Ambient air pollution increases the risk of respiratory mortality but evidence for impacts on lung function and chronic obstructive pulmonary disease (COPD)is less well established. The aim was toevaluatewhether ambient air pollution isassociated with lung function andCOPD, and explore potential vulnerability factors. We used UK Biobank data on 303,887 individuals aged 40-69 years, with complete covariate data and valid lung function measures. Cross-sectional analysesexamined associations ofLand Use Regression-based estimates ofparticulate matter (PM2.5, PM1035and PMcoarse) and nitrogen dioxide (NO2) concentrations withforced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), the FEV1/FVC ratio, and COPD (FEV1/FVC 37< lower limit of normal). Effect modificationwas investigated for sex, age, obesity, smoking status, household income, asthma status, and occupations previously linked to COPD.40Higher exposures to each pollutant weresignificantly associated with lower lung function. A 5 μg/m3increase in PM2.5concentrationwas associated with lower FEV1(-83.13 mL [95%CI: -92.50, -73.75]) and FVC (-62.62 mL [95%CI:-73.91, -51.32]). COPD prevalence was associated with higher concentrations of PM2.5 (OR 1.52 [95%CI: 1.,1.62], per 5 μg/m3),PM10 (OR 1.08 [95%CI: 1.00,1.16], per 5 μg/m3), andNO2(OR 1.12 [95%CI: 1.10, 1.14], per 10 μg/m3), but not with PMcoarse.Stronger lung functionassociations were 46seenfor males, individuals from lower income households,and ‘at-risk’ occupations, and higher COPD associations for obese, lower income,and non-asthmatic participants. Ambient air pollution wasassociated with lowerlung function and increased COPD prevalencein this large study.