Background The accumulation of various pollutants in confined and crowded metro carriages will deteriorate the air quality and affect passengers' health. Sanitary standards for metro carriages have not been established in China.

Objective The study is designed to investigate the concentration of inhalable particulate matters (PM₁₀) in metro carriages, and to provide evidence for proposing health control strategies and standards or limits for metro system.

Methods In this study, three metro lines with larger passenger flow and longer service years were selected from the 15 metro lines operating in the city by a 20% ratio. The concentration of PM₁₀ in metro carriages of the selected metro lines were continuously monitored during morning rush hours (2 h), evening rush hours (2 h), and non-rush hours (2 h) of working days in three seasons (summer, autumn, and winter) from May to December in 2018. The monitoring points were set at the center of a carriage in the middle and both ends of a train, the distance between two points was 60-80 m, and the height of monitoring points was 1.0-1.5 m from the floor. According to the Examination methods for public places-Part 2:Chemical pollutants, continuous monitoring was carried out with a direct-reading dust monitor that recorded once per minute.

Results The concentrations of PM₁₀ in the selected metro carriages ranged from 0.018-0.868mg·m^-3, and the median (P₂₅, P₇₅) concentration was 0.100 (0.063, 0.135) mg·m^-3. The PM₁₀ concentrations were mainly distributed in the range of 0.000-0.150 mg·m^-3, which accounted for 84.0% of the total results. In terms of seasonal variation, the concentrations of PM₁₀ were significantly different (P < 0.001):the winter concentration of PM₁₀M (P₂₅, P₇₅) was the highest0.134 (0.111, 0.159) mg·m^-3, followed by the concentration in autumn0.101 (0.071, 0.1280 mg·m^-3, and that in summer0.060 (0.048, 0.079) mg·m^-3 was the lowest; more than 32.6% of the winter samples exceeded 0.150 mg·m^-3. In terms of running hour variation, the concentrations of PM₁₀ were significantly different (P < 0.001); the concentration of PM₁₀ during morning rush hoursmedian (P₂₅, P₇₅) was the highest0.109 (0.062, 0.154) mg·m^-3, followed by the concentration in non-rush hours0.100 (0.061, 0.129)mg·m^-3, and that in evening rush hours0.097 (0.064, 0.123)mg·m^-3 was the lowest.

Conclusion Compared with other cities, the concentration of PM₁₀ in metro carriages in the selected city is at an average level. Through the analysis of measured data and comparison with previous studies and references, we propose the limit of PM₁₀ concentration in carriages of urban metro lines at ≤ 0.150 mg·m^-3. At the same time, we recommend taking into considerations increasing new air volume and purification capacity during carriage design to reduce the concentration of PM₁₀ in carriages effectively.