Personal exposure to nitrogen dioxide (NO2) and the severity of virus-induced asthma in children

Abstract

Background: A link between exposure to the air pollutant nitrogen dioxide (NO2) and respiratory disease has been suggested. Viral infections are the major cause of asthma exacerbations. We aimed to assess whether there is a relation between NO2 exposure and the severity of asthma exacerbations caused by proven respiratory viral infections in children.

Methods: A cohort of 114 asthmatic children aged between 8 and 11 years recorded daily upper and lower respiratory-tract symptoms, peak expiratory flow (PEF), and measured personal NO2 exposures every week for up to 13 months. We took nasal aspirates during reported episodes of upper respiratory-tract illness and tested for infection by common respiratory viruses and atypical bacteria with RT-PCR assays. We used generalised estimating equations to assess the relation between low (<7.5 microg/m3), medium (7.5-14 microg/m3 ), and high (>14 microg/m3) tertiles of NO2 exposure in the week before or after upper respiratory-tract infection and the severity of asthma exacerbation in the week after the start of an infection.

Findings: One or more viruses were detected in 78% of reported infection episodes, and the medians of NO2 exposure were 5 (IQR 3.6-6.3), 10 (8.7-12.0), and 21 microg/m3 (16.8-42.9) for low, medium, and high tertiles, respectively. There were significant increases in the severity of lower respiratory-tract symptom scores across the three tertiles (0.6 for all viruses [p=0.05] and >2 for respiratory syncytial virus [p=0.01]) and a reduction in PEF of more than 12 L/min for picornavirus (p=0.04) for high compared with low NO2 exposure before the start of the virus-induced exacerbation.

Interpretation: High exposure to NO2 in the week before the start of a respiratory viral infection, and at levels within current air quality standards, is associated with an increase in the severity of a resulting asthma exacerbation.

Figure 1

Mean change in respiratory-tract symptom scores and PEF rates after viral infection for children in medium and high NO₂ exposure tertiles compared with children in the low exposure tertile RS=respiratory syncytial. Bars show SE.

Figure 2

Upper and lower respiratory-tract symptoms and PEF rate in relation to virus detection RT=upper respiratory tract. LRT=lower respiratory tract. PEF=peak expiratory flow. RSV=respiratory syncytial virus. PV=picornavirus. COR=coronavirus. Temporal relation between infection and symptom variables in one child are shown. Symptom scores are based on an arbitrary scale, and only symptoms recorded for 2 or more consecutive days are shown.