Meta-analysis of the association between secondhand smoke exposure and stroke

Background Active smoking is a recognized risk factor for stroke. We determined the evidence for an association with secondhand smoke exposure. Methods A systematic review was undertaken according to PRISMA guidelines. Random effects meta-analysis provided a pooled estimate of risk, and heterogeneity quantified using I-2 values. Potential publication and study bias were assessed using a funnel plot and Eggers test. Meta-regression analyses were used to investigate sources of heterogeneity. Results The 20 eligible studies provided 35 estimates of risk derived from 885 307 participants, of whom 5894 (0.7) suffered a stroke. The pooled estimate of risk was 1.25 (95 CI: 1.121.38) with no evidence of significant publication or small-study bias. There was moderate heterogeneity (I-2 54.2, P 0.001) but no study characteristics were statistically significant in the meta-regression analysis. There was a non-linear dose relationship. The relative risk increased from 1.16 (95 CI: 1.061.27) for exposure to 5 cigarettes/day to 1.56 (95 CI: 1.251.96) for exposure to 40 cigarettes/day. Conclusions There is evidence of a strong, consistent and dose-dependent association between exposure to secondhand smoke and risk of stroke, suggestive of a causal relationship, with disproportionately high risk at low levels of exposure suggesting no safe lower limit of exposure.