In this study, we sought to determine the prevalence of reported neighborhood traffic (a proxy for TRAP exposure) and its association with childhood asthma/asthma like symptoms. In this ECHO cohort 15% of the children were exposed to significant neighborhood traffic, and there was a marked racial/ethnic disparity, with exposure rates being 39.5% in non-Hispanic black children and 34.9% in Hispanic children. Similarly, there was a marked racial/ethnic disparity in the prevalence of past or current asthma/asthma like symptoms.
The results from our statistical models indicate that the odds of having asthma/asthma like symptoms for children with reported high neighborhood traffic was 78% higher than the odds for children without reported high neighborhood traffic. These results are in similar direction as the Dutch Prevention and Incidence of Asthma and Mite Allergy (PIAMA) study, which examined associations between measured traffic-related air pollution and the development of asthma, allergy, and related symptoms in a prospective birth cohort.55 The authors found air pollution measurements, specifically PM2.5 concentrations, were associated with increased incidence of asthma [OR = 1.28 (95% CI: 1.10, 1.49)], prevalence of asthma [OR = 1.26; 95% CI: 1.04. 1.51)], and prevalence of asthma symptoms [OR = 1.15 (95% CI, 1.02–1.28)]. These associations were even stronger for children who remained at their birth residence. The authors stressed the need for more birth cohort studies, since the role of air pollution exposures in the development of childhood asthma, allergy, and related symptoms remains unclear.
There has been mounting evidence of a causal effect of TRAP exposures on asthma exacerbations.56 Our findings are consistent with the results of a prospective study in Sweden (n ~ 4000) which found positive associations between early life air pollution exposures (PM10 and NO2) and asthma exacerbations.57 The Cincinnati birth cohort study (n ~ 700) also detected associations between particulate air pollution exposure and wheezing during infancy.58 Another study reported significant associations between traffic-related pollution and asthma exacerbations in children born to parents with atopic dermatitis and in those suffering from recurrent wheezing or asthma.59, 60 Our results adds to the growing evidence on the importance of relevant windows of TRAP exposure.
A 2017 meta-analysis of 41 epidemiologic studies provided evidence for the hypothesis that childhood exposure to TRAP contributes to the development of asthma, and called for further studies.61 A recent review, based on seven studies, concluded that TRAP exposures may be associated with transient and persistent asthma/wheezing phenotypes in children,56 and also stressed the need for more studies on these phenotypes. As children’s early life and current exposures to TRAP appears to play a key role in asthma development,18 carefully designed studies are needed to understand this relationship and identify critical windows for intervention.
Previous studies have assessed exposure to TRAP by directly measuring air pollutants, modelling, or indirectly by using distance to busy roads and/or self-reported traffic intensity.58, 62–64 A prospective cohort study of air pollution and respiratory health among children reported that incident asthma was positively associated with TRAP measured outside children’s residences for two weeks during the summer and winter.65 Another birth cohort of racially diverse children (n = 24,608) reported an association between early-life mobile source models of air pollution and childhood asthma incidence.66 A more recent studied showed that decreased ambient air pollutants in Southern California between 1993 and 2014 was associated with a lower incidence of childhood asthma.67
In our study, we found that children of racial minority groups had a higher odds of asthma/asthma like symptoms when compared to non-Hispanic white children which is in agreement with prior reports.68–70 The odds of asthma/asthma like symptoms for non-Hispanic Black and Hispanic children were 153% and 125% higher than the odds for non-Hispanic White children, respectively. This difference remained even after adjusting for mother’s education as a proxy for socioeconomic status.71 A recent U.S. Centers for Disease Control and Prevention Report showed that asthma prevalence is more than twice as high among African American children (15.7%) and nearly twice as high in children of Puerto Rican (12.9%) descent compared with non-Hispanic white children (7.1%).72 73
Our results further showed that asthma/asthma like symptoms was strongly associated with being male and having a positive family history of asthma. The odds of asthma/asthma like symptoms for males was 53% higher than for females. Other studies have also reported increased risk of asthma and asthma-related symptoms in males, when compared to females. 74–76 In our study, children with asthma/asthma like symptoms had more exposure to current secondhand smoke/environmental tobacco smoke (15.7% vs 9.6%). However, after adjusting for demographic factors, this effect was no longer significant. We did not observe differences between asthma/asthma like symptoms and the presence of cats and dogs either during the child’s first year or in the last 12 months. These results are similar to a prior cohort study.59
We also found that obese children had 154% higher odds of having asthma/asthma like symptoms than non-obese children. Additionally, children with play equipment in the home environment/backyard had 60% higher odds of having asthma/asthma like symptoms than those without such items in the home environment. The presence of traffic can negatively affect physical activity in the neighborhood 77, 78 a key determinant of obesity.63 Two longitudinal studies have reported the possible contribution of air pollution exposure to the development of obesity in children63, 79 and animal studies support these observations.80, 81 Parents may opt to have play equipment within the home environment, such as in a backyard, to allow for physical activity, and that may be of benefit, especially for families residing in urban areas.82 However, parents may also need to consider potential TRAP exposures that could occur during outdoor physical activity.
This is the first study to report significant positive associations between asthma/asthma like symptoms and play equipment in the home environment/backyard, even after adjusting for obesity. Children who had play equipment in their backyards had higher odds of asthma like symptoms while those who had access to local parks did not when obesity was added to our regression models. There may be several reasons for this. The effect of green spaces on wheezing/asthma symptoms may depend on the size of the space, amount of vegetation, and types of pollution present. 83 84 85 In addition, parks must be safe for children to use. Studies have shown that regular physical activity has health benefits.86 Additionally, other studies have shown that regular physical activity has health benefits for children when they are in low pollution rather than high pollution environments.87, 88 While there is the possibility that playing in the backyard exposes a child to TRAP, further research is needed to examine the possible interplay of TRAP with exercise, types of green spaces, and obesity in relation to asthma/asthma like symptoms.89
Strengths of the study include the prospective longitudinal follow up of the children from mothers’ first trimester through up to 8 years of age and the racial/ethnic diversity of the cohort. The ISAAC questionnaire is a validated tool and the Pre-PAQ has also been reported to be both a valid and reliable measure of parental, family and neighborhood factors.49 As such, we expect little misclassification, if any.
Our study also has several limitations. First, data are from maternal response to questionnaires and therefore subject to recall bias. However, we believe this bias was equally distributed among children with and without reported asthma/asthma like symptoms, biasing effect estimates toward the null hypothesis. This is because the Pre-PAQ questions in this study assessed children’s physical activity and not specific environmental exposures. An analysis comparing the current participants in the study with the participants who were eligible for the study revealed statistically significant differences in maternal race-ethnicity and education; however, participants were similar with respect to maternal age, child sex, gestational age at delivery, year of birth and birthweight (Supplementary Table 1). We also believe that controlling for child’s race-ethnicity as well as maternal education mitigated any impact of potential selection bias and results remain informative. In our study, there was an association between reported asthma/asthma like symptoms and increased gestational age at delivery, but not for other factors such as mode of delivery, child’s use of antibiotic medication in the past year of child’s stay at the neonatal intensive care unit after birth. While early-term birth is a predictor of asthma, late term birth has also been associated with atopic dermatitis by 7 years of age 90. Atopic dermatitis often precedes other allergic diseases such as asthma 91, however, given the cross sectional nature of our study, we are limited in discussing this topic. We concur with the strong need to identify alternatives for disease prevention 91. There may also be genetic differences in the children enrolled in this study and this may affect susceptibility to TRAP. Since our analysis is cross-sectional, we are unable to determine whether exposure to TRAP preceded respiratory symptoms, or vice-versa, as such reverse causality is possible, as well as the possible time-related effects of obesity. Hence we had limited power to fully delineate some potential associations.
Finally, our exposure variable of interest, significant neighborhood traffic, was determined by questionnaire responses from mothers and we did not attempt to conduct air pollution exposure assessment outside their residences. Future directions for this work will involve investigations with historical records of ambient air pollution, particularly during the pre and post-natal periods, as well as lung function data. Clinicians should look closely at the contribution of an environmental factor such as traffic-related air pollution among patients who experience asthma/asthma like symptoms to prevent deteriorating pulmonary health. Additionally, public health professionals may also need to improve surveillance to help identify communities which may be at risk for increased TRAP concentrations.