Energy in the body is produced in cellular breathing process where ATP is created. A side effect of the aerobic cellular breathing process is the production of reactive oxygen species (ROS) such as peroxides, hydroxyl radical, superoxide, singlet oxygen and so on. Most of these side products are free radical, which are highly reactive molecules. It has been shown that prolonged exposure to high ROS levels causes non-specific tissue damage by attacking membrane phospholipids, proteins, and DNA. Damage from ROS has been suspected to be cause cancer, cardiovascular and neurological diseases, psychiatric diseases, lung and kidney disorders, liver and pancreatic diseases, hypertension, infertility, aging, and so on [Brieger et al. 2012; Rahman et al. 2012].

However, it has become clearer that low to intermediate ROS levels, singlet oxygen in particular, play an essential role in the regulation of the body's functions through the induction of low level stress [D'Autréaux & Toledano, 2007]. This results in a form of stress-response hormesis [Gems & Partridge, 2008], which is a term referring to beneficial effects of a treatment that at high levels is actually harmful [Southam & Ehrlich, 1943]. This effect stems from low level activation of the intrinsic cellular ROS defense mechanism, which deals with oxidative stress, in addition to increasing the activity of phase II response enzymes that protect from damage beyond the ROS. As a result, low ROS levels lead to stress resistance, which manifests itself as reduced damage to tissues and slower aging, and ultimately, to an extended life span [Ristow & Zarse, 2010]. At the cellular level, ROS will induce mechanisms that regulate growth, programmed cell death, and other cellular signaling. At the systems level they contribute to complex functions such as blood pressure regulation, improved cognitive and immune function, and prevention of the development of degenerative and chronic diseases [Brieger et al. 2012; Rahman et al. 2012]. These competing effects make it clear that the level of ROS needs to be maintained within a certain range in the body, since not only ROS levels that are too high will interfere with the body's functions in such a way that potentially can lead to a disease and even death, but also ROS levels that are too low will have a similar detrimental effects.