Developing Asthma in Childhood from Exposure to Secondhand Tobacco Smoke: Insights from a Meta-Regression

By Vork, Kathleen L.; Broadwin, Rachel L. et al. | Environmental Health Perspectives, October 2007 | Go to article overview

Developing Asthma in Childhood from Exposure to Secondhand Tobacco Smoke: Insights from a Meta-Regression


Vork, Kathleen L., Broadwin, Rachel L., Blaisdell, Robert J., Environmental Health Perspectives


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]

Asthma in childhood becomes a lifelong condition for many people, and there is evidence that its prevalence has increased over the past 50 years (Ross Anderson et al. 2007). Direct and indirect severe impacts on general health, well-being, and premature death can lead to large costs to the health care system for the management and treatment of asthma. The role of secondhand tobacco smoke (SHS) in asthma exacerbation is well accepted [U.S. Department of Health and Human Services (DHHS) 2006], whereas its role in childhood asthma induction is less well understood. Several reviews and meta-analyses have weighed the evidence of a causal relationship between exposure to SHS and the onset of asthma in children [Cook and Strachan 1999; Office of Environmental Health Hazard Assessment (OEHHA) 1997; Strachan and Cook 1998; U.S. DHHS 2006; U.S. Environmental Protection Agency (EPA) 1992].

These reviews and meta-analyses differ in their conclusions about the sufficiency of evidence to infer a causal relationship between SHS exposure and asthma induction in children. The U.S. EPA and California EPA concluded that SHS exposure is causally associated with an increase in the incidence of childhood asthma (OEHHA 1997; U.S. EPA 1992), based on studies of young children. Strachan and Cook (1998) observed elevated estimates of relative risk (RRs) from studies of preschool age and equivocal RRs from studies of older children. Their meta-analysis, originally conducted in the mid-1990s, was updated in the most recent Surgeon General's Report (SGR) on Health Effects from Involuntary Exposure to Tobacco Smoke (U.S. DHHS 2006). The SGR concluded that the evidence is suggestive, but not sufficient to infer a causal relationship between SHS and the induction of childhood asthma. The SGR's main reason for this conclusion is that the small number of studies that examined an association of SHS exposure from parental smoking with asthma incidence among older children (when there is reasonable diagnostic certainty) found inconsistent evidence of elevated RRs. …

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