BACKGROUND: Breast cancer is the most common cancer in women worldwide. The accumulated evidence on second-hand smoke (SHS) and risk of breast cancer will be re-evaluated by the International Agency for Research on Cancer during the 2025-2029 period. The purpose of this study was to examine prospectively, among never smokers, the association between daily SHS exposure at home during childhood, and the risk of breast cancer in the Multiethnic Cohort (MEC) Study.
METHODS: We analyzed data from 25.248 never-smoking, female participants, aged 56-95 years when they provided SHS information in a third follow-up survey (2008-2012). We identified invasive breast cancer cases via linkage to the Hawaii and California Surveillance, Epidemiology and End Results (SEER) Program cancer registries through December 2019. We used Cox proportional hazards models to estimate age-adjusted hazard ratios (HRs) and 95% confidence intervals (CIs), adjusting for race and ethnicity.
RESULTS: During a mean follow-up of 7.8 years, we identified 731 incident, invasive breast cancer cases. Women who reported SHS exposure daily during childhood had a borderline statistically significant 15% increased risk (HR = 1.15, 95% CI: 0.99–1.33) of breast cancer compared with those unexposed. A trend test indicated a dose response relationship of borderline statistical significance between duration of childhood SHS exposure and breast cancer risk (P_trend =0.052). The SHS exposure and breast cancer risk association did not differ when we stratified on breast cancer risk factors (birth cohort, education, age at menarche, parity, body mass index, physical activity, alcohol consumption, menopausal status), nor on race and ethnicity (all P_{heterogeneity} > 0.05).
CONCLUSIONS: Our findings support the notion that SHS exposure during childhood increases the risk of breast cancer. The results provide additional arguments for banning smoking in public and private places, including vehicles and homes when children are present.