**7.4 Singlet oxygen**

Singlet oxygen (1 O2) is an electronically excited form of oxygen which is well known to be formed when photosensitizers such as chlorophyll or the aromatic dye rose Bengal absorb light energy and transfer some of that energy to molecular oxygen [122, 123]. Various nonphotosensitized mechanisms for its formation have also been reported and suggested to occur in biological systems, but the importance of such endogenous singlet oxygen formation has had a controversial history [122, 124]. Ozone (O3) is best known as occurring in the stratosphere where it shields organisms on earth from ultraviolet C and much of ultraviolet B radiations, which are the most damaging UV components of solar radiations because they are readily absorbed by DNA [125, 126]. It is also known as a respiratory system-damaging pollutant in the troposphere and ironically as a therapeutic agent in alternative medicine [127]. More recently, it was shown that antibodies or amino acids catalyze the conversion of singlet oxygen (1 O2) to ozone (O3) and that this reaction occurs during the killing of bacteria by activated neutrophils [128, 129]. Since both singlet oxygen and ozone are highly reactive oxygen species, a full understanding of their mechanisms of formation and action *in vivo* is necessary. Hence, various reported mechanisms of the endogenous formation of these reactive oxygen species (ROS), the potential relevance of such pathways in human physiology, diseases and activity of these oxidants.
