Global associations between air pollutants and Chronic Obstructive Pulmonary Disease (COPD) exacerbations: a systematic review - Moore, EA; Chatzidiakou, L; Kuku, M-O; Jones, RL; Smeeth, L; Beevers, S; Kelly, FJ; Barratt, B; Quint, JK (European Respiratory Society, 2016)