From Medscape Education Clinical Briefs
Passive Smoke Exposure Associated With Wheeze, Asthma in Children
03/21/2012
CLINICAL CONTEXT
Secondhand smoke exposure has a profound effect on the risks for morbidity and mortality worldwide, according to a study by Oberg and colleagues published in the January 8, 2011, issue of the Lancet. They note that the largest proportion of the nonsmoking population exposed to secondhand smoke is children. Overall, approximately 1% of worldwide mortality may be the result of secondhand smoke, with ischemic heart disease accounting for most deaths. Similarly, approximately 0.7% of worldwide disability is caused by secondhand smoke, with most of this burden borne by children. Lower respiratory tract infections among children younger than 5 years account for the largest proportion of all disabilities related to secondhand smoke exposure.
New research has provided better data regarding the relationship between prenatal and passive smoke exposure and the risks for incident asthma and wheeze among children. The current systematic review and meta-analysis by Burke and colleagues provides an accurate assessment of these risks.
STUDY SYNOPSIS AND PERSPECTIVE
Children who are exposed to tobacco smoke prenatally or in their home after birth are at least 20% more likely to have wheezing episodes or develop asthma. The magnitude of the risks is higher than seen in previous estimates, according to a meta-analysis published online March 19 in Pediatrics.
Hannah Burke, BMBS, from the University of Nottingham in the United Kingdom, and colleagues conducted a systematic review and meta-analysis of 71 prospective epidemiologic studies that examined the association between passive smoke exposure and the incidence of pediatric wheeze and asthma.
After conducting an extensive literature search that included Medline, Embase, and the Cumulative Index to Nursing and Allied Health Literature, as well as conference abstracts, they identified and screened 5074 articles, yielding 70 articles with a total of 71 studies.
"We included all prospective epidemiologic studies assessing the association between passive smoke exposure and incidence of asthma or wheeze in children or young people up to the age of 18 years in which participants were free of disease (asthma or wheeze) at the start of the study and passive smoke exposure was documented at a time point before the incidence of disease was determined," the authors write.
The researchers analyzed the effects of 4 different types of smoke exposure on the development of wheezing and asthma: prenatal maternal smoking, maternal smoking, paternal smoking, and household smoke exposure.
Prenatal Maternal Smoking
Prenatal maternal smoke exposure was associated with a 40% increase in risk for wheeze in children aged 2 years or younger (odds ratio [OR], 1.41, 95% confidence interval [CI], 1.20 - 1.67; I2, 82.5%; 14 studies). Results were similar for children aged 3 to 4 years (OR, 1.28; 95% CI, 1.14 - 1.44; I2, 65.5%; 8 studies). This exposure also was associated with a 52% increased risk of wheezing in children aged 5 to 18 years (OR, 1.52; 95% CI, 1.23 - 1.87; I2, 21.1%; 5 studies).
In addition, prenatal smoke exposure was associated with an increased risk for asthma; this risk was highest in children aged 2 years or younger (OR, 1.85; 95% CI, 1.35 - 2.53; I2, 41.9%; 5 studies).
Risks declined progressively with age, but children aged 5 to 18 years still had an increased risk of developing asthma (OR, 1.23; 95% CI, 1.12 - 1.36; I2, 50%; 11 studies).
Maternal Smoking
Postnatal maternal smoke exposure was associated with an increased risk for wheezing episode in children aged 2 years or younger (OR, 1.70; 95% CI, 1.24 - 2.35; I2, 0.0%; 4 studies), in children aged 3 to 4 years (OR, 1.65; 95% CI, 1.20 - 2.28; I2, 48.5%; 4 studies), and in children aged 5 to 18 years (OR, 1.18; 95% CI, 0.99 - 1.40; I2, 1.4%; 3 studies).
Postnatal maternal smoke exposure was not associated with increased risk for asthma in children aged 4 years or younger, but there was a borderline association for children 5 to 18 years of age (OR, 1.20; 95% CI, 0.98 - 1.46; P = .08; I2, 65.3%; 8 studies).
Paternal Smoking
There were limited data on paternal smoking, with only 2 studies available for analysis of risk of wheezing in children 5 to 18 years of age (OR, 1.38; 95% CI, 1.05 - 1.85; I2, 0%; 2 studies).
No studies had data on the association between paternal smoke exposure and the risk for asthma in children 2 years of age of younger, and there was only 1 study with data on children 3 to 4 years of age. That study showed a significant effect of paternal smoking (OR, 1.34; 95% CI, 1.23 - 1.46).
There was no association between paternal smoke exposure and development of asthma in children aged 5 to 18 years (OR, 0.98; 95% CI, 0.71 - 1.36; I2, 0%; 4 studies).
Household Smoke Exposure
Exposure to household smoke was associated with an increased risk of wheezing in children aged 2 years or younger (OR, 1.35; 95% CI, 1.10 - 1.64; I2, 64.5%; 9 studies).
Household smoke exposure was not associated with an increased risk of wheezing in children aged 3 to 4 years (OR, 1.06; 95% CI, 0.88 - 1.27; I2, 54.5%; 4 studies), but it was associated with an increased risk of wheezing in children aged 5 to 18 years (OR, 1.32; 95% CI, 1.12 - 1.56; I2, 0%; 5 studies).
Exposure to household smoke was not associated with an increased risk for asthma in children 2 years of age or younger (OR, 1.14; 95% CI, 0.94 - 1.38; I2, 0.1%; 3 studies), but it was associated with an increased risk for asthma in children aged 3 to 4 years (OR, 1.21; 95% CI, 1.00 - 1.47; I2, 72.7%; 5 studies) and children aged 5 to 18 years (OR, 1.30; 95% CI, 1.04 - 1.62; I2, 37.7%; 5 studies).
Effects of Passive Smoke Higher Than Previous Estimates
"Our findings indicate that the effects of passive smoking on the incidence of wheeze and asthma are substantially higher than previously estimated, particularly for the effect of maternal postnatal smoking exposure," the authors write.
The authors note that this systematic review and meta-analysis is the largest reported review of this subject to date.
"Exposure to passive smoking is an important risk factor for the incidence of wheeze and asthma throughout childhood.... [I]t is important to limit children's exposure to passive smoke both during gestation and throughout the child's life," the authors write.
This study was supported by a project grant from Cancer Research UK and by core funding to the UK Centre for Tobacco Control Studies from the British Heart Foundation, Cancer Research UK, Economic and Social Research Council, Medical Research Council, and the Department of Health, under the auspices of the UK Clinical Research Collaboration. The authors have disclosed no relevant financial relationships.
Pediatrics. 2012;129:735-744.
STUDY HIGHLIGHTS
- Researchers identified prospective epidemiologic studies evaluating the relationship between passive smoke exposure and the incidence of new wheeze or asthma among children and adolescents. Studies published through February 2011 were included in the review.
- Studies included prenatal smoking exposure as well as passive smoke exposure during different stages of childhood.
- 180 studies received full-text review, and 70 articles were eligible for inclusion in the meta-analysis. The collective research was fairly evenly divided in the evaluation of childhood wheeze and asthma and in the assessment of the effects of prenatal and postnatal passive smoke exposure.
- The overall quality of the included studies was moderate. The main methodologic flaws in the research were a lack of objective measure of smoking and a lack of adjustment for confounding factors.
- Prenatal maternal smoking was associated with an OR for wheeze of 1.41 (95% CI, 1.20 - 1.67) among children at 2 years old or younger.
- The OR associated with prenatal maternal smoking for wheeze among children 3 to 4 years old was 1.28 (95% CI, 1.14 - 1.44). The respective OR for children 5 to 18 years old was 1.52 (95% CI, 1.23 - 1.87).
- The effect of postnatal maternal smoking increased the risk for wheeze among children 2 years or younger (OR, 1.70; 95% CI, 1.24 - 2.35), children 3 to 4 years old (OR, 1.65; 95% CI, 1.20 - 2.68), and children 5 to 18 years old (OR, 1.18; 95% CI, 0.99 - 1.40).
- More limited data were available regarding the association between paternal smoking and wheeze, but paternal smoking did significantly increase the risk for wheeze among children 15 to 18 years old.
- The collection of data demonstrated a generally weaker association between passive smoke exposure and the risk for asthma vs passive smoke exposure and the risk for wheeze. The strongest type of passive smoke exposure promoting a higher risk for asthma was prenatal maternal smoking in children 2 years or younger (OR, 1.85; 95% CI, 1.35 - 2.53).
- Other passive smoke exposures during childhood increased the risk for incident asthma by approximately 20%, and the sum of these results was statistically significant.
CLINICAL IMPLICATIONS
- Overall, approximately 1% of worldwide mortality may be caused by secondhand smoke, with ischemic heart disease accounting for most deaths. Children are the largest proportion of the nonsmoking population exposed to secondhand smoke, and they bear the greatest burden of disability caused by secondhand smoke. Lower respiratory tract infections among children younger than 5 years account for the largest proportion of all disability related to secondhand smoke exposure.
- The current study by Burke and colleagues suggests that passive smoke exposure increases the risks for wheeze and asthma in children by at least 20%.
