Active and passive smoking and the risk of breast cancer in women aged 36-45 years: a population based case-control study in the UK.

Abstract

Active smoking has little or no effect on breast cancer risk but some investigators have suggested that passive smoking and its interaction with active smoking may be associated with an increased risk. In a population based case-control study of breast cancer in women aged 36-45 years at diagnosis, information on active smoking, passive smoking in the home, and other factors, was collected at interview from 639 cases and 640 controls. Women were categorised jointly by their active and passive smoking exposure. Among never smoking controls, women who also reported no passive smoking exposure were significantly more likely to be nulliparous and to be recent users of oral contraceptives. Among those never exposed to passive smoking, there was no significant association between active smoking and breast cancer, relative risk (RR) of 1.12 (95% confidence interval (CI) 0.72-1.73) for past smokers and RR of 1.19 (95% CI 0.72-1.95) for current smokers, nor was there an association with age started, duration or intensity of active smoking. Compared with women who were never active nor passive smokers, there was no significant association between passive smoking in the home and breast cancer risk in never smokers, RR of 0.89 (95% CI 0.64-1.25), in past smokers, RR of 1.09 (95% CI 0.75-1.56), or in current smokers, RR of 0.93 (95% CI 0.67-1.30). There was no trend with increasing duration of passive smoking and there was no heterogeneity among any of the subgroups examined. In this study, there was no evidence of an association between either active smoking or passive smoking in the home and risk of breast cancer.

Full text

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Active and passive smoking and the risk of breast cancer in
women aged 36–45 years: a population based case–control study
in the UK
AW Roddam*,1, K Pirie1, MC Pike2, C Chilvers3, B Crossley1, C Hermon1, K McPherson4, J Peto5,6,
M Vessey7and V Beral1
1Cancer Research UK Epidemiology Unit, Richard Doll Building, University of Oxford, Old Road Campus, Headington, Oxford OX3 7LF, UK;2Department
of Preventive Medicine, Norris Comprehensive Cancer Center, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033, USA;
3Department of Health, E Floor, Government Office for the East Midlands, The Belgrave Centre, Stanley Place, Talbot Street, Nottingham NG1 5GG, UK;
4Nuffield Department of Obstetrics and Gynaecology, University of Oxford, Womens Centre, Level 3, John Radcliffe Hospital, Headington, Oxford OX3
9DU, UK;5Cancer Research UK Section of Epidemiology, Brookes Lawley Building, Institute of Cancer Research, Sutton, Surrey SM2 5NG, UK;
6Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK;7Unit of
Health Care Epidemiology, Department of Public Health, Old Road Campus, Headington, Oxford OX3 7LF, UK
Active smoking has little or no effect on breast cancer risk but some investigators have suggested that passive smoking and its
interaction with active smoking may be associated with an increased risk. In a population based case–control study of breast cancer in
women aged 36–45 years at diagnosis, information on active smoking, passive smoking in the home, and other factors, was collected
at interview from 639 cases and 640 controls. Women were categorised jointly by their active and passive smoking exposure. Among
never smoking controls, women who also reported no passive smoking exposure were significantly more likely to be nulliparous and
to be recent users of oral contraceptives. Among those never exposed to passive smoking, there was no significant association
between active smoking and breast cancer, relative risk (RR) of 1.12 (95% confidence interval (CI) 0.72–1.73) for past smokers and
RR of 1.19 (95% CI 0.72–1.95) for current smokers, nor was there an association with age started, duration or intensity of active
smoking. Compared with women who were never active nor passive smokers, there was no significant association between passive
smoking in the home and breast cancer risk in never smokers, RR of 0.89 (95% CI 0.64–1.25), in past smokers, RR of 1.09 (95% CI
0.75–1.56), or in current smokers, RR of 0.93 (95% CI 0.67–1.30). There was no trend with increasing duration of passive smoking
and there was no heterogeneity among any of the subgroups examined. In this study, there was no evidence of an association
between either active smoking or passive smoking in the home and risk of breast cancer.
British Journal of Cancer (2007) 97, 434–439. doi:10.1038/sj.bjc.6603859
Published online 19 June 2007
& 2007 Cancer Research UK
www.bjcancer.com
Keywords: breast cancer; smoking; passive smoking; case-control study
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Active smoking has little or no effect on a woman’s risk of
developing breast cancer (Collaborative Group on Hormonal
Factors in Breast Cancer, 2002a; IARC, 2004), although there
remains some uncertainty over whether or not there is an effect
with long duration of use (Terry and Rohan, 2002). There is
considerable controversy over the possible association between
passive smoking and breast cancer risk, with much uncertainty
over whether any association may be stronger for premenopausal
compared to postmenopausal women (Johnson, 2005; US Surgeon
General, 2006; Miller et al, 2007). Furthermore, it has been
suggested that the lack of an effect of active smoking on breast
cancer risk might be due to the reference group of never smokers
being confounded by passive smokers (Johnson, 2005).
We report here on the relationship between exposure to active
smoking and passive smoking within the home and breast cancer
risk in women aged 36–45 years using data from a UK population
based case–control study which was designed to investigate the
relationship between oral contraceptive use and breast cancer risk,
and also collected data on a range of lifestyle factors including
active and passive smoking history.
MATERIALS AND METHODS
Study population
This is a population based case–control study of breast cancer that
recruited women who were residents of the Thames, Oxford and
Yorkshire regions, aged 36–45 years who were diagnosed between
July 1987 and February 1990. The study was restricted to white
women with no previous malignancy (except non-melanoma skin
cancer), severe mental handicap or significant illness.
Received 12 March 2007; revised 24 May 2007; accepted 25 May 2007;
published online 19 June 2007
*Correspondence: Dr AW Roddam;
E-mail: andrew.roddam@ceu.ox.ac.uk
British Journal of Cancer (2007) 97, 434–439
& 2007 Cancer Research UKAll rights reserved 0007– 0920/07$30.00
www.bjcancer.com
Epidemiology

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Cases were women with an incident invasive breast cancer,
identified chiefly through the Regional Cancer Registries, supple-
mented by hospital discharge records from Hospital Activity
Analysis registers and patient lists kept at major treatment centres.
These latter two methods were used to help minimise the delay
between diagnosis and interview. All cases were confirmed by
cancer registration. Permission from hospital consultants and
general practitioners was sought before the cases’ identities were
disclosed from the Cancer Registry and before the women were
contacted. Breast cancer patients’ case-notes were reviewed,
confirming the diagnosis in 97% of the cases. Out of 838 eligible
cases, 644 (77%) were interviewed. Reasons for nonparticipation
were consultants or general practitioners’ refusal (6%), woman’s
own refusal (3%), death (11%) and relocation from the study
area (3%).
For every participating case, one age-matched control was
randomly selected from patients registered with the case’s general
practitioner. If a chosen control could not be interviewed, a further
age-matched control was selected. Of the 644 first eligible controls,
588 (91%) were interviewed. Reasons for nonparticipation of the
primarily selected controls were general practitioners’ refusal (3%)
or woman’s own refusal (6%).
Data collection
Study participants were interviewed in their homes, using a
standardised structured questionnaire. Each case–control pair was
interviewed by the same, trained, interviewer. The mean time
between diagnosis and interview was approximately 2 years (range
1–5 years). Women were asked whether they had ever smoked,
and if so, how old they were when they started. For each
subsequent year, women were asked to report the number of
months of smoking and the average number of cigarettes smoked
per day (including manufactured/hand-rolled cigarettes and small
cigars). In addition, the participants were asked to report for each
year from the age of 16 years if they were married or living with a
boyfriend, and if yes how many cigarettes per day he smoked at
home (none, 1–4, 5–14, 15–24, 25þ). In addition, information
was sought from each woman about her menstrual and
reproductive history and other risk factors for breast cancer.
Active and passive smoking exposure
Women were defined as current smokers if they reported being a
smoker in the year prior to interview, and as past smokers if they
had stopped smoking for at least 1 year prior to interview. Women
were categorised as being passive smokers if they reported living
for at least 1 year with a partner who smoked at home. Among
women who were passive smokers, the total number of years of
exposure and the cigarette-years of exposure were calculated, the
latter as the number of years of exposure multiplied by the number
of cigarettes smoked per day in the home.
Statistical analyses
Women were defined as peri/postmenopausal if they reported
having had a natural menopause (n¼14), bilateral oophorectomy
(n¼6) or were users of hormone replacement therapy (n¼73).
One control who did not have information on active smoking
exposure along with three controls and five cases who did not have
information on passive smoking exposure were excluded leaving
639 cases and 640 controls for analysis.
Differences in characteristics were assessed using either w2tests
for categorical variables or t-tests for continuous variables. Odds
ratios as estimates of relative risks (RRs) and 95% confidence
intervals (CIs) were calculated using conditional logistic regression
with stratification by age at diagnosis/pseudodiagnosis (single
year) and region of recruitment. All estimates are presented
stratified by age and region and additionally adjusted for
socioeconomic status of the woman (according to the Registrar
Generals classification: professional (SC1 or SC2), nonmanual
(SC3), manual/not employed (SC4/SC5/other)), alcohol consump-
tion(non-drinkers, 1–50gweek?1,
(o22kgm?2, 22–25kgm?2, 25þ kgm?2), parity and age at first
giving birth (nulliparous, parous and age at first giving birth o25
years, parous and age at first giving birth X25 years), use of oral
contraceptives (never user, ever user within the last 5 years, ever
user and last use more than 5 years ago), family history of breast
cancer (no history, mother and/or sister with breast cancer), age at
menarche (o13 and 13þ years) and menopausal status (pre-
menopausal, peri/postmenopausal). These adjustments were in-
cluded on the basis of a priori evidence of their effects on breast
cancer risk irrespective of their significance in this study. The
effect of passive smoking exposure was examined in a number of
subgroups known to be related to breast cancer risk – menopausal
status, alcohol consumption, oral contraceptive use, family history
of breast cancer, parity and age at first birth, socioeconomic status,
BMI and age at menarche. In these analyses, RRs were estimated
from conditional logistic regression models stratified by age and
region and adjusted for parity (nulliparous, parous) and oral
contraceptive use (never user, last use within 5 years, last use 5þ
years) where appropriate. Heterogeneities between estimates of
RRs were assessed using w2tests.
All analyses were performed in Stata version 8.1 (Stata
Corporation, College Station, TX, USA). All statistical tests were
two sided, and P-values less than 0.05 were regarded as statistically
significant.
50þ gweek?1),BMI
RESULTS
Characteristics of the cases and controls included in these analyses
are presented in Table 1. Cases and controls were of a similar
socioeconomic class, had a similar pattern of alcohol consumption,
history of oral contraceptive use, parity and menopausal status.
Cases were more likely to have a family history of breast cancer
and to have a lower mean BMI. Mean age at menarche and mean
age at first giving birth among parous women was similar between
cases and controls.
Among participants, 300 (23%) reported being a past smoker
and 372 (29%) reported being a current smoker, and a higher
proportion of ever smokers also reported exposure to passive
smoking in the home (42, 63 and 78% among never, past and
current smokers, respectively). Similar rates of exposure to passive
smoking in the home by case and control status were reported for
never smokers (41 and 44% in cases and controls, respectively)
and ever smokers (70 and 73% in cases and controls, respectively).
Among smokers, the average number of cigarettes per day,
duration of smoking and age started smoking were similar
between cases and controls. The duration of exposure to passive
smoking was similar between cases and controls for never, past
and current smokers.
Among controls who reported never smoking, there were no
significant differences between those with, and those without,
exposure to passive smoking
family history of breast cancer, BMI, age at menarche, menopausal
status and age at first giving birth (Table 2). There was also
no difference in their mean alcohol consumption; women
exposed to passive smoking consumed 4.3gday?1compared with
4.4gday?1in women not exposed. However, there were significant
differences between exposure to passive smoking and both
parity and oral contraceptive use (Po0.05, Table 2). Compared
with women who reported exposure to passive smoking, those
who were not exposed were significantly more likely to be
nulliparous, and to be either never or recent users of oral
contraceptives.
andsocioeconomicstatus,
Tobacco smoke and breast cancer risk
AW Roddam et al
435
British Journal of Cancer (2007) 97(3), 434–439
& 2007 Cancer Research UK
Epidemiology

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There was no association between active smoking and
breast cancer risk with an RR of 1.15 (95% CI 0.87–1.53) for
past smokers and an RR of 1.04 (95% CI 0.79–1.36) for current
smokers compared with never smokers (Table 3). Similarly,
there was no association between smoking intensity, duration of
smoking and age started smoking and breast cancer risk
either individually or after mutual adjustment (Table 3). Among
those participants never exposed to passive smoking, there
was no association between past or current smoking and
breast cancer risk with an RR of 1.12 (95% CI 0.72–1.73) for
past smokers and an RR of 1.19 (95% CI 0.72–1.95) for current
smokers. Compared with women who were never active smokers
nor exposed to passive smoking, there was no significant
association between exposure to passive smoking in the home
and breast cancer risk in never smokers with an RR of 0.89 (95%
CI 0.64–1.25), in past smokers with an RR of 1.09 (95% CI 0.75–
1.56) or in current smokers with an RR of 0.93 (95% CI 0.67–1.30).
There was no significant trend between the number of years of
exposure to passive smoking and breast cancer risk by active
smoking exposure (Table 3).
Among the never smokers, there was no significant hetero-
geneity between risk of breast cancer and exposure to passive
smoking for a range of participant characteristics (Table 4). The
association between passive smoking and breast cancer risk was
consistent in subgroups of menopausal status (although 93% were
premenopausal), alcohol consumption, oral contraceptive use,
family history of breast cancer, parity and age at first giving birth,
socioeconomic status, BMI and age at menarche.
Table 1Descriptive characteristics of all cases and controls
Characteristic
Cases (%),
n¼639
Controls
(%), n¼640P-value
Socioeconomic status
Professional
Nonmanual
Manual/not employed
157 (25)
219 (34)
263 (41)
170 (27)
212 (33)
258 (40)
0.71
Use of oral contraceptives
Never
Last use o5 years ago
Last useX5 years ago
110 (17)
117 (18)
412 (64)
130 (20)
112 (18)
398 (62)
0.37
Alcohol consumption
Non-drinkers
1–50gweek?1
50+gweek?1
210 (33)
261 (41)
168 (26)
206 (32)
270 (42)
164 (26)
0.89
History of breast cancer in first-degree relative
No
Yes
N/K
557 (87)
71 (11)
11 (2)
585 (91)
43 (7)
12 (2)
0.02
Mean height (cm)
Mean weight (kg)
Mean BMI (kgm?2)
Mean age at menarche (years)
163.8
62.5
23.3
12.5
163.1
63.5
23.9
12.6
0.06
0.09
0.01
0.16
Parity
Nulliparous
Parous
93 (15)
546 (85)
79 (12)
561 (88)
0.25
Mean age at first giving birth (years)
(among parous)
24.4 24.10.30
Menopausal status
Premenopausal
Peri/postmenopausal
591 (92)
48 (8)
595 (93)
45 (7)
0.74
Active smoking exposure
Never
Past
Mean cigarettes per day
Mean years smoked
Mean age started smoking
Current
Mean cigarettes per day
Mean years smoked
Mean age started smoking
297 (46)
156 (24)
11.3
10.8
17.8
186 (29)
14.0
23.2
17.6
310 (48)
144 (23)
10.7
10.6
17.7
186 (29)
14.1
22.6
17.7
Active and passive exposure
Never smoker, no passive
exposure
Never smoker, passive exposure
Mean years of exposure
Past smoker, no passive exposure
Past smoker, passive exposure
Mean years of exposure
Current smoker, no passive
exposure
Current smoker, passive exposure
Mean years of exposure
175 (27)175 (27)
122 (19)
12.2
58 (9)
98 (15)
12.0
44 (7)
135 (21)
13.3
52 (8)
92 (14)
12.9
38 (6)
142 (22)
16.2
148 (23)
14.8
Table 2
smoking among never smokers
Descriptive characteristics of controls by exposure to passive
Characteristic
Neither
active
smoker nor
exposed
to passive
smoking,
n¼175
Exposed to
passive but
not
active
smoking,
n¼135P-value
Socioeconomic status
Professional
Nonmanual
Manual/not employed
48 (27)
62 (35)
65 (37)
27 (20)
50 (37)
58 (43)
0.30
Use of oral contraceptives
Never
Last use o5 years ago
Last use X5 years ago
51 (29)
36 (21)
88 (50)
27 (20)
17 (13)
91 (67)
0.01
Alcohol consumption
Non-drinkers
1–50gweek?1
50+gweek?1
67 (38)
71 (41)
37 (21)
46 (34)
68 (50)
21 (16)
0.20
History of breast cancer in first-degree relative
No
Yes
N/K
161 (92)
10 (6)
4 (2)
122 (90)
11 (8)
2 (1)
0.62
Mean height (cm)
Mean weight (kg)
Mean BMI (kgm?2)
Mean age at menarche
(years)
163.0
64.9
24.5
12.5
161.9
62.6
23.4
12.7
0.15
0.09
0.28
0.24
Parity
Nulliparous
Parous
31 (18)
144 (82)
12 (9)
123 (91)
0.03
Mean age at first giving birth
(years) (among parous)
25.3 24.50.15
Menopausal status
Premenopausal
Peri/postmenopausal
167 (95)
8 (5)
126 (93)
9 (7)
0.42
Tobacco smoke and breast cancer risk
AW Roddam et al
436
British Journal of Cancer (2007) 97(3), 434–439
& 2007 Cancer Research UK
Epidemiology

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DISCUSSION
In this study, neither being an active smoker nor exposure to
passive smoking in the home from a partner was associated with
breast cancer risk among women aged 36–45 years. There was no
association between smoking intensity, duration or age started
smoking and breast cancer risk, neither was there a trend with
increasing duration of exposure of passive smoking. Adjustment
for established breast cancer risk factors made little difference to
risk estimates, and the lack of association between passive
smoking and breast cancer risk was consistent among a number
of subgroups including menopausal status, alcohol consumption,
oral contraceptive use, family history of breast cancer, parity and
age at first giving birth, socioeconomic status, BMI and age at
menarche.
The lack of an association between active smoking and breast
cancer risk in the current study is consistent with previous reviews
and a collaborative reanalysis of worldwide data (Collaborative
Group on Hormonal Factors in Breast Cancer, 2002a; Terry and
Rohan, 2002; IARC, 2004; US Surgeon General, 2006). Contrary to
some hypotheses (Terry and Rohan, 2002), the current study found
no evidence either of an association with increasing intensity or
duration of exposure to tobacco smoke, nor is there evidence of a
relationship between age at initiation of smoking and breast cancer
risk. Furthermore, it has been suggested that this lack of an
association may be due to the reference group of never smokers
being confounded by passive smoking exposure (Johnson, 2005).
In the current analysis, we investigated this question by setting the
reference group to be never smokers who had also reported no
passive smoking exposure and found there to be no association
between breast cancer risk and active smoking either with or
without additional passive smoking exposure. However, the use of
such a reference group creates certain problems in analysing and
interpreting results. In this study for example, never smoking
women who were also not exposed to passive smoking were more
likely to be nulliparous and recent users of oral contraceptives, two
factors known to affect the risk of breast cancer (Collaborative
Group on Hormonal Factors in Breast Cancer, 1996, 2002b).
There remains considerable controversy over the relationship
between passive smoking and breast cancer since the results from
previous studies are inconclusive; some suggested an increase in
risk with exposure to passive tobacco smoke (Wells, 1991; Smith
et al, 1994; Morabia et al, 1996; Millikan et al, 1998; Lash and
Aschengrau, 1999; Delfino et al, 2000; Johnson et al, 2000; Liu et al,
Table 3Relative risks of breast cancer by active and passive smoking
Stratified by age and region Additionally adjustedaAdditionally and mutually adjustedb
RR (95% CI) RR (95% CI) Exposure Cases ControlsRR (95% CI)
Active smoking exposure
Never smoker
Past smoker
Current smoker
297
156
186
310
144
186
1.00 1.00
1.13 (0.86–1.49)
1.04 (0.80–1.35)
1.15 (0.87–1.53)
1.04 (0.79–1.36)
1.00
0.90 (0.54–1.50)
Intensity (per 5 cigarettes per day)
Ever smoker 342330 1.02 (0.91–1.13) 1.00 (0.89–1.12)1.00 (0.88–1.12)
Duration (per 5 years smoked)
Ever smoker342 3301.02 (0.93–1.12)1.00 (0.91–1.11) 1.02 (0.86–1.22)
Age started smoking (per 5 years)
Ever smoker342 330 0.97 (0.80–1.19)0.97 (0.79–1.19)0.99 (0.76–1.28)
Active and passive exposure
Never smoker, no passive exposure
Never smoker, passive exposure
175
122
175
135
1.00 1.00
0.90 (0.65–1.25)0.89 (0.64–1.25)
Past smoker, no passive exposure
Past smoker, passive exposure
58
98
52
92
1.12 (0.73–1.72)
1.06 (0.75–1.51)
1.12 (0.72–1.73)
1.09 (0.75–1.56)
Current smoker, no passive exposure
Current smoker, passive exposure
44
142
38
148
1.15 (0.71–1.87)
0.96 (0.70–1.31)
1.19 (0.72–1.95)
0.93 (0.67–1.30)
Number of years of exposure to passive smoking at home
Never smoker, 0 years
Never smoker, 1–10 years
Never smoker, 11+ years
175
56
66
175
55
80
1.00 1.00
1.02 (0.66–1.56)
0.83 (0.56–1.22)
0.99 (0.64–1.53)
0.84 (0.56–1.25)
P-valuec¼0.31
Past smoker, 0 years
Past smoker, 1–10 years
Past smoker, 11+ years
58
42
56
52
38
54
1.12 (0.73–1.72)
1.10 (0.68–1.79)
1.04 (0.68–1.59)
1.12 (0.72–1.73)
1.12 (0.68–1.85)
1.06 (0.68–1.66)
P-valuec¼0.71
Current smoker, 0 years
Current smoker, 1–10 years
Current smoker, 11+ years
44
31
111
38
44
104
1.15 (0.71–1.87)
0.71 (0.43–1.17)
1.07 (0.76–1.50)
1.19 (0.72–1.96)
0.66 (0.39–1.10)
1.06 (0.74–1.53)
P-valuec¼0.58
aAdditionally adjusted for socioeconomic status, alcohol consumption, BMI, parity and age at first giving birth, use of oral contraceptives, family history of breast cancer, age at
menarche, menopausal status.bAdjustments as for (a), but with mutual adjustment for all active smoking-related variables among the ever smokers.cP-value for linear trend test
of increasing duration of passive smoking exposure.
Tobacco smoke and breast cancer risk
AW Roddam et al
437
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2000; Kropp and Chang-Claude, 2002) and others showed no effect
(Wartenberg et al, 2000; Nishino et al, 2001; Egan et al, 2002; Lash
and Aschengrau, 2002; Gammon et al, 2004; Reynolds et al, 2004;
Shrubsole et al, 2004; Bonner et al, 2005; Hanaoka et al, 2005;
Lissowska et al, 2006). Recent reviews of the literature have also
been inconsistent with two showing that there was an association
between passive smoking and breast cancer (Johnson, 2005; Miller
et al, 2007) and a further two suggesting that more evidence is
needed before any conclusions can be drawn (IARC, 2004; US
Surgeon General, 2006). The main reason for the difference in the
reviews is that the latter two included all published studies, but in
the other reviews various subanalyses were performed controlling
for quality measures of the exposure assessment – specifically
which aspects of passive smoking were assessed and what
assessment method of passive smoking was used.
The two reviews that suggested there was an association between
passive smoking and breast cancer also highlighted that the
relationship appeared stronger for premenopausal compared with
postmenopausal breast cancer (Johnson, 2005; Miller et al, 2007).
However, only about half of the published studies provided
results stratified by menopausal status (Wells, 1991; Millikan et al,
1998; Delfino et al, 2000; Johnson et al, 2000; Gammon et al, 2004;
Reynolds et al, 2004; Shrubsole et al, 2004; Bonner et al, 2005;
Hanaoka et al, 2005; Lissowska et al, 2006) and therefore the
possibility of publication bias cannot be ruled out. The current
study adds to the totality of the evidence suggesting that exposure
to passive smoking in the home is not associated with an increase
in the risk of breast cancer, and does not support the view that
there is an increased risk in premenopausal women. We were not
able to adequately examine the role of passive smoking in
postmenopausal women, as over 90% of the women in this
study were premenopausal and the majority (78%) of the peri/
postmenopausal women were classified as such on the basis of
their being current users of HRT.
As noted by previous reviews (Johnson, 2005; Miller et al, 2007),
one of the difficulties in studies investigating passive tobacco
smoking is both in the definition and measurement of the
exposure variable. It has been suggested that adult exposure
and childhood exposure could have different biological effects
and should, therefore, be investigated separately (IARC, 2004;
US Surgeon General, 2006). In some studies, like the current, the
focus is solely on exposure from partners smoking within
the home and did not quantify additional sources of exposure
such as during childhood or from the workplace. This can
lead to some misclassification of the exposure which is most
likely to be nondifferential; however, given that over 50% of
the study participants were exposed to passive smoking in the
home, we would not expect there to be substantial misclassifica-
tion. Nevertheless, such misclassification may have limited
our ability to detect an association and we are unable to exclude
a modest association between passive smoking and breast
cancer.
One of the limitations of the current study is its retrospective
nature leading to many possible biases in the assessment of
the exposure of interest and relevant confounding variables.
Women were asked about their smoking history, both active and
passive, after the diagnosis of breast cancer, and it is possible
that there are systematic differences in the reporting of responses
between cases (who knew they had breast cancer) and controls.
However, at the time this study was carried out, there was less
public concern over the dangers of passive smoking and therefore
less chance that such retrospective reporting bias would be an
issue. Since this study was performed, the prevalence of smoking
among women has fallen consistently year on year and the
likelihood of exposure to passive smoke both in the workplace
and the home has also fallen. However, neither of these changes
will have any impact on the generalisability of the results of
this study, since they only affect the prevalence of the exposure
variable. It is possiblethat
heavy persistent smokers who may have died prior to being
eligible for this study; however, since it recruited people before
the age of 45 years where tobacco-related death rates are low,
this is unlikely to be a major source of bias. A strength of the
study is that the data were collected during in-person interviews by
a small number of trained nurses and is complete. Furthermore,
the study obtained high-quality measurements of both the
smoking and passive smoking exposure, with a full history of
dates starting smoking, stopping smoking and numbers of
cigarettes per day being recorded whenever significant changes
occurred.
In summary, the current study finds no significant associations
between either active smoking or passive smoking in the home and
risk of breast cancer in women aged 36–45 years.
this studyfailed torecruit
Table 4
among never smokers by different characteristics
Relative risks of breast cancer by passive smoking exposure
Cases Controls
Subgroup Exposed/unexposedRR (95% CI)a
Menopausal status
Premenopausal
Peri/postmenopausal
112/170
7/4
126/167
7/5
0.83 (0.59–1.17)
1.51 (0.19–12.2)
P-valueb¼0.35
Alcohol consumption
Never drinkers
Drinkers
42/66
79/107
42/64
89/108
0.93 (0.51–1.69)
0.86 (0.56–1.30)
P-value¼0.98
Oral contraceptive use
Never user
Last use within 5 years
Last use greater than 5 years
ago
11/35
19/22
85/111
17/39
12/30
91/88
0.68 (0.25–1.91)
2.51 (0.90–6.99)
0.74 (0.49–1.12)
P-value¼0.11
Family history of breast cancer in mother or sister
No
Yes
108/154
8/9
122/161
9/7
0.89 (0.62–1.26)
1.12 (0.20–6.41)
P-value¼0.57
Parity and age at first giving birth
Nulliparous
Parous and age at first giving
birth o25 years
Parous and age at first giving
birth 25+ years
10/33
66/54
11/30
66/68
0.64 (0.21–1.91)
1.06 (0.63–1.78)
45/7857/74 0.68 (0.40–1.16)
P-value¼0.21
Socioeconomic status
Professional
Nonmanual
Manual/not employed
25/54
40/55
50/52
26/47
50/61
57/61
0.81 (0.40–1.63)
0.80 (0.45–1.43)
1.03 (0.58–1.85)
P-value¼0.82
Body mass index (kgm?2)
o25
25+
89/137
31/38
95/116
36/51
0.72 (0.48–1.07)
1.07 (0.54–2.14)
P-value¼0.18
Age at menarche (years)
o12
13+
61/82
59/90
56/81
74/86
1.09 (0.66–1.79)
0.67 (0.42–1.09)
P-value¼0.15
aRelative risks are stratified by age and region and additionally adjusted for parity and
oral contraceptive use.bP-values are for tests of heterogeneity in effect between
subgroup characteristics.
Tobacco smoke and breast cancer risk
AW Roddam et al
438
British Journal of Cancer (2007) 97(3), 434–439
& 2007 Cancer Research UK
Epidemiology