BMJ 1997;315:980-988 (18 October)
Papers
The accumulated evidence on lung cancer and environmental tobacco smoke
A K Hackshaw,
lecturer,a
M R Law,
reader,a
N J Wald,
professor aa Department of Environmental and Preventive Medicine, Wolfson Institute of Preventive Medicine, St Bartholomew’s and Royal London School of Medicine and Dentistry, London EC1M 6BQ
Correspondence to: Professor Wald njwald@mds.qmw.ac.uk
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Abstract |
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Objective: To estimate the risk of lung cancer in
lifelong non-smokers exposed to environmental tobacco smoke.
Design: Analysis of 37 published epidemiological
studies of the risk of lung cancer (4626 cases) in non-smokers who did and did not live
with
a smoker. The risk estimate was compared with that from linear extrapolation of the risk in
smokers
using seven studies of biochemical markers of tobacco smoke intake.
Main outcome measure: Relative risk of lung cancer
in lifelong non-smokers according to whether the spouse currently smoked or had never
smoked.
Results: The excess risk of lung cancer was
24% (95% confidence interval 13% to 36%) in non-smokers
who
lived with a smoker (P<0.001). Adjustment for the effects of bias (positive and negative) and
dietary confounding had little overall effect; the adjusted excess risk was 26% (7%
to
47%). The dose-response relation of the risk of lung cancer with both the number
of
cigarettes smoked by the spouse and the duration of exposure was significant. The excess risk
derived by linear extrapolation from that in smokers was 19%, similar to the direct
estimate
of 26%.
Conclusion: The epidemiological and biochemical
evidence on exposure to environmental tobacco smoke, with the supporting evidence of tobacco
specific carcinogens in the blood and urine of non-smokers exposed to environmental
tobacco
smoke, provides compelling confirmation that breathing other people’s tobacco smoke
is a
cause of lung cancer.
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Key messages
- A woman who has never smoked has an estimated 24% greater risk of lung cancer
if she lives with a smoker
- Neither bias nor confounding accounted for the association
- There is a dose-response relation between a non-smoker’s risk of lung
cancer and the number of cigarettes and years of exposure to the smoker
- The increased risk was consistent with that expected from extrapolation of the risk in
smokers using biochemical markers
- Tobacco specific carcinogens are found in the blood and urine of non-smokers
exposed
to environmental tobacco smoke
- All the available evidence confirms that exposure to environmental tobacco smoke causes
lung cancer
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Introduction |
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Ten years ago scientific committees and national organisations concluded that exposure
to
environmental tobacco smoke (also called passive smoking) is a cause of lung cancer.1 2 3 4 Substantial additional
evidence has since been published, and we report a new analysis. The additional data permit a
more
precise estimate of the size of the association, with a further assessment of whether it is cause and
effect by seeking a dose-response relation and examining whether sources of bias and
confounding could account for the association. We also compared the direct estimate of risk from
epidemiological studies with that from a low dose linear extrapolation of the risk in smokers
using
biochemical markers of exposure to tobacco smoke.
As before,5 the estimate of effect was the relative
risk
of lung cancer in lifelong non-smokers according to whether the spouse currently smoked
or
had never smoked. Spousal exposure is the best available measure: it is well defined and has been
validated using biochemical markers.6 7 8 9 It reflects exposure in general because non-smokers who live
with smokers tend to be more exposed to tobacco smoke from other sources, because they are
more
likely to mix socially with smokers.6 Workplace exposure
varies considerably and is difficult to measure.
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Methods |
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Direct estimate of risk of lung cancer from epidemiological studies
Studies of environmental tobacco smoke and lung cancer were identified from Medline,
the
citations in each study, and consultation with colleagues. We included studies comparing the risk
of lung cancer in lifelong non-smokers according to whether the spouse (cohabitees are
included in this term) currently smoked or had never smoked. There were five cohort and 34
case-control studies.10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Twenty nine studies
were
in peer reviewed journals, four in books with an ISBN number, two in peer reviewed doctoral
theses,
and three in published proceedings of scientific conferences; one study was an official report
from
a scientific organisation. We excluded studies with fewer than five cases of lung cancer (too few
to
calculate an odds ratio),49 50 those that did not report separate results in non-smokers
(the
proportionate effect of exposure to environmental tobacco smoke is much smaller in
smokers),51 52
53 54 55 56 57 58 and those that did not
have controls59 or had controls with smoking related
diseases.60 We excluded studies in which the effects of
exposure to environmental tobacco smoke and radon could not be distinguished61 and studies that were duplicate publications of the same
cases.62 63
64 65 66 67 68 69 We also excluded three
studies in which exposure from a spouse and exposure outside the home were not
distinguished70 71
72 and one unpublished study that had been submitted to
the
United States Occupational Safety and Hygiene Authority73;
inclusion of these four studies would have altered the summary relative risk estimate by less than
1%.
In 35 studies lung cancer was generally (86%) confirmed histologically or
cytologically; in four it was not stated how it was diagnosed.10 16 30 38 In the
case-control
studies controls were selected from the general population in 17 studies,14 19 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 47 69 and from patients with
non-smoking-related diseases in 17 studies.1
12 13 15 17 18 20 21 27 29 30 33 34 41 42 44 45 Age in years was
generally the same in cases and controls; the age adjusted estimate was used in three studies in
which
it was not.30 40
69 We used odds ratios unadjusted for potential
confounding
factors except in four studies, in which only adjusted estimates were available.19 30 40 69 For the cohort studies
we used the published age adjusted relative risks (and 95% confidence interval). The
relative
risk estimates from the studies were pooled using the method of DerSimonian and Laird,74 75 which allows for
heterogeneity between studies by weighting each study using the within and between study
variance.
If there is no heterogeneity, weighting is by the inverse of the variance (fixed effects).
Indirect estimate of risk by extrapolation from the risk in smokers
We estimated the risk of lung cancer in non-smokers exposed to environmental
tobacco smoke by extrapolating from the risk in smokers, using the urine or saliva concentrations
of cotinine and nicotine (both sufficiently tobacco specific) in each. A weighted average ratio
was
calculated from all such studies, identified using Medline.
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Results |
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Table 1 shows details of the 39 (five cohort and
34
case-control) studies. Seven showed a significant excess risk. The pooled relative risk of
lung
cancer from the 37 studies on women was 1.24 (95% confidence interval 1.13 to 1.36)
(P<0.001)a 24% excess risk among lifelong non-smokers with spouses
who
smoked. Inclusion of the nine studies of men and the two reporting only on men and women
combined made little difference (pooled relative risk 1.23 (1.13 to 1.34)).
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Table 1 Epidemiological studies of the
risk
of lung cancer in lifelong non-smokers whose spouses smoked relative to the risk in those
whose spouses did not smoke
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Heterogeneity
Table 2 shows that relative risk estimates of lung
cancer and exposure to environmental tobacco smoke did not significantly differ between men
and
women (P=0.31), between geographical regions (P=0.26), with year of
publication
(P=0.16), or between cohort and case-control studies (P=0.53). There was
heterogeneity across the studies of women (P=0.10), the studies from China and Hong
Kong
(P=0.01), the studies published between 1986 and 1990 (P=0.05), and the
case-control studies (P=0.06), but this was entirely due to the inclusion of one
study.35 When this study was excluded there was no
evidence of heterogeneity (P>0.20). Its effect on the pooled relative risk estimate was
negligible
(1.24 with it, 1.26 without it). This study suggested an implausible protective effect from
exposure
to environmental tobacco smoke (relative risk 0.79 (0.62 to 1.02)). The authors commented that,
in their study, the effect of environmental tobacco smoke was probably obscured by another
cause
of lung cancer, indoor cooking using open coal fires with little ventilation.35
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Table 2 Relative risk of lung cancer and
exposure to environmental tobacco smoke in the 39 epidemiological studies according to sex,
geographical region, year of publication, and study design
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The following results are based only on the 37 studies of women and exposure from their
husbands (cohabitants again included) because most of the cases of lung cancer (91%) and
most of the data necessary to quantify the effects of bias and confounding were in
women.
Figure 1 is a cumulative plot of the pooled relative
risk of lung cancer and exposure to environmental tobacco smoke in non-smokers from
all
studies available each year up to 1997. It shows that the addition of further studies over time has
not
materially changed the estimate, so our current estimate of 1.24 is robust.

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Fig 1 Cumulative pooled estimate of relative risk (bars show
95% confidence interval) of lung cancer from studies of women who were lifelong
non-smokers living with a smoker compared with those living with a non-smoker.
(Number of studies on which each pooled estimate is based is shown to right of
figure)
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Dose-response relation
Data on the dose-response relation between the number of cigarettes smoked by the
husband and the risk of lung cancer was reported in 16 studies.10 13 16 18 20 22 24 25 28 30 33 41 42 44 47 48 Figure 2 (a) shows data from one such study28 with a significant trend. For each study a linear regression
analysis
was performed between the relative risk (in logarithms) and the number of cigarettes smoked by
the
husband. The summary estimate (allowing for the within and between study variation74 76) shows a significant
dose-response relation. Risk increases by 23% (14% to 32%) for every
10 cigarettes smoked per day by the husband (88% if he smoked 30).

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Fig 2 Dose-response relation between the relative risk
(95% confidence interval) of lung cancer and (a) the number of cigarettes smoked daily
by
the spouse and (b) the number of years living with a spouse who smokes. (Data from study of
Geng
et al28)
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Eleven studies14 20 24 26 28 33 40 41 43 47 48 examined risk
according
to the number of years a woman lived with a smoker. Figure 2 (b) shows the results from one such study.28 The summary estimate showed a significant increase in risk with
increasing duration of exposure. Risk increases by 11% (4% to 17%) for
every
10 years of exposure (35% for 30 years’ exposure). These results were not
dependent
on constraining the regression line through the relative risk of 1.0 by the inclusion of women
with
spouses who did not smoke.
Histological type
Several studies reported on the histological type of lung cancer. The pooled relative risk
was
1.58 (1.14 to 2.19) for squamous and small cell carcinoma13
18 22 23 25 33 39 40 43 77 and 1.25 (1.07 to 1.46) for adenocarcinoma alone.17 18 19 27 33 39 40 43 Smoking itself is more
strongly related to squamous and small cell carcinoma than adenocarcinoma, so this difference,
though not significant (P=0.2), is consistent with the view that exposure to environmental
tobacco smoke is equivalent to low dose smoking.
Biases and confounding in epidemiological studies
The excess risk of lung cancer in non-smokers who live with smokers has been
claimed
to be entirely attributable to bias.77 We quantified two
sources of bias and one source of confounding that may affect the relative risk of lung cancer in
the
studies.
Misclassification bias
Some current or former smokers may say that they are lifelong non-smokers and
so
be misclassified in the studies. Because of their smoking status, they are more likely to develop
lung
cancer, and because smokers tend to live with smokers this particular bias will overestimate the
true
risk of lung cancer.5
Misclassification bias increases with increasing values of four determinants: the
prevalence
of reported smoking among women; the extent to which women who smoke are more likely to
have
a husband who smokes (aggregation ratio); the proportion of women who had ever smoked who
report themselves to be lifelong non-smokers; and the risk of lung cancer in misclassified
ever
smokers. The four determinants were estimated as follows:
Smoking prevalence was specified in individual studies; if it
was
not we used national data.
Aggregation ratioEstimates of the aggregation ratio
generally lie between 2 and 4.5 14 20 32 78 79 80 81 82 83 84 85 86 We used an estimate
of
3.
The proportion of ever smokers misclassified as never smokers
was derived by adding two separate estimates of misclassified current and former smokers. The
first
is the proportion of all ever smokers who are current smokers misclassified as never smokers,
which
is 3.1% and is estimated as follows. Of reported non-smokers, 2.0% are
likely
to be current smokers from nicotine or cotinine concentrations (table 3). Of British women, 53% report that they are never
smokers,
21% that they are former smokers, and 26% that they are current smokers.90 Hence, 1.48% of all women are current smokers who
report
being never or former smokers (2.0%x(53%+21%)); these are
3.1% of all ever smokers (1.48%/(21%+26%)). The second
is the proportion of all ever smokers who are former smokers misclassified as never smokers,
which
is 3.8% and is estimated from responses to surveys on smoking by the same people on two
separate occasions as follows. In one study of 1296 never smokers, 851 former smokers, and
1127
current smokers, 102 of the smokers claimed to be never smokers at a subsequent interview some
years later.91 Therefore, 5.1%
(102/(851+1127)) of ever smokers were former smokers who reported themselves as
never smokers. In another study of similar size the estimate was 2.5%92; the average of these two estimates is 3.8%. The
proportion
of all ever smokers in Great Britain who are misclassified as never smokers is, therefore, about
7% (3.1%+3.8%).
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Table 3 Cotinine and nicotine
concentrations
in reported non-smokers (never smoked and former smokers) as markers of probable
current
smoking
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The relative risk of lung cancer in misclassified ever smokers
was
obtained separately for misclassified current and former smokers. Women who currently smoke
but
report themselves as never smokers tend to be light smokers; the cotinine concentrations in
misclassified women in table 3 were about 30%
of
the mean or median concentration in reported active smokers. Their risk of lung cancer will be
correspondingly low. The relative risk in women is 12 on average,93 an 11-fold excess risk, so the excess risk in these women
is
3.3 (30%x11), and the relative risk is 4.3. Former smokers reporting to be never
smokers are likely to have given up smoking long ago and to have smoked less than continuing
smokers.77 Most gave up at least 10 years previously (N
Britten, personal communication), and even if they smoked 20 cigarettes a day their risk of lung
cancer would only be about 1.5.93 The estimated overall
relative risk for misclassified current and former smokers together is therefore 3.0 (average of
4.3
and 1.5, weighted by the proportions of current (26%) and former (21%)
smokers90).
These estimates of the determinants of misclassification bias are similar to those obtained
in
the previous analysis,5 despite the additional data now
available; they are therefore likely to be robust. The relative risk estimates from each of the 37
studies were adjusted for misclassification bias; the method, a modification of that used
before,5 is described in the Appendix 1. Table 4 shows
estimates of the overall adjusted relative risk estimate according to various combinations of the
determinants of misclassification bias. Implausibly high values are required to reduce the
observed
relative risk of 1.24 to a value that is not significant. With the most likely values (listed above),
the
observed relative risk in the 37 studies of 1.24 (1.13 to 1.36) is reduced to 1.18 (1.06 to 1.30)
(P<0.001).
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Table 4 Sensitivity analysis of the effect
of
misclassification bias: relative risk (95% confidence interval) of lung cancer and exposure
to environmental tobacco smoke in the epidemiological studies after adjusting for
misclassification
of smokers (past or present) as never smoking. (With no misclassification, relative risk is 1.24
(1.13
to 1.36))
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Bias due to exposure to environmental tobacco smoke in reference group
In the epidemiological studies the reference groupnon-smoking women
living
with non-smokerswas taken to have no exposure and no increase in risk. Some
of
these women would have been exposed to environmental tobacco smoke from other sources. The
average urinary cotinine in non-smokers with a non-smoking spouse is not
zero,6 7 8 9 yet nicotine from tobacco
smoke is, for practical purposes, the only source of cotinine. This increase in risk in the reference
group will dilute (reduce) the relative risk estimate.
This can be corrected by using data on urinary cotinine concentrations.5 In four studies urinary cotinine concentration in non-smokers
living with smokers was, on average, three times that in non-smokers living with
non-smokers.6 7
8 9 If x is the excess risk of lung cancer in non-smokers living with
a
non-smoker, then 3x is the excess risk in
non-smokers living with a smoker. The observed relative risk (1.24) is equal to
(1+3x)/(1+x), so x=0.14. The risk
in non-smokers living with a smoker relative to non-smokers with no exposure to
environmental tobacco smoke (urinary cotinine zero) is thus 1+3x=1.42 (1.21 to 1.66).
Dietary confounding
In nine studies of women who had never smoked,29
33 94 95 96 97 98 99 100 low fruit and
vegetable consumption was associated with a higher risk of lung cancer, suggesting that nutrients
in fruit and vegetables may protect against lung cancer. Smokers eat less of these foods than do
non-smokers,101 102 103 104 and non-smokers who live with smokers eat less of
them
than do non-smokers who live with non-smokers.48 105 106 107 108 Part of their excess lung cancer risk could, therefore, arise
through dietary confounding.
Most of the studies did not record data on diet, and we estimated its confounding effect
indirectly. From a pooled regression analysis of the studies of fruit and vegetable consumption
and
lung cancer in non-smoking women, the relative risk associated with a decrease in
consumption of one standard deviation was 1.20 (this corresponds to a relative risk of 1.5 in the
quarter of the population with the lowest consumption compared with the quarter with the
highest).
The difference in the consumption of fruit and vegetables between non-smokers who do
and
do not live with smokers was estimated in three studies (about 185 000 non-smokers in
total)48 104
108; the largest difference was 0.12 standard
deviations. The relative risk of lung cancer corresponding to this difference is 1.02 (antilog (log
1.20x0.12)). Of the overall 24% excess risk of lung cancer in non-smokers
exposed to environmental tobacco smoke, only 2% is accounted for by dietary differences.
The estimated relative risk due to exposure to environmental tobacco smoke allowing for
confounding is thus 1.21 (1.24/1.02). The eight epidemiological studies of exposure to
environmental tobacco smoke and the risk of lung cancer that directly recorded data on diet
confirmed the negligible effect of dietary confounding.33
43 48 65 98 100 109
Overall estimate of risk of lung cancer
Adjustment of the observed relative risk of 1.24 (1.13 to 1.36) for misclassification bias
reduced it to 1.18 (1.07 to 1.31); adjustment for dietary confounding further reduced it to 1.16
(1.04
to 1.27), but adjustment for exposure to environmental tobacco smoke in the reference group
increased it to 1.26 (1.06 to 1.47). The effects tend to cancel, and the unadjusted (observed)
pooled
relative risk is a valid estimate of the true risk.
Indirect estimate of risk of lung cancer by extrapolation from the risk in smokers
The relative risk of lung cancer in men who currently smoke compared with never
smokers
is about 20 (excess risk 19).93 110 (The lower risk in women reflects fewer years of smoking, but
they have been exposed to environmental tobacco smoke from men for longer.) The relation
between
the intake of tobacco smoke and the risk of lung cancer is quadratic, but it is almost linear up to
about 25 cigarettes a day.111 It is, therefore, possible to
estimate the risk due to exposure to environmental tobacco smoke by linear extrapolation.
Table 5 summarises the results of seven studies
measuring the urine or saliva concentration of nicotine and cotinine. In non-smokers
exposed
to environmental tobacco smoke, marker concentrations are about 1.0% of those in
smokers.
As cotinine and nicotine are tobacco specific, non-smokers exposed to environmental
tobacco
smoke have about 1% of the exposure to tobacco smoke of smokers and therefore
1%
of the excess risk of lung cancer19% (1% of 19). This is similar to the
estimate of 20% from a low dose extrapolation based on an analysis of pooled data from
nine
large cohort studies of the risk of lung cancer according to cigarette consumption.116 The indirect (19%) and direct (26%) estimates
of
excess risk are similar.
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Table 5 Estimates of concentrations of
nicotine and cotinine in urine and saliva of non-smokers, whether or not exposed to
environmental tobacco smoke (ETS), and of smokers
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Discussion |
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Carcinogens in environmental tobacco smoke are inhaled and pass into the blood.
Experimental exposure of non-smokers to tobacco smoke increased the urinary
concentration
of a tobacco specific carcinogen,117 and
non-smokers exposed to environmental tobacco smoke have raised blood concentrations
of
tobacco specific carcinogen adductsfor example, DNA and haemoglobin
adducts.118 119 120 121 It is therefore to
be expected that exposure to environmental tobacco smoke causes cancer.
This analysis compared with the previous one5 uses
three times as many studies (37 v 13), with seven times
as
many cases of lung cancer (4626 v 676). The pooled
estimate
of the excess risk (24%) is more precise (95% confidence interval 13% to
36%). Despite additional data on the two sources of bias, the previous estimates of their
size
did not materially change, and are therefore robust.5 Bias
and
confounding do not explain the effect, and the adjusted estimate of the excess risk was
26%.
The finding of a significant dose-response relation between the risk of lung cancer and the
extent of exposure to environmental tobacco smoke adds weight to the evidence that the
association
between exposure and lung cancer is causal.
There was no evidence of publication bias against negative studies. Seven studies found
a
significant (P<0.05) positive result. If there were no association, the probability of such a
result
arising by chance is 1 in 40, so a total of 280 studies (7x40) would be required to generate
the seven significant ones. It is implausible that there should be as many as 241 unpublished
studies
to 39 published ones.
The similarity of the direct estimate of lung cancer due to environmental tobacco smoke
and
the indirect estimate from extrapolating from the risk in smokers, the evidence of a
dose-response relation, the inability of bias or confounding to explain the association, and
the
presence of tobacco specific carcinogens in the blood and urine of non-smokers lead to an
inescapable conclusion that exposure to environmental tobacco smoke is a cause of lung cancer.
The
estimated excess risk of 26% corresponds to several hundred deaths per year in Great
Britain.
Our review corroborates and strengthens earlier conclusions that environmental tobacco smoke
causes lung cancer.
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Acknowledgements |
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We thank Richard Doll, Robert Carter, Robert Curnow, and Richard
Peto for their helpful comments on an earlier version of this article that was submitted to the
Scientific Committee on Tobacco and Health at the Department of
Health.
Funding: Department of Health. The views expressed are those of the authors and not
necessarily those of the Department of Health.
Conflict of interest: None.
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Appendix |
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Adjustment for misclassification bias
The method used to adjust for misclassification bias was that previously used5 with two modifications (P Lee, personal communication, 1986).
The
proportion of true ever smokers misclassified as never smokers was used; previously it was
applied
to reported ever smokers, and the risk of lung cancer in misclassified smokers was expressed
relative
to that in all reported non-smokers; previously it was relative to true non-smokers
married to non-smokers. The difference in results is minor. In the previous
analysis5 the relative risk adjusted for misclassification was 1.30; with the
modification, the estimate was 1.28.
The method involves first obtaining estimates of the proportions of individuals in a
population according to the reported and true smoking statuses of husbands and wives (table
6). The values S1,
S2, N1, N2,
s1, s2, n1, and
n2 can be estimated using (a) the
aggregation ratio
C=(S1xN2)/(S2xN1) (our best estimate was C=3) and (b) the proportion of true female ever smokers misclassified as never
smokers (D), where
S1=(1D)xs1
and S2=(1D)xs2, (our best
estimate was D=7%).
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Table A1 Proportion of individuals in
population according to reported and true smoking statuses of husbands and wives
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Table 7 shows the proportion of women who
are
reported non-smokers according to their true smoking status and their risk of lung cancer.
The
observed relative risk from each study (RRobs) and the relative risk in
smoking women who are misclassified as non-smokers compared with all reported
non-smokers (RRmis) are known (see box).
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Table A2 Proportion of women who are
reported non-smokers according to their true smoking status and risk of lung
cancer
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The equations for RRobs and RRmis simplify
to
two simultaneous equations from which the values of E1 and
E2 can be calculated. 1+E1 is then the relative
risk
of lung cancer due to exposure to environmental tobacco smoke in true non-smoking
women.
The standard error of the adjusted relative risk was obtained by adjusting the upper 95%
confidence limit for the observed (unadjusted) relative risk in the same way as described above.
The
adjusted relative risks were then pooled as before.25
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