Free radicals are unstable molecules that have an unpaired electron, making them highly reactive. They are generated as natural byproducts of cellular metabolism, but can also be produced by external factors such as UV radiation, pollution, smoking, and poor diet. Free radicals are known to damage cellular structures, including DNA, proteins, and lipids, through a process called oxidative stress. In the skin, oxidative stress caused by free radicals can lead to premature aging, including wrinkles, fine lines, sagging, and age spots.