自由基是一種缺少電子的不穩定分子，它通過「搶奪」周圍分子的電子來穩定自身，這個過程叫做氧化。皮膚細胞被自由基氧化，DNA 受損，膠原蛋白分解，細胞膜被破壞——這是皮膚老化最重要的機制之一。
紫外線是最主要的自由基來源，其次是污染顆粒、香菸煙霧、某些化學物質，甚至正常的新陳代謝過程。沒有人能完全避開自由基，但可以提升皮膚的抗氧化防禦力。
抗氧化劑的工作原理，是自願「捐出」電子給自由基，讓它們穩定，從而阻斷連鎖氧化反應。維他命 C（抗壞血酸）、維他命 E（生育酚）、白藜蘆醇、輔酶 Q10 都是研究最充分的皮膚用抗氧化劑。
需要注意的是，維他命 C 本身也容易被氧化。暴露在空氣和光線中的維他命 C 精華，如果顏色從透明或淡黃色變成深橙甚至棕色，意味著成分已經氧化失效。活性維他命 C 應該儲存在遮光容器中，開封後在三個月內使用完畢。
使用抗氧化精華的最佳時機是早晨：白天接觸的 UV 和污染是自由基的主要來源，早上塗抹能夠最大化保護效果。
Free radicals are unstable molecules missing an electron. They stabilize by "stealing" electrons from surrounding molecules, initiating a chain reaction of oxidation. When skin cells are oxidized, DNA is damaged, collagen is broken down, and cell membranes are compromised — this is one of the most significant mechanisms of skin aging.
UV radiation is the primary source of free radicals, followed by pollution particles, cigarette smoke, certain chemicals, and even normal metabolic processes. Free radical exposure cannot be entirely avoided, but the skin's antioxidant defenses can be strengthened.
Antioxidants work by voluntarily donating electrons to free radicals, stabilizing them before they can initiate damage. Vitamin C (ascorbic acid), Vitamin E (tocopherol), resveratrol, and CoQ10 are among the best-researched topical antioxidants.
One important caveat: Vitamin C is itself prone to oxidation. If a Vitamin C serum changes from clear or pale yellow to deep orange or brown, the active ingredient has degraded and is no longer effective. Active Vitamin C should be stored in opaque packaging and used within three months of opening.
The optimal time to apply antioxidants is morning. Daytime UV and pollution exposure are the primary sources of free radical generation; morning application maximizes protective effect throughout the day.