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Meta-analysis of studies of passive smoking and lung cancer: effects of study type and continent.

BACKGROUND
To calculate a pooled estimate of relative risk (RR) of lung cancer associated with exposure to passive smoking in never smoking women exposed to smoking spouses. This study is an updated meta-analysis that also assesses the differences between estimated risks according to continent and study type using meta-regression.


METHODS
From a total of 101 primary studies, 55 studies are included in this meta-analysis, of which, 7 are cohort studies, 25 population-based case-control and 23 non-population-based case-control studies. Twenty previously published meta-analyses are also reviewed. Fixed and random effect models and meta-regression are used to obtain pooled estimates of RR and P-value functions are used to demonstrate consistency of results.


RESULTS
The pooled RR for never-smoking women exposed to passive smoking from spouses is 1.27 (95% CI 1.17-1.37). The RR for North America is 1.15 (95% CI 1.03-1.28), Asia, 1.31 (95% CI 1.16-1.48) and Europe, 1.31 (1.24-1.52). Sequential cumulative meta-analysis shows no trend. There is no strong evidence of publication bias.


CONCLUSIONS
The abundance of evidence, consistency of finding across continent and study type, dose-response relationship and biological plausibility, overwhelmingly support the existence of a causal relationship between passive smoking and lung cancer.