Free radicals are unstable and highly reactive molecules that contain one or more unpaired electrons, and are generated by metabolism, environmental pollutants, smoking and sunlight.

• 自由基是不稳定并且具高度反应力的分子，含有一个或者一个以上不成对的电子，经由新陈代谢、环境污染、抽烟或者太阳光所产生。