The relationship between air pollution from heavy traffic and allergic sensitization, bronchial hyperresponsiveness, and respiratory symptoms in Dutch schoolchildren - Children's Health

Environmental Health Perspectives, Sept, 2003 by Nicole A.H. Janssen, Bert Brunekreef, Patricia van Vliet, Francee Aarts, Kees Meliefste, Hendrik Harssema, Paul Fischer

Data Analysis

Statistical analysis was performed using the multilevel software package MlwiN (Rasbash et al. 2001). We used a random intercept model to account for the hierarchical structure of the data (children are clustered within schools). We used restricted iterative generalized least squares and second-order penalized quasi-likelihood. Only children who lived within 1,000 m of the motorway were included in the analysis.

Odds ratios (ORs) for traffic density and air pollution concentrations were expressed as the difference between the maximum and the minimum of the exposure indicator. ORs for distance to motorway, for both the school and the home address, were expressed as the difference between 100 and 400 m. The logarithm of distance was used because from general dispersion models an exponential decay in the contribution from the road with distance can be expected.

Results were adjusted for known potential confounders age, sex, non-Dutch nationality, cooking on gas, current parental smoking, current pet possession, parental education level (as a proxy for socioeconomic status), number of persons in the household, presence of an unvented water heater in the kitchen, questionnaire not filled out by the mother, and presence of mold stains in the kitchen, living room, or bedroom. Parental respiratory symptoms were not included as confounders in the main analysis because these symptoms could also be related to exposure to traffic-related air pollution, instead, additional adjustment for parental asthma and hay fever was made in a sensitivity analysis. Additional sensitivity analysis included restricting the population to children living within 500 m of the motorway and restricting the population to children who had lived at their current address and had been attending the present school for > 1 year, and additional adjustment for bronchitis and for severe cold or flu in the 3 weeks preceding the study.

To evaluate the possibility of a stronger effect of traffic-related air pollution among susceptible children, we also conducted analyses separately for children with a positive SPT and for children with a positive BHR test. Effect estimates for these two subgroups were compared with those found for children with a negative result for both tests.

Results

Traffic characteristics and annual average air pollution levels at the participating schools are given in Table 1. There was a 4-5-fold range in the traffic densities of the various motorways. For soot, there was a 2.5-fold difference between the school with the highest and lowest annual average concentration. For P[M.sub.2.5] and N[O.sub.2], the range was smaller (1.4-1.7). Associations among traffic characteristics and air pollution concentrations are described elsewhere (Janssen et al. 2001). By design, car and truck traffic densities were weakly correlated (R = 0.31). Distance from the school to the motorway was moderately correlated with truck traffic (R = 0.52) but not with car traffic. Distance from the homes to the motorway was not correlated with any of the other traffic characteristics. Concentrations of P[M.sub.2.5] and soot significantly increased with increasing truck traffic density and significantly decreased with increasing distance of the school to the highway. Outdoor N[O.sub.2] concentrations significantly increased with increasing total traffic density.

Response rates were 65% for the questionnaire, 62% for the lung function test, 58% for the bronchial challenge, 49% for the SPT, and 43% for the blood sampling. Of 2,509 children for which a completed questionnaire was obtained, 2,083 (83%) lived within 1,000 m of the motorway. Percentages of children living within 1,000 m for the other parts of the study were similar (82-85%).

Table 2 shows the prevalence of selected allergic and respiratory symptoms, elevated total IgE, SPT reactivity, low lung function, and BHR. Of the 318 children with a positive SPT, 170 (14.9% of the total population, 53.5% of the SPT-positive children) children responded to outdoor allergens, and 250 children (21.9% of the total population, 78.6% of the SPT-positive children) responded to indoor allergens; 64 (20.1%) children responded to outdoor allergens alone, and 144 (45.3%) children responded to indoor allergens alone. About 63% of the children studied had a negative SPT and a negative bronchial challenge test. Prevalences of respiratory symptoms for children who participated in the bronchial challenge, SPT, and/or blood sampling did not differ significantly (p < 0.05) from the prevalences for children who did not participate in these tests (results not shown).