RATIONALE: Epidemiological research strongly supports an association between air pollution and COPD exacerbations. Numerous mechanisms may underlie any association as pollutants are toxic to pulmonary cells and may increase susceptibility to respiratory infections. The relationship between ambient pollution and exacerbation etiology has not been studied. OBJECTIVES: To evaluate the characteristics of pollution-associated exacerbations and whether the association is specific to exacerbations of infective or non-infective etiology. METHODS: We analyzed the effect of preceding ambient PM10, NOx and O3 on characterized COPD exacerbations in a regression model adjusted for temperature, seasonality and long-term trend. We specifically examined associations with exacerbations of suspected viral and/or bacterial, or non-infective etiology. For the associations identified we further examined the characteristics of pollution-associated exacerbations. MEASUREMENTS AND MAIN RESULTS: 4173 exacerbations occurred over the 20 year study period. Higher ambient NOx was consistently associated with increased viral-type exacerbations at 2-4 days lag (p=0.010). Recovery for viral-type exacerbations following higher ambient NOx was significantly prolonged. These findings were consistent in the subset of 2841 exacerbations treated with oral corticosteroids or antibiotics, with recovery 1.29 (95% CI 1-17-1.42; P<0.001) times longer with 'viral-type' exacerbations of onset 3 days after above versus below median ambient NOx. A likely bimodal association of PM10 with infective exacerbations was also evident, and supported by a daily time-series analysis. CONCLUSIONS: Higher levels of ambient NOx are associated with prolonged exacerbations of likely viral etiology, supporting toxicological effects of air pollution that increase susceptibility to, and severity of, infection.