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Air quality inside transportation carriages has become a public concern. A comprehensive measurement campaign was conducted to examine the commuters’ exposure to PM 2.5 (d p ⩽ 2.5 μm) and CO 2 in Shanghai metro system under different conditions. The PM 2.5 and CO 2 concentrations inside all the measured metro lines were observed at 84 ± 42 μg/m 3 and 1253.1 ± 449.1 ppm, respectively. The factors that determine the in-carriage PM 2.5 and CO 2 concentrations were quantitatively investigated. The metro in-carriage PM 2.5 concentrations were significantly affected by the ventilation systems, out-carriage PM 2.5 concentrations and the passenger numbers. The largest in-carriage PM 2.5 and CO 2 concentrations were observed at 132 μg/m 3 and 1855.0 ppm inside the carriages equipped with the oldest ventilation systems. The average PM 2.5 and CO 2 concentrations increased by 24.14% and 9.93% as the metro was driven from underground to overground. The average in-carriage PM 2.5 concentrations increased by 17.19% and CO 2 concentration decreased by 16.97% as the metro was driven from urban to the suburban area. It was found that PM 2.5 concentration is proportional to the on-board passenger number at a ratio of 0.4 μg/m 3 ·passenger. A mass-balance model was developed to estimate the in-carriage PM 2.5 concentration under different driving conditions. [ABSTRACT FROM AUTHOR]