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

Summary

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 μg/m³), medium (7·5–14 μg/m³), and high (>14 μg/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 μg/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.