Title : Secondhand smoke exposure and human health: An umbrella review
Abstract:
Background: Secondhand smoke (SHS) exposure remains a major global health concern, containing more than 7,000 toxic chemicals, many of which are carcinogens. Although numerous systematic reviews and meta-analyses have been published, their findings have not been synthesized at the umbrella review level, leaving the overall impact of SHS on health outcomes unclear.
Methods: We conducted a systematic umbrella review following PRISMA guidelines. PubMed, Cochrane Library, and Web of Science were searched up to December 23, 2024, with an update covering studies published until August 24, 2025. Systematic reviews with meta-analyses assessing associations between SHS exposure and health outcomes in nonsmokers (fetuses, children, and adults) were included. Methodological quality of the included studies were assessed using AMSTAR 2. Evidence credibility was evaluated using GRADE and Evidence class. We also assessed the robustness of the findings by calculating Fail-safe N and m_nominal. The study was registered with PROSPERO (CRD: 42024583424).
Results: As of the latest update, more than 95 meta-analyses covering over 120 unique health outcomes are included. All of the included outcomes will be classified into defined systems, including adverse pregnancy outcomes, congenital malformations, circulatory system disease, respiratory system disease, digestive system diseases, neurological and psychiatric diseases, head and neck diseases, genitourinary diseases, systemic diseases, bone and joint diseases, and other outcomes. After applying Evidence class and GRADE assessments, we will focus on presenting the most reliable evidence, based on the highest evidence levels.
Conclusion: This umbrella review provides the first comprehensive synthesis of SHS-related health effects across multiple organ systems. Despite the overall low certainty of evidence, the observed dose–response relationships strengthen causal inference. These findings highlight the urgent need to reinforce tobacco control policies, reduce SHS exposure environments, and implement early preventive interventions.
