Table 1.

Applications of hydroxy acids

Types/classes of HAs	Safety evaluation	Photocarcinogenesis	Mechanism of biological action
αHAs	Not mutagenic or carcinogenic, not reproductive or developmental toxins, not skin sensitizers11	No alteration of photocarcinogenesis induced by SSR13	Reduced Ca ion concentration in the epidermis disrupts cellular adhesions by removing Ca ions from the cell adhesions by chelation allowing for exfoliation; promoting cell growth and retarding cell differentiation19
Glycolic acid	Increased SSR sensitivity in human skin9,10 Increased epidermal thickness, epidermal and dermal levels of hyaluronic acid, and collagen gene expression23	Increased epidermal proliferation in mice14	Acceleration of collagen synthesis by fibroblasts and also modulation of matrix degradation and collagen synthesis through keratinocyte-released cytokines22 Accelerated epidermal turnover and inhibition of melanin formation in melanocytes by directly inhibiting tyrosinase activity34 Pretreatment increased UV-induced pigmentation35
PHA		Photoprotective (gluconolactone)33	Function as a chelating agent and exhibits potency in scavenging free radicals33
βHA	–	–	–
SA	Enhances percutaneous penetration, not a photosensitizer, not phototoxic, no change in SSR sensitivity in human skin9 UV effects on skin sensitivity should be considered due to its keratolytic action on human skin12	Photoprotective, reducing the carcinogenicity of SSR13,24 Increased epidermal proliferation and thickness in mice14	Acts at the level of transcription to downregulate the production of fibrinogen, fibronectin, and α-hemolysin virulence factors necessary for bacterial replication in host tissues28