Oxygen can accept single electrons to form unstable derivatives, referred to as reactive oxygen species (ROS). This ROS can seriously damage living cells
Examples of ROS : superoxide radical, hydrogen peroxide, the hydroxyl radical, and singlet oxygen.
ROS formation is usually kept to a minimum by antioxidant defense mechanisms. Oxidative stressExamples of circumstances that may cause serious oxidative damage :
overconsumption of certain drugs
exposure to intense radiation,
repeated contact with certain contaminants (e.g., tobacco smoke).