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 did not observe an association between pet exposures and childhood asthma, findings that are consistent with several birth cohort studies (Lau et al. 2000; Nafstad et al. 2001; Ronmark et al. 2002). Although some studies have found positive associations (McConnell et al. 2002; Zheng et al. 2002), others have found pets to be protective (Hesselmar et al. 1999; Remes et al. 2001). In a review of 32 articles, presence of pets in the first 2 years of life was associated with a nonsignificant 11% increase in asthma risk (Apelberg et al. 2001). It is difficult to explore the association of pet exposure and childhood asthma even in prospective studies because of concerns over temporality and other lifestyle factors that might be associated with pet keeping. For example, families with asthma or allergic disease might avoid keeping pets. Further prospective studies are needed to examine the associations between childhood asthma and age at pet exposure, duration of pet exposure, and measured levels of allergens and endotoxin.

Although studies have observed positive associations between asthma and pesticide and herbicide use in adults (Bener et al. 1999; Hoppin et al. 2002), data on pesticides and childhood asthma are limited. We found that exposure to either pesticides or herbicides, beginning in the first year of life, was associated with an increased risk for early-onset persistent asthma. The exposures occurred in both farm and nonfarm settings in our study. Our results are consistent with a previous study that reported > 3-fold increased risk of asthma in children between 7 and 10 years of age who had at least 0.3 [micro]g/L of the organochlorine compound dichlorodiphenyldichloroethene in their blood (Karmaus et al. 2001b). It has been suggested that children's hand-to-mouth behavior, closeness to the playground, low ratio of skin surface to body mass, reduced ability to detoxify toxic substances, and increased sensitivity of cholinergic receptors to pesticides make them more vulnerable to the toxic effects of pesticides, especially during their early lives (Ernst 2002; Faustman et al. 2000; Landrigan et al. 1999). Moreover, immature respiratory systems and immune systems as well as developing nervous systems may be more vulnerable to the deleterious effects of pesticide and herbicide. Given the widespread use of pesticides and herbicides in the home and farm environments and the magnitude of the observed risks, additional studies of the role these exposures in asthma etiology across childhood are needed.

Several European cross-sectional studies have suggested a reduced risk of asthma with early-life exposures to a farming environment (Riedler et al. 2001; Von Ehrenstein et al. 2000). It has been suggested that exposure to a farming environment (e.g., livestock, dust, crop) causes higher levels of bacterial endotoxin exposure, and the latter eventually leads to the production of several cytokines (e.g., interleukin-12, interferon-[gamma]) that tip the balance toward the [T.sub.H]1- over [T.sub.H]2-mediated immunity, thereby reducing asthma risk (Braun-Fahrlander 2001). However, we did not see such an inverse association with early-life farm exposures in our study. In fact, our results suggest an increased risk for early-onset persistent asthma with farm-related exposure, and we observed a significant increased risk of asthma in children who were exposed to farm animals, crops, or dust in their first year of life. Other studies in the United States and Canada have found that growing up in a farming environment is associated with increased risk of asthma and that endotoxin exposures may increase asthma risk. Explanations for this discrepancy between studies include differences in farming practices, crops, and differences in dietary, lifestyle, and other unrecognized "rural" factors that might influence this risk reduction in Europe but not in California and other regions of the world. Moreover, it has been suggested that proximity of the stables to the home and time spent in such stables might be important determinants for assessing asthma risk in the European studies (Braun-Fahrlander 2001). We were not able to address these issues because we did not have appropriate information on our study subjects.