Repeated exposure to an ambient level of NO2 enhances asthmatic response to a nonsymptomatic allergen dose

We investigated the effects of NO2 and allergen on lung function in a repeated exposure model. For 4 subsequent days, 16 subjects with mild asthma and allergy to birch or grass pollen were exposed at rest to either purified air or 500 microg x m(-3) NO2 for 30 min in an exposure chamber. Four hours later, an individually determined nonsymptomatic allergen dose was inhaled. Lung function (forced expiratory volume in one second (FEV1)) was measured by a portable spirometer at early phase (EP) 15 min after allergen and at late phase (LP) 3-10 h after allergen. Subjective symptoms and medication were followed by diary cards. Asthmatic response was significantly increased after repeated exposure to NO2 and allergen compared to air and allergen. The 4-day mean fall in FEV1 after NO2 was at EP -25% versus -0.4% for air (p=0.02) and at LP -4.4% versus -1.9% for air (p=0.01, ANOVA). An increase in EP response was seen already after a single NO2 exposure (p=0.03). There was a tendency (p=0.07) towards increased night-time symptoms of asthma after NO2 plus allergen. Although the effects were small, the results indicate that a repeated short exposure to an ambient level of NO2 enhances the airway response to a nonsymptomatic allergen dose.