Maternal Smoking, Asthma, and Bronchiolitis: Clear-Cut Association or Equivocal Evidence?

ALTHOUGH there is no doubt that secondhand environmental tobacco smoke exposure contributes to lower respiratory tract infection (LRTI) in infants and young children in developing countries1 and industrialized nations,2,3 the evidence for a specific effect on respiratory syncytial virus (RSV) LRTI is less clear.4 It is the equivocal nature of the evidence for the role of smoking affecting the severity of RSV bronchiolitis that has, in part, prevented the American Academy of Pediatrics from strongly advocating smoking as a risk factor when considering prophylaxis with palivizumab for previously premature infants of 32 to 35 weeks’ gestation.5–7 In this issue of Pediatrics, Carroll et al8 present probably the largest population-based study of term infants with bronchiolitis to determine the association between maternal asthma, maternal smoking, and the incidence and severity of bronchiolitis. In their cohort of 100 000 mother-infant dyads, 20% of the infants had 1 health care visit for bronchiolitis. Maternal smoking increased the risk of bronchiolitis by 14%, maternal asthma raised it by 39%, and both together raised it by 47%. Infants of mothers with asthma who smoked had a higher risk for emergency department visits and hospitalizations. Although maternal smoking increased the risk of prolonged hospitalization by 19%, if the mother was smoking and had asthma the increased risk was 38%. Given the enormous size of the study encompassing the vast majority of children, infants, and mothers enrolled in the Tennessee Medicaid program between 1995 and 2003, this would seem to be compelling evidence for an association between maternal smoking and bronchiolitis. Despite the enormous size of this study, the results must be tempered with an understanding of the study limitations, which were acknowledged by the authors. The major limitation is that other risk factors that are well known to be associated with hospitalization for RSV or bronchiolitis—such as an index of crowding that includes the number of sibling and adults in the household, day-care exposure, or day-care exposure of siblings or the presence of siblings 5 years of age, birth during the first half of the RSV season, and a putative risk factor (breastfeeding)—were not included in these analyses.4 In an examination of atopic risk factors, although a maternal history of asthma was considered, that of other relatives or other conditions such as atopic dermatitis or allergic rhinitis in first-degree relatives was not included in the analysis.9 Finally, a diagnosis of bronchiolitis does not equate to RSV, although in RSV season the majority of bronchiolitis admissions (perhaps 60%–80%) would be caused by RSV. An initial study in Rochester, New York,10 conducted between 1974 and 1976, suggested that there is a significant difference between children hospitalized with RSV and controls in the amount of environmental tobacco smoke exposure (76% vs 40%, respectively; P .01). In that study, unfortunately, no multivariate analysis was performed. A subsequent case-control study of 53 subjects with bronchiolitis and 106 matched controls showed an increased risk of bronchiolitis in families with 1 smoker.11 In this study, a multivariate analysis that included a socioeconomic index, breastfeeding, crowd-