3) Stockley, R.A., O'Brien, C., Pye, A., and Hill, S.L. 2000. Relationship of sputum color to nature and outpatient management of acute exacerbations of COPD. Chest 117:1638-1645.
4) 1983. Intermittent positive pressure breathing therapy of chronic obstructive pulmonary disease. A clinical trial. Ann Intern Med 99:612-620.