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Quick answer: Free radicals are highly reactive molecules, and oxidative stress occurs when reactive oxygen species outpace the skin’s antioxidant defenses. Ultraviolet radiation is a major source. Over time, oxidative processes can affect lipids, proteins, DNA and structural components involved in visible photoaging.
A free radical has an unpaired electron, which makes it reactive. Reactive oxygen species include free radicals and related molecules. The body creates them during normal metabolism, while ultraviolet light, tobacco smoke and some pollutants can increase their formation.
Skin uses enzymes and antioxidant molecules to manage reactive species. Oxidative stress describes an imbalance in which reactive activity exceeds those defenses. It is a process measured in research—not a condition a person can diagnose from a dull complexion.
Ultraviolet exposure can activate signaling pathways that increase breakdown of collagen and other extracellular-matrix components. It can also contribute to inflammation and uneven pigmentation. This is one reason chronic sun exposure is linked with wrinkles, texture change and spots, but intrinsic aging, genetics and lifestyle also matter.
Antioxidants donate electrons or otherwise interrupt oxidative reactions. Vitamin C, vitamin E and plant polyphenols are used topically for this reason. Laboratory antioxidant activity does not guarantee a clinical benefit: the molecule must remain stable, reach the relevant skin compartment and be present in an effective finished formula. See Antioxidants for skin: what the evidence shows.
For a routine rather than the chemistry, read How to protect skin from oxidative stress and environmental damage.
Vitamin C is a water-soluble antioxidant with roles in normal collagen biology. Topical L-ascorbic acid is best studied, while derivatives differ in stability and evidence. Read Vitamin C for skin: benefits, forms and use before choosing a formula.
