Table 1 Policy summary

Background

Although smoking rates have declined globally, SHS is a major public health issue—with over one-third of the world’s population exposed and health effects disproportionately borne by women, children and people in low- and middle-income countries. Comparability across SHS–response associations is constrained by considerable variability across exposure definitions and measurement, study design and results.

In the present meta-analysis, we systematically applied the Burden of Proof methodology to synthesize evidence investigating the association between SHS and nine outcomes related to cardiovascular disease, neoplasms and respiratory conditions—in addition to type 2 diabetes and otitis media.

Main findings and limitations

We found statistically significant associations between SHS and all nine outcomes examined, suggesting that SHS exposure is irrefutably harmful to human health. When incorporating measures of known and unexplained between-study heterogeneity to generate conservative estimates of SHS-related health risk consistent with available evidence, the strongest relationships were seen for cardiovascular conditions that include IHD and stroke (the two major causes of disease burden worldwide), along with type 2 diabetes and lung cancer; for these four outcomes, SHS exposure was conservatively estimated to increase disease risk by at least around 8%, 5%, 1% and 1%, respectively. The strength of the evidence on the relationship between SHS and breast cancer, COPD, lower respiratory infections, asthma and otitis media is rated as weak, and can benefit from additional higher-quality studies.

Inconsistencies in case definitions and exposure measures and definitions used in the input data may limit the accuracy and generalizability of our findings. Moreover, to standardize results across studies, we modeled SHS exposure as a dichotomous variable, which may have oversimplified SHS risk profiles by discounting effects related to intensity and frequency of exposure. Additionally, the nine disease outcomes we investigated are unlikely to capture the full disease burden associated with SHS exposure.

Policy implications

Our meta-analysis of attributable health risks experienced by nonsmokers exposed to SHS suggests that SHS should be an area of concern for policymakers, health professionals and individual citizens. Although some of the SHS–disease associations we estimated were relatively weak, this is due in part to inconsistencies in methods and results across input studies. Moreover, the relatively high prevalence of SHS—and of the disease outcomes it is associated with—magnifies the need to prioritize reducing SHS exposure through a combination of public health policies and education initiatives. In addition to supporting strategies that promote active smoking cessation and noninitiation, it is essential to continue enacting, implementing and enforcing laws that establish smoke-free public areas. It is likewise imperative to raise awareness of the adverse consequences of SHS exposure in order to promote voluntary smoking restrictions in private homes, where women and children are disproportionately affected.