Oxidative stress in cells and tissues produces Free radical and singlet oxygen oxidations of linoleic acid to generate 13-HpODEs, 9-HpODEs, 13-HODEs, and 9-HODEs; these non-enzymatic reactions produce or are suspected but not proven to produce approximately equal amounts of their S and R stereoisomers.  Free radical oxidations of linoleic acid also produce 13-EE-HODE, 9-hydroxy-10E,12-E-octadecadienoic acid, 9-hydroxy-10E,12-Z-octadecadienoic acid, and 11-hydroxy-9Z,12Z-octadecaenoic acid while singlet oxygen attacks on linoleic acid produce (presumably) racemic mixtures of 9-hydroxy-10E,12-Z-octadecadienoic acid, 10-hydroxy-8E,12Z-octadecadienoic acid, and 12-hydroxy-9Z-13-E-octadecadienoic acid.  4-Hydroxynonenal (i.e. 4-hydroxy-2E-nonenal or HNE) is also a peroxidation product of 13-HpODE. Since oxidative stress commonly produces both free radicals and singlet oxygen, most or all of these products may form together in tissues undergoing oxidative stress.  Free radical and singlet oxygen oxidations of linoleic acid produce a similar set of 13-HODE metabolites (see 9-Hydroxyoctadecadienoic acid).  Studies attribute these oxidations to be major contributors to 13-HODE production in tissues undergoing oxidative stress including in humans sites of inflammation, steatohepatitis, cardiovascular disease-related atheroma plaques, neurodegenerative disease, etc. (see oxidative stress).