Recently, while making bedside rounds in the ICU, I asked a medical student on my team to describe the incidence of a finding in a disease that the team was discussing. The student had difficulty answering the question, and the senior resident on our service quickly replied “ten percent.” He then turned to the flustered student and said “always answer ten percent or ninety percent; you’ll usually be right!” The “10 and 90” rule doesn’t always work, of course, but clinicians do like round numbers. One such number that fellows in my clinic become familiar with is that about fifteen percent of smokers (between ten and twenty percent, depending on the study) develop clinical signs and symptoms of COPD.1
Why only a fraction of cigarette smokers develop clinical manifestations of COPD is an intriguing and important question. One obvious conclusion is that there is some factor or factors specific to the subpopulation of smokers who develop COPD that is different from other smokers. Efforts to identify these important risk factors have been a focus of research in COPD.