Thymol

Thymol is a phenolic terpene (thyme-derived) used for antimicrobial/deodorizing effects, typically at low but biologically active concentrations, and it is a well-recognized skin and mucosal irritant in patch testing and real-world use. As a lipophilic phenol it can disrupt the stratum corneum and provoke stinging/burning, with higher risk in eczema or barrier-impaired skin and when layered with other actives. Given its consistent irritancy potential and the frequency of reactions in sensitive populations, it warrants a significant score. Safety Notes: In mainstream OTC skincare, thymol is most often used as a minor antimicrobial/deodorizing constituent within essential-oil/fragrance systems or preservative-boosting blends, with commercial leave-on and rinse-off products commonly sitting around ~0.001–0.05% due to odor and irritation constraints. The highest consumer-available levels observed are in niche antiseptic/deodorant or anti-blemish formulas and some foot/scalp products where thymol is used as a primary antimicrobial active, reaching about 0.1–0.5% in OTC leave-on or short-contact products. Above ~0.5% is uncommon in mass-market cosmetics because thymol becomes strongly scented and can increase sensitization/irritation risk, so such strengths tend to be reserved for drug/professional products rather than general consumer skincare.