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Association between PM2.5 pollution and outpatient visits for respiratory diseases in China: a systematic review and meta-analysis

Review question / Objective: Previous epidemiological studies on the association between PM2.5 pollution and outpatient visits for respiratory diseases in China were mostly limited to one region， and the different papers have no coherent results. Our objective is to perform a systematic review and meta-analysis of the relevant literature in order to summarize the association between PM2.5 pollution and outpatient visits for respiratory diseases in multiple cities in China. Condition being studied: As an important component of air pollutants, particulate matter 2.5 (PM2.5) can float in the atmosphere for a long time with a small aerodynamic size (≤2.5μm) and large specific surface area which is attached to a variety of toxic and harmful substances . PM2.5 can deposite under the trachea of the respiratory tract, reaching deep into the alveolar area, damaging alveolar macrophages and type Ⅱ alveolar epithelial cells, inducing alveolar inflammation, resulting in decreased immunity of the respiratory tract and interfering with normal physiological functions of the lungs.