Mammalian tissues have large amounts of available ATP which are generated by oxidative phosphorylation in mitochondria. To maintain the body, large amounts of oxygen are required to regenerate the ATP molecules. A small fraction of the inspired oxygen is converted to superoxide radicals and related metabolites even under physiological conditions. Most reactive oxygen species react rapidly with a variety of molecules, thereby interfering with cellular functions and various diseases.
Sato, E. F., & Inoue, M. (2002, January). Reactive oxygen. Japanese Journal of Hygiene. https://doi.org/10.1016/b978-012103520-4/50019-2