Residential traffic-related pollution exposures and exhaled nitric oxide in the children's health study

Abstract

Background: The fractional concentration of nitric oxide in exhaled air (FeNO) potentially detects airway inflammation related to air pollution exposure. Existing studies have not yet provided conclusive evidence on the association of FeNO with traffic-related pollution (TRP).

Objectives: We evaluated the association of FeNO with residential TRP exposure in a large cohort of children.

Methods: We related FeNO measured on 2,143 children (ages 7-11 years) who participated in the Southern California Children's Health Study (CHS) to five classes of metrics of residential TRP: distances to freeways and major roads; length of all and local roads within circular buffers around the home; traffic densities within buffers; annual average line source dispersion modeled nitrogen oxides (NOx) from freeways and nonfreeway roads; and predicted annual average nitrogen oxide, nitrogen dioxide, and NOx from a model based on intracommunity sampling in the CHS.

Results: In children with asthma, length of roads was positively associated with FeNO, with stronger associations in smaller buffers [46.7%; 95% confidence interval (CI), 14.3-88.4], 12.4% (95% CI, -8.8 to 38.4), and 4.1% (95% CI, -14.6 to 26.8) higher FeNO for 100-, 300-, and 1,000-m increases in the length of all roads in 50-, 100-, and 200-m buffers, respectively. Other TRP metrics were not significantly associated with FeNO, even though the study design was powered to detect exposures explaining as little as 0.4% of the variation in natural log-transformed FeNO (R2 = 0.004).

Conclusion: Length of road was the only indicator of residential TRP exposure associated with airway inflammation in children with asthma, as measured by FeNO.

Figure 1

Estimated percent difference in Fe_NO^a and 95% CI associated with an increase^b in each length of road metric: (A) adjusting for asthma status and (B) fitting separate models by asthma status. ^aAdjusted for race/ethnicity, sex, asthma status (A), asthma medication, rhinitis history, age at collection, BMI percentile, secondhand tobacco smoke, parental education, month and hour of Fe_NO collection, outdoor testing, and community of residence. ^bExposure contrasts: 100 m, 300 m, and 1,000 m for length of roads in a 50-m, 100-m, and 200-m buffer, respectively.

Figure 2

Smooth association of length of local roads in a 50-m buffer with Fe_NO for children with asthma^a (black solid line: smooth fitted value for a child with average values for the adjustment covariates; black dashed line is the 95% CI). The estimated degrees of freedom (EDF) and the p-value testing the association of the smooth with Fe_NO are displayed. On the y-axis, values are plotted on the natural log scale and labeled on the original scale. ^aAdjusted for race/ethnicity, sex, asthma medication, rhinitis history, age at collection, BMI percentile, secondhand tobacco smoke, parental education, month and hour of Fe_NO collection, outdoor testing, and community of residence.