Background: Previous studies do not provide a clear picture of the relationship between nitrogen dioxide (NO(2)) exposure and asthma.
Methods: Eighteen schools using unflued gas heating in winter were randomly allocated to either retain their heaters (10 control schools) or to have replacement flued gas or electric heaters installed at the beginning of winter (8 intervention schools). Fortnightly telephone interviews were used to record daily individual asthma symptoms that occurred over 12 weeks (including winter). Lung function and histamine challenge tests were performed at baseline and the end of the study. NO(2) was measured in each school classroom on 9 days and in each household on 3 days spread over the study period.
Results: From 199 primary school children that met the eligibility criteria, 45 intervention and 73 control children agreed to participate. Baseline characteristics were similar between groups. Difficulty breathing during the day (Relative Risk [RR] = 0.41; 95% CI: 0.07, 0.98) and night (RR = 0.32; 95% CI: 0.14, 0.69), chest tightness during the day (RR = 0.45; 95% CI: 0.25, 0.81), and daytime asthma attacks (RR = 0.39; 95% CI: 0.17, 0.93) were significantly reduced in the intervention group. Percentage predicted forced expiratory volume in one second (FEV(1)), the concentration of histamine inducing a 20% fall in FEV(1) (PD(20)), and the dose-response slope (DRS) were similar between groups at follow-up. Mean (standard deviation) NO(2) levels were 15.5 (6.6) parts per billion (ppb) and 47.0 (26.8) ppb in the intervention and control schools respectively (P < 0.001).
Conclusions: Asthma symptoms were reduced following a replacement intervention that removed high exposure to NO(2). Such replacement should be considered a public health priority for schools using unflued gas heating during winter.