Exposure to second-hand smoke and breast cancer risk in non-smoking women: a comprehensive systematic review and meta-analysis

Abstract

Through the use of an innovative method to identify original publications, we conducted a meta-analysis of all epidemiological studies evaluating the association between second-hand smoke (SHS) exposure and breast cancer risk among female non-smokers published in English up to October 2022. Pooled relative risks (RR) were obtained through the use of random-effects models. Dose-response relationships were derived using log-linear functions. Out of 73 identified eligible studies, 63 original articles were included in the meta-analysis. The pooled RR for breast cancer for overall exposure to SHS was 1.24 (95% confidence interval, CI, 1.15-1.34, number of articles, n = 52). Regarding the setting of exposure, RRs were 1.17 (95% CI 1.08-1.27, n = 37) for SHS exposure at home, 1.03 (95% CI 0.98-1.08, n = 15) at the workplace, 1.24 (95% CI 1.11-1.37, n = 16) at home or workplace, and 1.45 (95% CI 1.16-1.80, n = 13) for non-specified settings. The risk of breast cancer increased linearly with higher duration (RR 1.29; 95% CI 1.04-1.59 for 40 years of SHS exposure, n = 12), intensity (RR 1.38; 95% CI 1.14-1.67 for 20 cigarettes of SHS exposure per day, n = 6), and pack-years (RR 1.50; 95% CI 0.92-2.45 for 40 SHS pack-years, n = 6) of SHS exposure. This meta-analysis shows a statistically significant excess risk of breast cancer in women exposed to SHS.

Fig. 1. Flowchart for the seletion of articles.

Flowchart for the selection of the original studies on the association between second-hand smoke (SHS) exposure and breast cancer risk included in the review and meta-analysis.

Fig. 2. Forest plot of the overall analysis* and pooled relative risk (RR) of breast cancer for women exposed to second-hand smoke (SHS) compared with non-exposed women by setting or source of exposure.

AA African-American ethnicity, CI confidence intervals, HIS Hispanic ethnicity, NHW non-Hispanic women of Caucasian ethnicity, Pre pre-menopausal women, Pre-peri pre- or peri-menopausal women, Post post-menopausal women, RR relative risk, WH White ethnicity. Non-specified settings (a) Including clarifications about weekly or daily minimum duration of exposure. Non-specified settings (b) Without clarifications about weekly or daily minimum duration of exposure. *Considering one source or setting of exposure for each article.

Fig. 3. Relative risk (RR) for the dose–response relationships between second-hand smoke (SHS) exposure duration, intensity, and pack-years and the risk of breast cancer:

a Years of SHS exposure. b Cigarettes per day (cig/day) of SHS exposure. c Pack-years of SHS exposure. Linear model from a random-effects dose–response model; 95% confidence interval of the linear model; RR for non-exposed women; RR for various exposure categories in each study included in the analysis. The area of the circle is proportional to the precision (i.e., to the inverse variance) of the RR.