The relevance in adults of air-flow obstruction, but not of mucus hypersecretion, to mortality from chronic lung disease. Results from 20 years of prospective observation. by Peto, R, Speizer, F, Cochrane, A, Moore, F, Fletcher, C, Tinker, C, Higgins, I, Gray, R, Richards, S, Gilliland, J, Norman-Smith, B