Isopentyldiol

Isopentyldiol is a low-molecular-weight diol used mainly as a humectant/solvent and penetration/texture aid, typically around ~1–5% (sometimes higher) in leave-on products. Available patch/HRIPT data and broad cosmetic use history suggest it is generally well tolerated with a low rate of irritant reactions, though stinging can occur in highly compromised barriers due to its solvent/humectant nature and potential to increase permeability of co-ingredients. For patient safety in severe sensitivity/eczema populations, I score it as very gentle rather than inert. Safety Notes: Isopentyldiol is used in commercial skincare primarily as a humectant/solvent and preservative-booster, with low-end use (~0.1–0.5%) in leave-on emulsions and cleansers to support preservation and feel. Most mass-market leave-on products sit around ~1–5%, while higher-strength consumer-available toners/essences, gels, and acne-care/antimicrobial-support formulas commonly reach ~10% and, more rarely, up to ~20% where it functions as a major polyol solvent/humectant. There is no specific EU/FDA maximum concentration limit for isopentyldiol in cosmetics, so practical upper levels are driven by sensorial tack, irritation potential, and formula compatibility rather than regulation.