| Developing Asthma in Childhood from Exposure to Secondhand Tobacco Smoke: Insights from a Meta-Regression Kathleen L. Vork, Rachel L. Broadwin, and Robert J. Blaisdell Air Toxicology and Epidemiology Branch, Office of Environmental Health Hazard Assessment, California Environmental Protection Agency, Oakland, California, USA Abstract
Objective: Studies have identified associations between household secondhand tobacco smoke (SHS) exposure and induction of childhood asthma. However, the true nature and strength of this association remains confounded in many studies, producing inconsistent evidence. To look for sources of potential bias and try to uncover consistent patterns of relative risk estimates (RRs) , we conducted a meta-analysis of studies published between 1970 and 2005. Data sources: Through an extensive literature search, we identified 38 epidemiologic studies of SHS exposure and the development of childhood asthma (that also controlled for atopy history) from 300 potentially relevant articles. Data synthesis: We observed substantial heterogeneity within initial summary RRs of 1.48 [95% confidence interval (CI) , 1.32–1.65], 1.25 (1.21–1.30) , and 1.21 (1.08–1.36) , for ever, current, and incident asthma, respectively. Lack of control for type of atopy history (familial or child) and child's own smoking status within studies and age category altered summary RRs in separate meta-regressions. After adjusting for these confounding characteristics, consistent patterns of association emerged between SHS exposure and childhood asthma induction. Our summary RR of 1.33 (95% CI, 1.14–1.56) from studies of incident asthma among older children (6–18 years of age) is 1.27 times the estimate from studies of younger children and higher than estimates reported in earlier meta-analyses. Conclusions: This new finding indicates that exposure duration may be a more important factor in the induction of asthma than previously understood, and suggests that SHS could be a more fundamental and widespread cause of childhood asthma than some previous meta-analyses have indicated. Key words: childhood asthma, environmental tobacco smoke, ETS, meta-analysis, meta-regression, relative risk, secondhand tobacco smoke, SHS. Environ Health Perspect 115:1394–1400 (2007) . doi:10.1289/ehp.10155 available via http://dx.doi.org/ [Online 28 June 2007]
Address correspondence to K. Vork, Office of Environmental Health Hazard Assessment, California Environmental Protection Agency, 1515 Clay St., 16th Floor, Oakland CA 94612 USA. Telephone: (510) 622-3158. Fax (510) 622-3210. E-mail: kvork@oehha.ca.gov Supplemental Material is available online at http://www.ehponline.org/docs/2007/10155/suppl.pdf The authors gratefully acknowledge S. Kheradpir, who provided important technical support, and M. Lipsett and C. Steinmaus for their helpful comments on early drafts of this manuscript. The authors declare they have no competing financial interests. Received 8 February 2007 ; accepted 28 June 2007.
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