(Circulation. 1998;97:1870.)
© 1998 American Heart Association, Inc.
Passive Smoking and Coronary Heart Disease in Women
Prof Dr F. Adlkofer
Berlin, Germany
To the Editor:
In a recent article by Kawachi et al,1 the
differing prevalence of cardiovascular risk factors and
other lifestyle variables in women with and those without tobacco
smoke exposure is attributed to the lower socioeconomic status of the
female passive smokers. In order not to impute the socially determined
elevated coronary heart disease (CHD) risk to the passive
smokers, the authors adjusted their data by standardizing for these
factors. In so doing, however, they may have compounded another error,
casting serious doubt on the validity of their results. They overlooked
the fact that women from lower social classes in general pay far less
attention to their health and visit their doctors less frequently than
do women from higher social classes. This is particularly true in the
United States, where financial reasons also play a role. The exclusion
of about 3500 women with a history of CHD before the study commenced,
as well as the exclusion of an unspecified number of women with CHD in
the course of the study inevitably led to a higher prevalence of
undiagnosed CHD cases in the group of passive smokers, so that it is
really not surprising that a higher incidence of myocardial infarction
was found in the women exposed to tobacco smoke. Given the selection
bias, however, a causal relation is not substantiated in this
study.
Selection bias is supported also by another finding reported in the
article by Kawachi et al. Tobacco smoke exposure at the workplace must
have decreased during the study (1982 to 1992) not only for the reasons
given by the authors but also because of the aging of the study
population. The oldest volunteers who, by virtue of their age were most
at risk of developing the disease, were already 61 years old when the
study began and, at 71 years of age, had long since retired from the
workplace at the time the study was concluded. Despite this
ever-decreasing tobacco smoke exposure, the relative CHD risk of
passive smokers increased steadily in the course of the study, rising
from 1.6 after 4 years to 2.0 after 6 years and eventually to 2.3 after
10 years. This finding, too, is best explained by the selection bias
described above. Again, it appears to be justified to assume that there
remained from the very beginning and in the course of the study a
larger number of women with undiagnosed CHD in the group exposed to
tobacco smoke than in the control group. Therefore, passive smoking
need not necessarily play a role in this increase in CHD risk.
To support the plausibility of a CHD risk of 1.91 from regular passive
smoking, Kawachi et al refer to a study of their
own,2 which showed a fourfold to fivefold
increase in CHD risk in active female smokers compared with female
nonsmokers. In other, much better-known studies, such as that of the
American Cancer Society Study (CPS II),3 the CHD
risk of 1.8 for active female smokers was actually slightly below that
reported by Kawachi et al for female passive smokers. Relative CHD
risks of active smokers of a similar order of magnitude as shown in the
CPS II study have been observed also in the Framingham
Study4 and in the study on British
doctors.5 It is difficult to believe that this
discrepancy between the CHD risks as found in these major studies on
active smokers and the Kawachi et al study can be explained, as Kawachi
et al claim, by an increased CHD risk in the control groups because
these studies included exposed and nonexposed nonsmokers, thus
narrowing the gap between the CHD risk of smokers and nonsmokers. The
credibility of Kawachi and colleagues' hypothesis would improve
substantially if the selection bias discussed above were to be
excluded. The authors could do this by reporting the number of women
with CHD they removed from each exposure group at each time point.
Asked for a possible conflict of interest, I declare categorically that
I am not in any way, financially, economically, or otherwise, linked to
the cigarette industry.
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]
-
Kawachi I, Colditz GA, Stampfer MJ, Willett WC, Manson JE,
Rosner E, Speizer PE, Henneckens CH. Smoking cessation and time course
of decreased risks of coronary heart disease in middle-aged
women. Arch Intern Med.. 1994;154:169–175.[Abstract]
-
Surgeon General Report. Reducing the Health Consequences of
Smoking: 25 Years of Progress. US Department of Health and Human
Services, 1989; Publication No. CDC: 89-8411.
-
Kannel WB, Higgins M. Smoking and hypertension as predictors of
cardiovascular risk in population studies. J
Hypertens. 1990;8:S3–S8.
-
Doll R, Peto R, Wheatley K, Gray R, Sutherland I. Mortality in
relation to smoking: 40 years' observation on male British doctors.
BMJ.. 1994;309:901–911.[Abstract/Free Full Text]
