Copper Sulfate

Copper sulfate is an inorganic salt used mainly as an antimicrobial/astringent and, at functional levels, it can be corrosive and strongly irritating to skin and mucosa; irritant reactions are well-documented in occupational and patch-testing contexts. Even at low cosmetic-relevant concentrations, it carries meaningful risk for stinging, erythema, and barrier disruption, with heightened danger in eczema or compromised/post-procedure skin and in leave-on routines where cumulative irritation increases. Given its known irritancy profile and narrow safety margin for sensitive users, I score it as high irritation potential. Safety Notes: In consumer skincare, copper sulfate is most commonly encountered at trace levels (around 0.0001–0.01%) as a colorant/impurity-associated copper source or as a minor antimicrobial/astringent component in niche toners, acne products, and rinse-off cleansers. Higher-strength OTC availability is uncommon due to irritation/toxicity concerns and regulatory safety expectations, but specialty astringent/antimicrobial-type formulations marketed to the general public (typically rinse-off or very limited-area use) have been observed up to ~0.5%. Leave-on products generally stay at the low end of the range, while the upper end is more consistent with rinse-off or spot-use products.