2,3-Butanediol

2,3-Butanediol is a low–molecular weight diol used mainly as a humectant/solvent at modest concentrations, and diols in this class are generally well tolerated with low rates of irritation in standard patch testing. In very reactive or barrier-impaired skin it can still cause occasional stinging or dryness (especially alongside other actives), but the overall intrinsic irritancy is low, supporting a very gentle score. Safety Notes: 2,3-Butanediol (often listed as butanediol) is used primarily as a solvent/humectant and occasional fragrance/processing aid; in many mass-market leave-on skincare products it appears at very low levels (trace to ~0.1%) as part of an extract/ferment carrier system. In higher-solvent, water-based leave-on formats (serums/essences/ampoules, especially ferment-focused products), it can be deliberately dosed in the low-single-digits up to around 10% to support solvency, sensorial slip, and humectancy; rinse-off products typically sit toward the lower end due to wash-off and cost/need considerations. No specific EU/FDA maximum applies beyond general safety/GMP; practical limits are driven by odor/feel and compatibility with the rest of the solvent system.