Oxidative stress comes from environmental exposure to air pollution, ultraviolet light, and tobacco smoke as well as from natural body functions such as food digestion, and cell respiration. Oxidative stress also occurs when we are sick, fighting an infection, have muscle inflammation, or when we are healing from an injury. This oxidative stress is caused by free radicals and singlet oxygen which can result in damage to the body by interacting with normal cells to change their chemical structure.