmetic safety requirements.