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Asthma Morbidity and Ambient Air Pollution: Effect Modification by Residential Traffic-Related Air Pollution

Delfino, Ralph J.a; Wu, Juna,b; Tjoa, Thomasa; Gullesserian, Sevan K.c; Nickerson, Bruced; Gillen, Daniel L.c

doi: 10.1097/EDE.0000000000000016
Air Pollution

Background: Ambient air pollution has been associated with asthma-related hospital admissions and emergency department visits (hospital encounters). We hypothesized that higher individual exposure to residential traffic-related air pollutants would enhance these associations.

Methods: We studied 11,390 asthma-related hospital encounters among 7492 subjects 0–18 years of age living in Orange County, California. Ambient exposures were measured at regional air monitoring stations. Seasonal average traffic-related exposures (PM2.5, ultrafine particles, NOx, and CO) were estimated near subjects’ geocoded residences for 6-month warm and cool seasonal periods, using dispersion models based on local traffic within 500 m radii. Associations were tested in case-crossover conditional logistic regression models adjusted for temperature and humidity. We assessed effect modification by seasonal residential traffic-related air pollution exposures above and below median dispersion-modeled exposures. Secondary analyses considered effect modification by traffic exposures within race/ethnicity and insurance group strata.

Results: Asthma morbidity was positively associated with daily ambient O3 and PM2.5 in warm seasons and with CO, NOx, and PM2.5 in cool seasons. Associations with CO, NOx, and PM2.5 were stronger among subjects living at residences with above-median traffic-related exposures, especially in cool seasons. Secondary analyses showed no consistent differences in association, and 95% confidence intervals were wide, indicating a lack of precision for estimating these highly stratified associations.

Conclusions: Associations of asthma with ambient air pollution were enhanced among subjects living in homes with high traffic-related air pollution. This may be because of increased susceptibility (greater asthma severity) or increased vulnerability (meteorologic amplification of local vs. correlated ambient exposures).

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From the aDepartment of Epidemiology, School of Medicine, University of California, Irvine, Irvine, CA; bProgram in Public Health, College of Health Sciences, University of California, Irvine, Irvine, CA; cDepartment of Statistics, School of Information and Computer Sciences, University of California, Irvine, Irvine, CA; and dDivision of Pulmonary Medicine, Children’s Hospital of Orange County, Orange, CA.

The project described was supported by California Air Resources Board contract number 10-319, and by South Coast Air Quality Management District contract number BPG-46329 (BP West Coast Products LLC) under the settlement agreement dated March 2005, concerning past air quality rule violations at the company’s Carson, California, refinery. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the funding agencies and no official endorsement should be inferred. The authors report no conflict of interests.

Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article ( This content is not peer-reviewed or copy-edited; it is the sole responsibility of the author.

Correspondence: Ralph J. Delfino, Department of Epidemiology, School of Medicine, University of California, Irvine, 224 Irvine Hall, Irvine, CA 92697-7555. E-mail:

© 2014 by Lippincott Williams & Wilkins, Inc