Postmenopausal women who smoke or used to smoke are at an increased risk of developing breast cancer compared to their counterparts who
never lit up, finds research published on bmj.com today.
The study also says that women who have had extensive
exposure to passive smoking, either as children or in adulthood, may also have
an excess risk of developing breast cancer.
While some previous studies have indicated that smoking
increases the risk of breast cancer, the theory that passive smoking is also a
risk factor, remains controversial.
The researchers, led by Dr Juhua Luo from West Virginia
University and Dr Karen Margolis from the HealthPartners Research Foundation in
Minneapolis, decided to carry out a large scale study following participants
over a long period of time to investigate the issue further.
The research team used data from the 1993-98 Women's Health
Initiative Observational study to determine links between smoking, passive
smoking and breast cancer.
They analysed data for almost 80,000 women, aged between 50
and 79 years, across 40 clinical centres in the United States. In total, 3,250
cases of invasive breast cancer were identified by the researchers during ten
years of follow-up.
The participants were asked a range of questions about their
smoking status, for example whether they had ever smoked or were former or
current smokers. Current or former smokers were asked the age at which they
started smoking and the number of cigarettes smoked a day. Former smokers were
asked the age at which they quit.
Questions on passive smoking related to whether the
participants lived in smoking households as children and/or as adults, and whether
they had worked in smoking environments.
The results show that smokers have a 16% increased risk of
developing breast cancer after the menopause. The increased risk for former
smokers is 9%. The highest breast cancer risk was found among women who had
smoked for over 50 years or more compared with lifetime non-smokers. Women who
started smoking as teenagers were also at particularly high risk. An increased
risk of breast cancer continued for up to 20 years after an individual stopped
The findings also reveal that among non-smoking women, those
who had been exposed to extensive passive smoking, for example over 10 years'
exposure in childhood; over 20 years' exposure as an adult at home and over 10
years' exposure as an adult at work; had a 32% excess risk of breast cancer.
The authors stress, however, that their analysis of the link
between breast cancer and secondhand smoke was restricted to the most extensive
passive smoking category and therefore more research is needed to confirm these
"Our findings highlight the need for interventions to
prevent initiation of smoking, especially at an early age, and to encourage
smoking cessation at all ages", Dr Margolis concludes.
In an accompanying editorial, Professor Paolo Boffetta from
the Mount Sinai School of Medicine in New York, says Margolis' study "supports
the hypothesis that smoking increases the risk of breast cancer, in particular
when the habit starts early in life".
However, Boffetta adds that the data needs to placed
in the context of the overall evidence, some of which found no increase in
risk. He also agrees that the evidence on secondhand smoke is not conclusive
and further studies are required.