Having completed Phase I study at University of Michigan, we planned to do the Phase II efficacy study under separate NCT number. As of 2016 this phase II study has not begun. Based on the initial study, we plan to choose the highest quercetin dose tolerated with no adverse events and the dose (500 mg of quercetin per day) that was found to increase plasma quercetin levels by 4 fold over baseline in healthy volunteers to examine the efficacy of quercetin in reducing inflammatory and oxidative stress markers and improving lung function in COPD subjects. In the second phase, we will enroll a total of 75 subjects and randomized into three arms; placebo (15 subjects) or one of the two doses of quercetin (30 subjects per arm). All enrolled subjects will be asked to avoid quercetin rich foods throughout the study period. One week after enrollment (run-in), subjects will be either supplemented with either placebo or one of the two doses of quercetin for 4 weeks. All participants will be blinded for study agents. Plasma and sputum quercetin levels, lung function, and markers of oxidative stress and inflammation will be determined at the start of the study (following run-in period), at the end of 4 weeks treatment period.