In a recently published study natural medicine, Researchers evaluate and quantify the adverse health effects of secondhand smoke (SHS) exposure.
study: Health effects associated with secondhand smoke exposure: a burden of proof study. Image source: Namning/Shutterstock.com
The continuing threat of secondhand smoke
Tobacco use is a major global health risk, accounting for more than 229.8 million disability-adjusted life years and 8.7 million deaths in 2019. Secondhand smoke exposure affects approximately 37% of the global population and is particularly harmful to non-smokers, with women and children often affected and at greater risk of exposure.
Although smoking rates have declined, the health impacts of secondhand smoke remain significant, particularly in low- and middle-income countries. In fact, the 2019 Global Burden of Disease Study (GBD) attributed 1.3 million deaths to secondhand smoke.
Further research is needed to address gaps in evidence quality and study heterogeneity, to better understand the full health impacts of secondhand smoke, and to effectively inform and enhance global tobacco control policy and public health interventions.
About the study
The researchers used a burden-of-risk function (BPRF) approach to estimate the association between secondhand smoke exposure and nine health outcomes while assessing the strength of supporting evidence. SHS was defined as current exposure to smoke from any combustible tobacco product in nonsmokers, consistent with definitions used in previous GBD studies.
The BPRF framework, previously used to assess the health effects of smoking and dietary factors, utilizes the meta-regression Bayesian, regularization and pruning (MR-BRT) tool to estimate pooled relative risks (RR) and uncertainty intervals. This approach accounts for systematic bias, within-study correlations, and unexplained between-study heterogeneity.
A systematic review was used to extract data from relevant studies, estimate pooled RRs comparing SHS exposure risk while adjusting for systematic bias, quantify unexplained heterogeneity between studies, assess publication and reporting bias, and estimate BPRF to generate conservative risk estimates and Corresponding Risk-Outcome Score (ROS).
The BPRF reflects the minimal harmful effects of risk exposure consistent with available evidence. ROS is a signed value of log RR that reflects the effect size and strength of evidence associated with each risk outcome, which is then converted into a star rating scale for interpretation.
The study did not disaggregate RR by sex, geography, or age, except for breast cancer and asthma, which focused on women and children, respectively. Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines and Guidelines for Accurate and Transparent Reporting of Health Estimates (GATHER) recommendations were followed and approved by the University of Washington Institutional Review Board.
The systematic review process included searching PubMed and Web of Science for studies published between January 1970 and July 2022, where researchers screened studies based on inclusion criteria and extracted data from selected publications. Priority was given to effect sizes that closely matched the GBD risk definition and subsequent meta-regression analysis using the MR-BRT tool was performed to generate pooled RRs for health outcomes in populations exposed to SHS.
Deviations between study designs and characteristics were tested and adjusted using the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach. Any remaining between-study heterogeneity was quantified using linear mixed-effects models. Publication and reporting bias were assessed using funnel plots and Egger’s regression tests.
Sensitivity analyzes were performed to determine the strength of the main findings. These analyzes involve imposing restrictions on data input and ensuring reproducibility by providing data and code.
A total of 410 publications from 9,081 records were used in the systematic review. Among the included studies, 125 were on asthma, 104 on lung cancer (104), 21 on chronic obstructive pulmonary disease (COPD), and 9 on 2 type diabetes research. This resulted in 623 observations from multiple locations.
For cardiovascular disease, 37 studies or 59 observations assessed the association between secondhand smoke exposure and ischemic heart disease (IHD), while 20 studies or 26 observations assessed its association with stroke. The reported RRs for IHD and stroke were 1.26 and 1.16, respectively, thus indicating that exposure to secondhand smoke increases the risk of IHD and stroke by 8% and 5%, respectively.
Cancer-related outcomes showed a weak association between secondhand smoke exposure and lung cancer, with an RR of 1.37, and breast cancer, with an RR of 1.22. Both associations were rated weak, with lung cancer receiving a two-star rating and breast cancer receiving a one-star rating in the BPRF framework. Sensitivity analyzes maintained these weak associations and no significant publication bias was detected.
For respiratory diseases such as asthma, lower respiratory tract infections and chronic obstructive pulmonary disease, the evidence is consistently rated as weak. After adjusting for self-reported diagnosis and other biases, the RRs for these conditions were 1.21, 1.34, and 1.44, respectively. Sensitivity analyzes and publication bias tests confirmed these weak associations.
Other health outcomes assessed included type 2 diabetes and otitis media. Secondhand smoke exposure was reported to have a weak detrimental effect on the risk of type 2 diabetes and otitis media, with RRs of 1.16 and 1.12, respectively. Both outcomes are associated with a one-star rating, so the evidence for a correlation is weak.
- Flower, LS, Anderson, JA, Ahmed, N. et al. (2024). Health effects associated with secondhand smoke exposure: a burden of proof study. natural medicine. doi:10.1038/s41591-023-02743-4