Isopropylideneglycerol

Isopropylideneglycerol is primarily used as a solvent/humectant-like vehicle in leave-on products, typically at low concentrations, and is generally considered well tolerated in routine cosmetic use. However, as a small, water-miscible solvent it can increase penetration of other ingredients and can sting on compromised barriers (e.g., eczema or post-procedure), so I do not score it as “very gentle.” Overall it fits best as a gentle ingredient with low but non-zero irritation potential in highly reactive skin. Safety Notes: Isopropylideneglycerol (commonly used as a solvent/humectant and delivery/feel modifier) is typically present at trace-to-low levels in mass-market leave-on creams/lotions/serums where it functions as a supporting solvent, often around 0.05–0.5%. In higher-solvent specialty leave-on products (e.g., treatment serums, boosters, or hydroalcoholic-style systems) it can be used at multi-percent levels to aid solubilization and sensorial profile, with observed consumer-available maxima around ~5%. Rinse-off products generally sit toward the low end because the functional benefit is less critical and cost/irritation constraints favor lower use levels.