IMPORTANCE While air pollutants at historical levels have been associated with cardiovascular and respiratory diseases, it is not known whether exposure to contemporary air pollutant concentrations is associated with progression of emphysema. OBJECTIVE To assess the longitudinal association of ambient ozone (O 3), fine particulate matter (PM 2.5), oxides of nitrogen (NO x), and black carbon exposure with change in percent emphysema assessed via computed tomographic (CT) imaging and lung function. DESIGN, SETTING, AND PARTICIPANTS This cohort study included participants from the Multi-Ethnic Study of Atherosclerosis (MESA) Air and Lung Studies conducted in 6 metropolitan regions of the United States, which included 6814 adults aged 45 to 84 years recruited between July 2000 and August 2002, and an additional 257 participants recruited from February 2005 to May 2007, with follow-up through November 2018. EXPOSURES Residence-specific air pollutant concentrations (O 3 , PM 2.5 , NO x , and black carbon) were estimated by validated spatiotemporal models incorporating cohort-specific monitoring, determined from 1999 through the end of follow-up. MAIN OUTCOMES AND MEASURES Percent emphysema, defined as the percent of lung pixels less than −950 Hounsfield units, was assessed up to 5 times per participant via cardiac CT scan (2000-2007) and equivalent regions on lung CT scans (2010-2018). Spirometry was performed up to 3 times per participant (2004-2018). RESULTS Among 7071 study participants (mean [range] age at recruitment, 60 [45-84] years; 3330 [47.1%] were men), 5780 were assigned outdoor residential air pollution concentrations in the year of their baseline examination and during the follow-up period and had at least 1 follow-up CT scan, and 2772 had at least 1 follow-up spirometric assessment, over a median of 10 years. Median percent emphysema was 3% at baseline and increased a mean of 0.58 percentage points per 10 years. Mean ambient concentrations of PM 2.5 and NO x , but not O 3 , decreased substantially during follow-up. Ambient concentrations of O 3 , PM 2.5 , NO x , and black carbon at study baseline were significantly associated with greater increases in percent emphysema per 10 years (O 3 : 0.13 per 3 parts per billion [95% CI, 0.03-0.24]; PM 2.5 : 0.11 per 2 μg/m 3 [95% CI, 0.03-0.19]; NO x : 0.06 per 10 parts per billion [95% CI, 0.01-0.12]; black carbon: 0.10 per 0.2 μg/m 3 [95% CI, 0.01-0.18]). Ambient O 3 and NO x concentrations, but not PM 2.5 concentrations, during follow-up were also significantly associated with greater increases in percent emphysema. Ambient O 3 concentrations, but not other pollutants, at baseline and during follow-up were significantly associated with a greater decline in forced expiratory volume in 1 second per 10 years (baseline: 13.41 mL per 3 parts per billion [95% CI, 0.7-26.1]; follow-up: 18.15 mL per 3 parts per billion [95% CI, 1.59-34.71]). CONCLUSIONS AND RELEVANCE In this cohort study conducted between 2000 and 2018 in 6 US metropolitan regions, long-term exposure to...

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