| Abstract
| - Background Nitrogen dioxide (NO2) has been related to respiratory infections in experimental studies but its role remains controversial in general population studies of children. We aim to assess the association between indoor NO2 and lower respiratory tract infections (LRTI) during the first year of life in a multicentre prospective cohort study. Methods Children (n = 1611) were recruited prior to birth for the Asthma Multicentre Infants Cohort Study (AMICS). Three concurrent cohorts (Ashford, Kent [UK]; Barcelona city, and Menorca Island [Spain]) followed the same research protocol. NO2 was measured with passive diffusion tubes placed in the living room for 2 weeks when infants were approximately 3 months old. Doctor-diagnosed LRTI during the first year of life (as well as antibiotic use) were measured by questionnaire, and in Ashford validated by the examination of clinical records. In Barcelona, direct measurements using nasopharyngeal lavage and cultures within a continuous surveillance system were done. Results The cumulative rates of LRTI (39% in Ashford, 28% in Barcelona, and 45% in Menorca) were unrelated to NO2 levels (corresponding medians 6, 46, and 12 ppb, respectively) in all three centres (all odds ratios being around 1). Similarly, the rates of LRTI in Barcelona measured with the continuous record showed no association with NO2 (all rate ratios being below 1). In addition, there was no association between rate of antibiotics courses per year per child (2.4 in Ashford, 1.7 in Barcelona, 0.9 in Menorca) and NO2 levels. Conclusions Indoor NO2 at current levels does not seem to be involved in increasing respiratory infections by itself in infants, suggesting that the effects observed in studies on outdoor air are probably due to other copollutants.
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