Researchers recently conducted a study, published in Nature Medicine, that evaluates and quantifies the health damaging effects of exposure to secondhand smoke (SHS).
Study: Health impacts associated with passive smoking: an evidence-based study. Image credit: Namning / Shutterstock.com
The continued threat of passive smoking
Tobacco consumption, a leading global health risk, contributed to over 229.8 million disability-adjusted life years and 8.7 million deaths in 2019. Secondhand smoke exposure, affecting about 37% of the world’s population, particularly harms nonsmokers, with women and children often at a higher risk of exposure.
Despite declining smoking rates, the health impacts of SHS remain significant, particularly in low- and middle-income countries. In fact, the Global Burden of Diseases Study (GBD) attributed 1.3 million deaths to SHS in 2019.
Further research is needed to address gaps in evidence quality and study heterogeneity, better understand the full impacts of SHS on health, and effectively inform and improve global tobacco control policies and public health interventions.
About the study
The researchers used the BPRF (Burden of Proof Risk Function) methodology to assess the relationship between SHS exposure and nine health outcomes while also evaluating the strength of supportive evidence. SHS was defined as current exposure of nonsmokers to the smoke from a burning tobacco product, consistent with definitions used in previous GBD studies.
The BPRF framework, previously used to evaluate the health impacts of smoking and dietary factors, utilizes a Meta-Regression Bayesian, Regularized, Trimmed (MR-BRT) tool to estimate pooled relative risks (RRs) and uncertainty intervals. This approach accounts for systematic bias, correlation within the study, and unexplained heterogeneity between studies.
A systematic review was employed to extract data from relevant studies, estimate pooled RRs for comparing SHS exposure risks while accounting for systematic bias, quantify unexplained heterogeneity between studies, evaluate publication and reporting biases, and estimate the BPRF to create conservative risk estimates and the corresponding Risk-Outcome Strength (ROS) values.
The BPRF reflects the smallest harmful effect of a risk exposure consistent with available knowledge. The ROS, a signed value of the log RR, reflects the effect size and strength of evidence for each risk-outcome association, which is then translated into a stars rating scale for interpretation.
In the study, the RRs were not disaggregated by gender, geography, or age, except for breast cancer and asthma which focused on exclusively female populations or children. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and the Guidelines for Accurate and Transparent Health Estimates Reporting (GATHER) were followed with the approval of the Institutional Review Board of the University of Washington.
The systematic review process involved searching for studies published between January 1970 and July 2022 in PubMed and Web of Science, with the researchers reviewing studies based on inclusion criteria and extracting data from selected publications. Effect sizes closely aligned with GBD risk definitions were prioritized, and the MR-BRT tool was subsequently used for the meta-regression analysis, generating pooled RRs for health outcomes in SHS-exposed individuals.
Bias between study designs and features were tested and adjusted using the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) approach. Remaining study heterogeneity was quantified using a linear mixed-effects model. Publication and reporting biases were evaluated using funnel plots and Egger’s regression tests.
Sensitivity analyses were conducted to determine the strength of the primary findings. These analyses involved applying data entry restrictions and ensuring reproducibility by providing data and code.
Study results
A total of 410 publications were used for the systematic review from a pool of 9,081 records. Of the included studies, 125 were on asthma, 104 on lung cancer, 21 on chronic obstructive pulmonary disease (COPD), and nine on type 2 diabetes, resulting in 623 observations from multiple locations.
For cardiovascular diseases, 37 studies or 59 observations examined the link between SHS exposure and ischemic heart disease (IHD), while 20 studies or 26 observations focused on the association with stroke. RRs of 1.26 and 1.16 were reported for IHD and stroke, respectively, indicating that SHS exposure increases the risk of IHD and stroke by 8% and 5%, respectively.
Cancer-related outcomes showed a weak association between SHS exposure and lung cancer, with an RR of 1.37, while the RR for breast cancer was 1.22. Both associations were rated as weak, with lung cancer receiving a two-star rating in the BPRF framework and breast cancer receiving a one-star rating. Sensitivity analyses confirmed these weak associations, and no significant publication bias was found.
Evidence for respiratory diseases such as asthma, lower respiratory tract infections, and COPD was consistently rated as weak. RRs for these diseases were 1.21, 1.34, and 1.44, accounting for self-reported diagnoses and other biases. Sensitivity analyses and tests for publication bias confirmed these weak associations.
Other health outcomes examined included type 2 diabetes and otitis media. Weak harmful effects of SHS exposure were reported for the risk of type 2 diabetes and otitis media, with RRs of 1.16 and 1.12, respectively. Both outcomes were associated with a one-star rating, indicating weak evidence of an association.
Journal reference:
- Flor, LS, Anderson, JA, Ahmad, N. et al. (2024). Health impacts associated with passive smoking: an evidence-based study. Nature Medicine. doi:10.1038/s41591-023-02743-4