Early-life environmental risk factors for asthma: findings from the children's health study - Children's Health

Environmental Health Perspectives, May, 2004 by Muhammad Towhid Salam, Yu-Fen Li, Bryan Langholz, Frank Davis Gilliland

We found no associations with exclusive breast-feeding and any asthma outcome. We found no significant differences in the associations of breast-feeding with asthma in children by history of maternal or family history of allergy or asthma.

Family or maternal history of asthma, secondhand tobacco exposure, maternal smoking during pregnancy, gestational age, yearly family income, health insurance coverage, and maternal education level did not confound the association between any of the early-life exposures and asthma outcomes. Therefore, these variables were not included in the final models. Furthermore, none of the associations between the exposures and early-life asthma varied by family or maternal history of asthma or allergy.

Discussion

In our population-based study of early-life environmental exposures and asthma, we found that exposures to cockroach; wood/oil smoke, soot, or exhaust; pesticide; herbicide; farm environment; and early day care attendance were associated with increased risk for early-life asthma. The associations were strongest when children were exposed beginning in their first year of life or, in the case of day care attendance, in the first 4 months of life. Thus, the present study, in the context of emerging evidence, suggests that the etiology of childhood asthma is complex and may include early-life environmental exposures as well as factors related to early allergic sensitization.

The effect of wood or oil smoke and cockroaches on childhood asthma was largely restricted to children with early transient wheezing. Combustion of wood liberates nitrogen dioxide, carbon monoxide, sulfur dioxide, and particulate matter, which have been associated with increased occurrence of respiratory illnesses (Larson and Koenig 1994). Oil smoke exposure has been shown to increase asthma risk significantly (Chen et al. 2002). Particulate matter from wood combustion significantly reduced lung function in elementary school children (Koenig et al. 1993). Similarly, exposure to cockroach allergen was associated with almost a 2-fold increased risk of wheeze in the first year of life (Belanger et al. 2003; Gold et al. 1999). In recent reports, cockroach allergen was found to alter bronchial airway epithelial cell permeability by induction of vascular endothelial growth factor (Antony et al. 2002) and was significantly associated with specific serum immunoglobulin E (IgE) levels (Bener et al. 2002). Our results are consistent with observations that early transient wheezing is associated with reduced lung function and/or increased reactivity of the airways in infancy and that exposure to combustion products and/or cockroach allergen may be important in these pathophysiologic processes.

The associations between asthma and the environmental exposures examined in this study were not due to confounding by SES. Although lower SES was significantly associated with sensitization to cockroach exposure and asthma prevalence in previous studies (Sarpong et al. 1996; Stevenson et al. 2001), most of our subjects belonged to middle socioeconomic class as evidenced by the relatively higher maternal education level and annual family income > $30,000, and most had health insurance coverage. In this SES-homogeneous population, we did not observe any significant association between asthma and cockroach exposure with SES or race. In addition, subjects were matched on community of residence, further restricting the variability in SES.