Passive smoking associated with significant increases in the risk of nine health outcomes including lung and breast cancer

By Vijay Kumar MalesuReviewed by Benedette Cuffari, M.Sc.Jan 12 2024

In a recent study published in Nature Medicine, researchers assess and quantify the adverse health effects of second-hand smoke (SHS) exposure.

Study: Health effects associated with exposure to secondhand smoke: a Burden of Proof study. Image Credit: Namning / Shutterstock.com

The persistent threat of second-hand smoke

Tobacco use, which is a leading global health risk, contributed to over 229.8 million disability-adjusted life years and 8.7 million deaths in 2019. SHS exposure, which affects about 37% of the global population, particularly harms nonsmokers, with women and children often at a greater risk of exposure.

Despite reduced smoking rates, the health impact of SHS remains significant, especially in low- and middle-income countries. In fact, the Global Burden of Diseases Study (GBD) in 2019 attributed 1.3 million deaths to SHS.

Further research is needed to address the gaps in evidence quality and study heterogeneity, better understand the full impact of SHS on health, as well as effectively inform and enhance global tobacco control policies and public health interventions.

About the study

Researchers employed the Burden of Proof Risk Function (BPRF) methodology to estimate the association between SHS exposure and nine health outcomes while also assessing the strength of the supporting evidence. SHS was defined as current exposure among nonsmokers to smoke from any combustible tobacco product, which aligns with definitions used in previous GBD studies.

The BPRF framework, which was previously used for evaluating the health effects of smoking and dietary factors, utilizes a meta-regression-Bayesian, regularized, and trimmed (MR-BRT) tool to estimate pooled relative risks (RRs) and uncertainty intervals. This approach accounts for systematic bias, within-study correlation, and unexplained between-study heterogeneity.

A systematic review was employed to extract data from relevant studies, estimate pooled RRs comparing SHS exposure risks while adjusting for systematic biases, quantify unexplained between-study heterogeneity, evaluate publication and reporting biases, and estimate the BPRF to generate conservative risk estimates and corresponding risk-outcome scores (ROS).

The BPRF reflects the smallest harmful effect of a risk exposure that is consistent with available evidence. The ROS, which is 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 star-rating scale for interpretation.

The study did not disaggregate RRs by sex, geography, or age, except for breast cancer and asthma, which focused on female-only populations and children, respectively. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and Guidelines for Accurate and Transparent Health Estimates Reporting (GATHER) recommendations were followed, with approval from the University of Washington's institutional review board.

The systematic review process included searching PubMed and Web of Science for studies published between January 1970 and July 2022, wherein researchers screened studies based on inclusion criteria and extracted data from selected publications. Effect sizes that closely matched the GBD risk definition were prioritized and the MR-BRT tool was subsequently utilized for meta-regression analysis, thereby generating pooled RRs for health outcomes in those exposed to SHS.

Biases across study designs and characteristics were tested and adjusted using the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) approach. Any remaining between-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 performed to determine the strength of primary findings. These analyses involved applying restrictions on data input and ensuring reproducibility by making data and code available.

Study findings

A total of 410 publications from a pool of 9,081 records were used for the systemic review. Of the included studies, 125 were conducted on asthma, 104 on lung cancer (104), as well as 21 on chronic obstructive pulmonary disease (COPD) and nine studies on type 2 diabetes. This resulted in 623 observations from multiple locations.

For cardiovascular diseases, 37 studies or 59 observations assessed the relationship between SHS exposure and ischemic heart disease (IHD), whereas 20 studies or 26 observations evaluated its association with stroke. RRs of 1.26 and 1.16 were reported for IHD and stroke, respectively, thus suggesting that SHS exposure increases the risk of IHD and stroke by 8% and 5%, respectively.

Cancer-related outcomes had a weak association between SHS exposure and lung cancer, with an RR of 1.37, whereas the RR with breast cancer was 1.22. Both associations were rated as weak, with lung cancer receiving a two-star rating and breast cancer a one-star rating in the BPRF framework. Sensitivity analyses maintained these weak associations, and no significant publication bias was detected.

For respiratory conditions like asthma, lower respiratory infections, and COPD, the evidence was consistently rated as weak. The RRs for these conditions were 1.21, 1.34, and 1.44, respectively, with adjustments made for self-reported diagnoses and other biases. Sensitivity analyses and tests for publication bias confirmed these weak associations.

Other health outcomes evaluated included type 2 diabetes and otitis media. Weak harmful effects of SHS exposure on the risk of type 2 diabetes and otitis media were reported, with RRs of 1.16 and 1.12, respectively. Both outcomes were associated with a one-star rating, thus indicating weak evidence of an association.