image: Vitacost N-acetylcysteine (NAC) Improves COPD Outcomes Oxidative stress (imbalance between oxidants and antioxidants) is part of the story of how COPD causes symptoms of cough and shortness of breath. Cigarette smoke is the main source of oxidation damage in the lungs leading to COPD, but even after they quit smoking, people with COPD still have a so-called "redox imbalance" that contributes to their daily COPD symptoms.