Healthy e-cigarette users have been found to have erythematous and irritable airway mucosa,55 and cases of more serious bronchial injury have been reported.56 Increased levels of the MUC5AC mucin have been found both in bronchial epithelia and in airway secretions, although notably many of these e-cigarette users were former smokers.55, 57 Increased mucin levels inversely correlate with the decline in lung function in COPD patients and are a biomarker of chronic bronchitis, indicating that mucins are a validated biomarker of harm.58, 59. Here, MUC5AC is linked to chronic obstructive pulmonary disease.