Highly reactive free radicle chemicals formed from oxygen as part of normal cellular metabolism. Examples are Peroxides (hydrogen peroxide) superoxide, hydroxyl radicle (OH–) and singlet oxygen. Normally involved in cell signaling processes these molecules can cause damage to organelles within the cell and to cell membranes and DNA if they build up to excessive levels. Under normal conditions ROS are scavenged by the antioxidant systems before accumulating to harmful levels. The principal antioxidants are the vitamins E & C which work by donating a hydrogen ion rendering the ROS harmless.