A Cohort Study of Indoor Nitrogen Dioxide and House Dust Mite Exposure in Asthmatic Children

Objective: The purpose of this study was to investigate dose–response relationships between asthma symptoms and indoor nitrogen dioxide (NO₂) and house dust mite allergen (HDM) in children. Methods: Asthmatic children from 18 primary schools in Adelaide, Australia, kept a daily symptoms diary over 12 weeks. Home and classroom NO₂ levels were measured repeatedly in winter 2000. HDM levels were obtained from beds. Lung function tests were performed at the beginning and at the end of the study period. Results: Data on exposure and respiratory outcomes were gathered for 174 children. For school exposure, the estimated relative symptom rate (RR) for a 10-ppb increase in NO₂ for difficulty breathing during the day was 1.09 (95% confidence interval [CI] = 1.03-1.15), at night 1.11 (95% CI = 1.05-1.18), and for chest tightness at night 1.12 (95% CI = 1.07-1.17). Significant symptom rate increases were also found for kitchen NO₂ exposure. This was supported by a negative dose–response relationship between percentage predicted forced expiratory volume in 1 second and NO₂ (-0.39%; 95% CI = -0.76 to -0.02) for kitchen exposure. Significant threshold effects using a 10-μg/g cutoff point for HDM exposure were established in the sensitized children for nighttime wheeze (RR = 3.62, 95% CI = 1.49-8.77), daytime cough (RR = 1.64, 95% CI = 1.14-2.36), and daytime asthma attack (RR = 1.95, 95% CI = 1.06-3.60). Conclusion: This study has established reliable risk estimates for exacerbations of asthma symptoms in children based on dose-response investigations of indoor NO₂ and HDM.