A meta-analysis of published epidemiological studies has found that exposure to second-hand smoke can significantly increase the risk of breast cancer in women who do not smoke.
The study is published in the British Journal of Cancer.
Review Article: Exposure to second-hand smoke and breast cancer risk in non-smoking women: a comprehensive systematic review and meta-analysis. Image Credit: Studio Peace / Shutterstock
Background
Breast cancer is the most frequently diagnosed malignancy in women, accounting for about 15% of all female cancer-related mortality. Approximately 2.3 million new breast cancer cases have been reported in 2020. Notably, it accounts for one-third of cancer diagnoses in women aged below 50 years.
Besides non-modifiable risk factors, such as advanced age and genetic mutations, breast cancer is associated with various modifiable risk factors, including smoking, alcohol intake, lack of physical activity, post-menopausal weight gain, and use of contraceptives or hormone therapies.
Second-hand smoke is considered a toxic air pollutant with carcinogenic potency. Exposure to second-hand smoke occurs when a person involuntarily inhales tobacco smoke present in the surrounding environment. According to the World Health Organization, second-hand smoke is associated with approximately 1.2 million deaths annually worldwide.
The current meta-analysis aimed to provide a comprehensive understanding of the association between second-hand smoke exposure and the risk of breast cancer in female non-smokers.
Study design
The scientists conducted a thorough literature search through distinct electronic databases using an innovative methodology that relies on a combination of umbrella reviews (a review of published systematic reviews or meta-analyses) and traditional reviews (a review of original studies on a given topic).
The final screening led to the identification of 73 original case-control or cohort studies published in English between 1984 and 2022. All studies provided information on the association between second-hand smoke exposure and the risk of breast cancer in female non-smokers.
Of 73 identified studies, 63 were included in the meta-analysis, and 10 were excluded because of duplicate data. These studies included more than 35,000 breast cancer cases.
Important observations
The meta-analysis of selected studies showed that female non-smokers are at a 24% increased risk of developing breast cancer when exposed to second-hand smoke. The risk of breast cancer was significantly higher in case-control studies than in cohort studies.
Considering different exposure settings, a significantly increased breast cancer risk was observed for women who were exposed to second-hand smoke at home, at home and workplace, and in non-specified settings.
Further stratification showed that non-smoking women exposed to second-hand smoke from a partner are at a 16% increased risk of developing breast cancer. Moreover, a 5% increased risk of breast cancer was observed in women with second-hand smoke exposure during childhood.
The dose-response analysis showed that the risk of breast cancer increases linearly with increasing duration, intensity, and pack-years (amount of smoke exposure over a long period of time) of second-hand smoke exposure.
Specifically, a 30% increased risk of breast cancer was observed in women who were exposed to second-hand smoke for more than 40 years.
Study significance
The study identifies second-hand smoke exposure as a potent risk factor for breast cancer development in non-smoking women.
Alcohol intake is considered to be one of the most significant modifiable risk factors for breast cancer. Existing literature indicates that moderate alcohol consumption is associated with a 23% increased breast cancer risk.
In this meta-analysis, exposure to second-hand smoke has been found to be associated with a 24% increased risk of breast cancer, placing this lifestyle risk factor among one of the main modifiable risk factors for breast cancer.
Exposure to second-hand smoke is particularly relevant as it affects approximately 35% of women globally, as compared to alcohol intake, which affects approximately 25% of women.
It has previously been hypothesized that second-hand smoke-induced breast cancer might be associated with N-acetyltransferase 2-related gene polymorphisms. However, the current meta-analysis could not find the significant impact of N-acetyltransferase 2 polymorphisms on the observed associations.
Second-hand smoke exposure is known to have a higher effect on breast cancer risk in pre-menopausal women compared to that in post-menopausal women. The current meta-analysis supports these observations.
As mentioned by the scientists, the accuracy of current estimates may be affected by the possibility of misclassification of second-hand smoke exposure in selected studies.
Although the quality of selected studies was not assessed here, the scientists mentioned that high-quality studies that provided adjusted estimates had also shown a significantly higher risk of breast cancer due to second-hand smoke exposure.
Overall, the strength, consistency, temporality, dose-response relationship, and biological plausibility of the observed association collectively suggest a causal association between second-hand smoke exposure and breast cancer risk in non-smoking women.
These findings highlight the urgent need for promoting smoke-free environments, particularly at home and in other private settings and increasing public awareness about the health risks associated with second-hand smoke exposure.