Background: Asthma can be worsened by exposure to air pollution. Exercise-induced bronchoconstriction (EIB) in children is usually evaluated with a free running test outdoors.

Aims and objectives: Our aim was to investigate the effect of short-term exposure to air pollution on EIB in children.

Methods: A retrospective analysis of reliable free running tests outdoors in children aged 6-16 years (mean 10.9 years, n=321) at Tampere University Hospital, Finland, was conducted. Pulmonary function was evaluated with spirometry. Outdoor air pollutant levels during the exercise tests were collected from the Tampere city registry.

Results: Nitrogen dioxide (NO₂) concentrations were low during the exercise tests (median 14, range 2-78 µg/m³) and there were no values over the World Health Organization recommendation for short-term limit (200 µg/m³). NO₂ concentration correlated with the relative decrease in forced expiratory volume at 1 second (FEV₁) in linear regression (coefficient 0.22, 95% CI [0.05-0.38], p-value = 0.011). This correlation remained significant in multivariate regression analysis adjusted with age, gender, Immunoglobulin E-mediated sensitization and absolute humidity of air (coefficient 0.19, 95% CI [0.01-0.38], p-value = 0.036). Ozone (O₃) or particulate matter diameter < 2.5 µm (PM_2.5) concentrations did not correlate with relative decrease in FEV₁ (p-values 0.586-0.687).

Conclusion: Relatively low NO₂ levels are associated with more severe obstruction after exercise in children. Our results link to the growing evidence that asthmatic children are susceptible to pollution and even low levels could worsen asthma morbidity in children.