Most common precipitating factor for COPD is -

Cigarette smoking is the most impoant risk factor for moality from COPD.There is a dose-response relationship curve between reduced pulmonary function and cigarette smoking,which accounts for the higher prevalence rates for COPD with increasing age.The intensity of cigarette smoking is expressed as pack years(average number of packs of cigarettes smoked per day multiplied by the total number of years of smoking.) Reference:Harrison's medicine-18th edition,page no:2151.