To the Editor:
As a cardiologist at work and a passive smoker at home, I read the study of Kawachi et al1 with more than usual interest. As is often the case with studies of this type, I had some difficulty putting the results in perspective. It is easy enough to figure out that long-term passive smoking approximately doubles one’s risk of developing coronary heart disease, but when advising my patients (or my wife) about what their smoking habits are doing to their families and coworkers, it is necessary to use measurements that are absolute, not relative, and that are easily understood by the layperson. One such measurement that I have found useful is days (or hours, weeks, months, and so on) of good health. One may ask, “Based on the data of Kawachi et al, how many days of good health are sacrificed by living or working in a smoke-filled environment?” The data may then be analyzed as follows.
Over the course of 10 years there were 135 coronary events in 25 959 passive smokers. There were 17 events in 6087 nonpassive smokers. Had the latter group been as large as the former (that is, 25 959 individuals), there would have been 72.5 expected events (17×25 959/6087).
When coronary events are plotted against time in years, the area under the curve represents patient-years of good health lost because of coronary disease. If it is assumed that the coronary event rate is a linear function of time, then the 25 959 passive smokers lost 10×135/2=675 person-years of good health during 10 years of follow-up. The normalized group of 25 959 nonpassive smokers lost 10×72.5/2=362.5 person-years of good health. The difference between the two groups is 312.5 person-years, or 312.5/25 959=0.012 years per person, or 4.4 days per person.
On the basis of the data of Kawachi et al, the average passive smoker loses 4.4 days of good health every 10 years because of coronary heart disease. A reasonable person, looking at the data expressed in these terms, might conclude that this is a trivial loss notwithstanding the fact that it is statistically significant.
The ability to measure even the tiniest molehill exceedingly accurately does not make it into a mountain.
- Copyright © 1998 by American Heart Association