Smoking and the development of progressive airflow obstruction.

An overall relation between cigarette smoking and the development of progressive airflow obstruction is established but only a minority of smokers ever develop severe airway narrowing; with the striking but rare exception of homozygous alpha 1-antitrypsin deficiency, the causes of this variation in susceptibility between smokers are uncertain. Possible associated risk factors may be classified under three broad headings--exogenous irritants, broncho-pulmonary infections and endogenous or 'host' factors--and will be briefly reviewed in this paper. But first the natural history of progressive airflow obstruction must be considered because this determines how potential risk factors can be assessed.