Effect of Domestic Concentrations of Nitrogen Dioxide on Airway Responses to Inhaled Allergen in Asthmatic Patients

Nitrogen dioxide is a common indoor pollutant. In the light of suggestions that outdoor air pollution can harm people with asthma, we investigated the effect of 1 h exposures to domestic concentrations of nitrogen dioxide on the airway response to house-dust mite (HDM) allergen in ten patients with mild asthma. Each subject breathed air, 100 ppb nitrogen dioxide, or 400 ppb nitrogen dioxide for 1 h, in double-blind, random order, then immediately underwent a fixed-dose HDM challenge. Baseline forced expiratory volume in 1 s (FEV1) was not affected by any of the gas mixtures. The mean early asthmatic response (maximum percentage change in FEV1 during first 2 h after challenge) was -14.62% (SD 8.03) after air, -14.41% (7.86) after 100 ppb nitrogen dioxide, and -18.64% (7.28) after 400 ppb nitrogen dioxide. The difference between air and 400 ppb (-4.01%) was significant (95% CI -1.34 to -6.69%, p < 0.009), but those between air and 100 ppb and between 100 and 400 ppb were not (0.21 [-3.10 to 3.53]% and -4.23 [-8.75 to 0.29]%). The mean late asthmatic response (maximum percentage change in FEV1) to challenge after air was -2.85% (3.95), after 100 ppb nitrogen dioxide -7.76% (6.92), and after 400 ppb -8.13% (6.64). The difference in means between the air and 400 ppb exposures was significant (-5.28 [-0.73 to -9.83]%, p < 0.02) but those between air and 100 ppb (-4.90 [-10.60 to 0.78]%) and 100 and 400 ppb (0.37 [3.06 to 3.80]%) were not. These findings suggest that nitrogen dioxide, at concentrations encountered in the home environment, can potentiate the specific airway response of patients with mild asthma to inhaled HDM allergen, although the effect is small.