Majid Ezzati and colleagues report the findings of a systematic review and meta-analysis that probes the association between environmental exposure to secondhand smoke and the epidemiology of pediatric invasive bacterial disease.
A number of epidemiologic studies have observed an association between secondhand smoke (SHS) exposure and pediatric invasive bacterial disease (IBD) but the evidence has not been systematically reviewed. We carried out a systematic review and meta-analysis of SHS exposure and two outcomes, IBD and pharyngeal carriage of bacteria, for Neisseria meningitidis (N. meningitidis), Haemophilus influenzae type B (Hib), and Streptococcus pneumoniae (S. pneumoniae).
Methods and Findings
Two independent reviewers searched Medline, EMBASE, and selected other databases, and screened articles for inclusion and exclusion criteria. We identified 30 case-control studies on SHS and IBD, and 12 cross-sectional studies on SHS and bacterial carriage. Weighted summary odd ratios (ORs) were calculated for each outcome and for studies with specific design and quality characteristics. Tests for heterogeneity and publication bias were performed. Compared with those unexposed to SHS, summary OR for SHS exposure was 2.02 (95% confidence interval [CI] 1.52–2.69) for invasive meningococcal disease, 1.21 (95% CI 0.69–2.14) for invasive pneumococcal disease, and 1.22 (95% CI 0.93–1.62) for invasive Hib disease. For pharyngeal carriage, summary OR was 1.68 (95% CI, 1.19–2.36) for N. meningitidis, 1.66 (95% CI 1.33–2.07) for S. pneumoniae, and 0.96 (95% CI 0.48–1.95) for Hib. The association between SHS exposure and invasive meningococcal and Hib diseases was consistent regardless of outcome definitions, age groups, study designs, and publication year. The effect estimates were larger in studies among children younger than 6 years of age for all three IBDs, and in studies with the more rigorous laboratory-confirmed diagnosis for invasive meningococcal disease (summary OR 3.24; 95% CI 1.72–6.13).
When considered together with evidence from direct smoking and biological mechanisms, our systematic review and meta-analysis indicates that SHS exposure may be associated with invasive meningococcal disease. The epidemiologic evidence is currently insufficient to show an association between SHS and invasive Hib disease or pneumococcal disease. Because the burden of IBD is highest in developing countries where SHS is increasing, there is a need for high-quality studies to confirm these results, and for interventions to reduce exposure of children to SHS.
Please see later in the article for the Editors' Summary
The deleterious health effects of smoking on smokers are well established, but smoking also seriously damages the health of nonsmokers. Secondhand smoke (SHS), which is released by burning cigarettes and exhaled by smokers, contains hundreds of toxic chemicals that increase the risk of adults developing lung cancer and heart disease. Children, however, are particularly vulnerable to the effects of SHS exposure (also known as passive smoking) because they are still developing physically. In addition, children have little control over their indoor environment and thus can be heavily exposed to SHS. Exposure to SHS increases the risk of ear infections, asthma, respiratory symptoms (coughing, sneezing, and breathlessness), and lung infections such as pneumonia and bronchitis in young children and the risk of sudden infant death syndrome during the first year of life.
Why Was This Study Done?
Several studies have also shown an association between SHS exposure (which damages the lining of the mouth, throat, and lungs and decreases immune defenses) and potentially fatal invasive bacterial disease (IBD) in children. In IBD, bacteria invade the body and grow in normally sterile sites such as the blood (bacteremia) and the covering of the brain (meningitis). Three organisms are mainly responsible for IBD in children—Streptococcus pneumoniae, Haemophilus influenzae type B (Hib), and Neisseria meningitidis. In 2000, S. pneumonia (pneumococcal disease) alone killed nearly one million children. Here, the researchers undertake a systematic review and meta-analysis of the association between SHS exposure in children and two outcomes—IBD and the presence of IBD-causing organisms in the nose and throat (bacterial carriage). A systematic review uses predefined criteria to identify all the research on a given topic; meta-analysis is a statistical method that combines the results of several studies. By combining data, it is possible to get a clearer view of the causes of a disease than is possible from individual studies.
What Did the Researchers Do and Find?
The researchers identified 30 case-control studies that compared the occurrence of IBD over time in children exposed to SHS with its occurrence in children not exposed to SHS. They also identified 12 cross-sectional studies that measured bacterial carriage at a single time point in children exposed and not exposed to SHS. The researchers used the data from these studies to calculate a “summary odds ratio” (OR) for each outcome—a measure of how SHS exposure affected the likelihood of each outcome. Compared with children unexposed to SHS, exposure to SHS doubled the likelihood of invasive meningococcal disease (a summary OR for SHS exposure of 2.02). Summary ORs for invasive pneumococcal disease and Hib diseases were 1.21 and 1.22, respectively. However, these small increases in the risk of developing these IBDs were not statistically significant unlike the increase in the risk of developing meningococcal disease. That is, they might have occurred by chance. For bacterial carriage, summary ORs for SHS exposure were 1.68 for N. meningitidis, 1.66 for S. pneumonia (both these ORs were statistically significant), and 0.96 for Hib (a nonsignificant decrease in risk).
What Do These Findings Mean?
These findings indicate that SHS exposure is significantly associated with invasive meningococcal disease among children. However, the evidence that SHS exposure is associated with invasive pneumococcal and Hib disease is only suggestive. These findings also indicate that exposure to SHS is associated with an increased carriage of N. meningitidis and S. pneumoniae. The accuracy and generalizability of these findings is limited by the small number of studies identified, by the lack of studies from developing countries where SHS exposure is increasing and the burden of IBD is high, and by large variations between the studies in how SHS exposure was measured and IBD diagnosed. Nevertheless, they suggest that, by reducing children's exposure to SHS (by, for example, persuading parents not to smoke at home), the illness and death caused by IBDs among children could be greatly reduced. Such a reduction would be particularly welcome in developing countries where vaccination against IBDs is low.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000374.
The US Centers for Disease Control and Prevention provides information on secondhand smoke, on children and secondhand smoke exposure, on meningitis, and on Hib infection
The US Environmental Protection Agency also provides information on the health effects of exposure to secondhand smoke (in English and Spanish) and a leaflet (also in English and Spanish) entitled Secondhand Tobacco Smoke and the Health of Your Family
The US Office of the Surgeon General provides information on the health consequences of involuntary exposure to tobacco smoke
The World Health Organization provides a range of information on the global tobacco epidemic
The World Health Organization has information on meningococcal disease (in English only) and on Hib (in several languages)
The US National Foundation for Infectious Diseases provides a fact sheet on pneumococcal disease