MPO and chronic obstructive pulmonary disease: Clinical studies have demonstrated that H2O2, 8-isoprostane, 4HNE, myeloperoxidase (MPO) and malondialdehyde (MDA) (endogenous oxidants as biomarkers of oxidative stress) are increased in exhaled breath condensate in patients with COPD [13,24,25,26,27], compared to healthy individuals; and these makers are more elevated during exacerbations [28].