PORTSMOUTH, UK—A new study finds that asthmatic children exposed to high levels of nitrogen dioxide (NO2) in the week before a respiratory viral infection are likely to experience more severe asthma exacerbations than would typically be expected.[1] Anoop J. Chauhan, MD, the lead investigator, said that “while viruses may induce the exacerbations, the amount of NO2 pollution before the asthma flare determines how severe the exacerbation will be.” By implication, he said, effective treatment against common cold viruses (which account for about 85% of all asthma exacerbations in children) in parallel with control of NO2 exposure may reduce the number and severity of asthma attacks in children. He added that there is no reason to discount a similar NO2 effect in adults with asthma or on other respiratory conditions, such as COPD.

The 114 children in the study were followed for up to 13 months. Each day, the children’s morning and evening prebronchodilator peak expiratory flow (PEF) measurements and respiratory symptoms were recorded. When upper respiratory tract symptoms were sufficiently severe to suggest a cold, a nasal aspirate sample was taken and DNA-tested for common respiratory viruses. Diffusion tubes were worn externally on the clothing to track NO2 exposure. Children who lived with smokers were excluded from participation.

A total of 280 upper respiratory tract infections developed in 99 children; a causative organism could be identified in 78% of the episodes. Among the infected children, lower respiratory tract symptoms increased in severity and PEF scores declined as NO2 exposure rose. The association between increasing NO2 exposure and worsening health status was found not only in the overall cohort of children but also in the subgroups with documented picornavirus or respiratory syncytial virus infection.

Dr. Chauhan said that little is known about how NO2 interacts with these viruses in the etiology of asthma, but one hypothesis is that NO2 inflames the tissues of the nose and lungs, increasing a person’s susceptibility to lower respiratory infection and amplifying the pathologic effects of the infection that develops.

According to the World Health Organization, the safe threshold of NO2 exposure is an annual average of 40 µg/m3. The median exposure levels in this study, which ranged between 5 and 21 µg/m3, were much lower, yet an adverse impact on health was seen. Dr. Chauhan said this discrepancy reveals a “major problem” in using outdoor air quality standards to define acceptable indoor standards, especially for pollutants such as NO2, which have important outdoor and indoor sources. Moreover, some individuals spend the bulk of their time indoors, making indoor levels of exposure more relevant.

The study identified the highest personal levels of NO2 exposure in children whose families used gas for cooking. Opening windows or installing extractor fans above the gas hobs, Dr. Chauhan said, can substantially reduce this source of NO2. Future research, he said, will need to show that lowering NO2 levels actually ameliorates asthma flares.

—Verna L. Schwartz, MS