(Circulation. 1998;97:1871.)
© 1998 American Heart Association, Inc.
Passive Smoking and Coronary Heart Disease
F. James Brennan, MD
Kingston General Hospital,
Kingston, Ontario, Canada
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
(17x25 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 10x135/2=675 person-years of good health
during 10 years of follow-up. The normalized group of 25 959
nonpassive smokers lost 10x72.5/2=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.
References
-
Kawachi I, Colditz GA, Speizer FE, Manson JE, Stampfer MJ,
Willett WC, Hennekens CH. A prospective study of passive smoking and
coronary heart disease. Circulation. 1997;95:2374–2379.[Abstract/Free Full Text]
