reactive oxygen species

Reactive oxygen species (ROS) include oxygen ions,  free radicals and peroxides both inorganic and organic. They are generally very small molecules and are highly reactive due to the presence of unpaired valence shell electrons. ROSs form as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling. However, during times of environmental stress ROS levels can increase dramatically, which can result in significant damage to cell structures. This cumulates into a situation known as oxidative stress.
See more at Wikipedia.org...

Reactive oxygen species, n molecules and ions of oxygen that have an unpaired electron, thus rendering them extremely reactive. [more]Reactive oxygen species - Community and Resources

Forms of oxygen that have enhanced chemical reactivity compared with the normal oxygen molecule; many are free radicals (see also free radical).Risk factorPhysical condition or lifestyle that in epidemiological studies appears to increase susceptibility to a particular disease.