Inclusion Criteria:  man and woman at the age of ≥40 years old Diagnosed with COPD within the past 1 year based on pulmonary function test (post bronchodilator FEV1/FVC<0.7) COPD Assessment Test (CAT) phlegm score ≥2 Smoking history ≥10 pack-years* (*Pack-year: number of packs of cigarettes smoked per day X number of years the individual has smoked) Patient who is to be treated with Mucomyst nebulizer therapy to alleviate difficulty of expectoration symptoms Voluntary written informed consent to study participation and to use of personal information for this study  Exclusion criteria  History of hypersensitivity reactions to any of the components of NAC or contraindicated to NAC therapy according to the product label Treated with Mucomyst within the past 4 weeks Newly treated with 'drugs for the alleviation of expectoration symptoms (antitussives/expectorants)' or dose changed for ongoing therapy within the past 4 weeks Pregnant and lactating woman Participating in another trial with investigational product treatment at the time of enrollment to the present study Ineligible for study participation in the opinion of the investigator