Rationale: The effect of indoor air pollutants on respiratory morbidity among patients with chronic obstructive pulmonary disease (COPD) in developed countries is uncertain. Objectives: The first longitudinal study to investigate the independent effects of indoor particulate matter (PM) and nitrogen dioxide (NO 2 ) concentrations on COPD morbidity in a periurban community. Methods: Former smokers with COPD were recruited and indoor air was monitored over a 1-week period in the participant's bedroom and main living area at baseline, 3 months, and 6 months. At each visit, participants completed spirometry and questionnaires assessing respiratory symptoms. Exacerbations were assessed by questionnaires administered at clinic visits and monthly telephone calls. Measurements and Main Results: Participants (n ¼ 84) had moderate or severe COPD with a mean FEV 1 of 48.6% predicted. The mean (6 SD) indoor PM 2.5 and NO 2 concentrations were 11.4 6 13.3 µg/m 3 and 10.8 6 10.6 ppb in the bedroom, and 12.2 6 12.2 µg/m 3 and 12.2 6 11.8 ppb in the main living area. Increases in PM 2.5 concentrations in the main living area were associated with increases in respiratory symptoms, rescue medication use, and risk of severe COPD exacerbations. Increases in NO 2 concentrations in the main living area were independently associated with worse dyspnea. Increases in bedroom NO 2 concentrations were associated with increases in nocturnal symptoms and risk of severe COPD exacerbations. Conclusions: Indoor pollutant exposure, including PM 2.5 and NO 2 , was associated with increased respiratory symptoms and risk of COPD exacerbation. Future investigations should include intervention studies that optimize indoor air quality as a novel therapeutic approach to improving COPD health outcomes.Keywords: indoor air; chronic obstructive pulmonary disease; particulate matter; nitrogen dioxide; exacerbations Chronic obstructive pulmonary disease (COPD), the third leading cause of death in the United States and the fifth leading cause worldwide, is expected to become increasingly prevalent in upcoming decades (1, 2). Most COPD is caused by environmental exposures; in developed countries, this exposure is primarily cigarette smoke. After COPD begins, evidence indicates that it can be worsened by other environmental exposures. For example, outdoor particulate matter (PM) concentrations have been associated with an increase in COPD hospitalizations and mortality (3, 4). Similarly, outdoor nitrogen dioxide (NO 2 ) exposure has been linked to worse COPD morbidity, including higher rates of exacerbations (4-6).Although substantial evidence shows that outdoor air pollutants impact COPD, there is much less evidence for the impact of indoor air on COPD, especially in developed countries. Although the Global Initiative for Chronic Obstructive Lung Disease guidelines identify indoor air pollution resulting from burning wood and other biomass fuels as a major risk factor for COPD (7), exposures under these conditions are two to three orders of magnitude higher ...