ROS are chemically-reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons. ROS form as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling. However, during times of environmental stress (e.g. UV or heat exposure) ROS levels can increase dramatically. This can result in significant damage to cell structures. This cumulates into a situation known as oxidative stress.