Childhood exposure to second-hand smoke (SHS) and risk of breast cancer in postmenopausal never smokers: the Multiethnic Cohort (MEC) study

Neither active smoking nor second-hand smoke (SHS) are established causes for breast cancer. We examined the association between daily SHS exposure at home during childhood and postmenopausal breast cancer risk among never smokers in the Multiethnic Cohort (MEC) Study. We analyzed data from 24,261 never-smoking postmenopausal female MEC participants, aged 60–89 years in 2008–2012, when they provided information on SHS exposure during childhood in the forth questionnaire (QX4). We identified invasive breast cancer cases and tumour receptor status via linkage to the Hawaii and California Surveillance, Epidemiology and End Results 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. During a mean follow-up of 7.8 years, we identified 709 incident, invasive breast cancer cases. Women reporting any childhood SHS exposure had a 17% higher risk (HR = 1.17, 95% CI: 1.00–1.36) of breast cancer compared with those reporting no childhood exposure, after adjustment for age at QX4 and race and ethnicity. We found higher HRs for breast cancer for those exposed ≥ 3 h/day (HR = 1.24, 95% CI: 1.02–1.51), those exposed ≥ 18 years (HR = 1.26, 95% CI: 1.04–1.53), and those with the highest joint exposure (≥ 3 h/day & ≥18 years) (HR = 1.28, 95% CI: 1.00–1.63). A trend test, with increasing integers from those not exposed to those most exposed, showed a dose response relationship between duration and breast cancer risk for all three associations (all Ptrend ≤0.03). The childhood SHS exposure and breast cancer risk association did not differ, (all Pheterogeneity ≥0.17), when we stratified on nine established breast cancer risk factors (year of birth, age at QX4, race and ethnicity, education years, age at menarche, parity status, BMI -, physical activity -, and alcohol consumption all at QX4). Our findings, in this diverse population, add prospective data that daily SHS exposure during childhood may increase the risk of postmenopausal breast cancer. Our results provide additional arguments to establish SHS as a cause of breast cancer and to implement smoking bans in public and private places, when children are present.