Secondhand smoke exposure and human health: an umbrella review

Secondhand smoke exposure (SHSE) poses risks to human health, yet the evidence remains fragmented. To address this, we conducted an umbrella review (Registration No. CRD42024583424 ) to synthesize and evaluate epidemiological evidence across the lifespan. We systematically searched four databases: PubMed, the Cochrane Library, Web of Science and Embase up to 24 August 2025, for systematic reviews with meta-analyses of observational studies that examined any health outcome in relation to SHSE. Methodological quality was assessed using AMSTAR 2, while evidence credibility and robustness were evaluated through evidence classification criteria, fail-safe number and GRADE framework. A total of 111 systematic reviews with meta-analyses involving approximately 253.3 million participants, encompassing 130 health outcomes and 226 independent associations across developmental stages from fetal development and childhood to adulthood, were included in the reanalysis. Convincing evidence demonstrated a dose-dependent association between prenatal maternal smoking and adverse pregnancy outcomes, such as infant low birth weight (11-19 cigarettes per day, odds ratio [OR] = 2.05, 95% confidence interval [CI] 1.89-2.23, k = 7, p < 0.001). Highly suggestive evidence also showed associations between SHSE and lung cancer (relative risk [RR] = 1.24, 95% CI 1.16-1.32, k = 82, p < 0.001), cardiovascular diseases (OR = 1.24, 95% CI 1.16-1.31, k = 52, p < 0.001) and depressive symptoms (OR = 1.39, 95% CI 1.28-1.51, k = 24, p < 0.001) with evidence of dose-response relationships. Despite limitations of observational data, this review distinguishes evidence saturation areas from key research gaps, underscores health risks especially for pregnant women and children, and may inform tobacco control interventions.