Background Carbon dioxide (CO₂) is a main component of gaseous pollutants in metro carriages. The high concentration of CO₂ in carriages not only affects comfortability, but also easily tends to cause fatigue, dizziness, and even breathing difficulties for passengers. With the continuous increase of metro operating mileage and the continuous extension of time passengers spend in carriages, the concentration of CO₂ in carriages deserves attention.

Objective This study investigates the concentration of CO₂ in metro carriages in a city and put forward corresponding control measures.

Methods From September to November 2017, the concentration of CO₂ were monitored in carriages of seven metro lines (A-G) with a daily passenger flow of more than 500 000 in the selected city in rush hours (7:00-9:30 or 17:00-19:00) and non-rush hours (9:30-17:00) of working days. Each monitoring sample set covered an actual continuous one-way end-to-end running time of a sampled train. Each CO₂ concentration at monitoring sites was recorded before carriage door open after arriving at a stop. The instantaneous concentration and hourly average concentration of CO₂ were calculated and compared with the national standard Hygienic indicators and limits for public places (GB 37488-2019) (CO₂ instantaneous concentration, ≤ 0.15%) and Hong Kong's Practice Note for Managing Air Quality in Air-conditioned Public Transport Facilities-Railways (Practice Note, thereafter; CO₂ hourly average concentration level 1, ≤ 0.25%), and then the sources of differences were analyzed and relevant policy suggestions were proposed.

Results The median of CO₂ instantaneous concentration in carriages of selected seven metro lines was 0.132%, and the range was 0.059%-0.295%; the median of CO₂ hourly average concentration was 0.152%, and the range was 0.088%-0.204%. More than half (63.50%) of the samples met the requirements for CO₂ instantaneous concentration stipulated by GB 37488-2019, and all samples met the requirement for CO₂ hourly average concentration level 1 specified by Practice Note. Except the instantaneous concentrations of CO₂ in rush hours of line A and line B and the hourly average concentration of CO₂ in rush hours of line C, the medians of instantaneous concentration and hourly average concentration of CO₂ in rush hours of each line were greater than 0.15%. Except the CO₂ hourly average concentration of line F, the CO₂ instantaneous concentration and hourly average concentration in rush hours of each line were higher than those in non-rush hours (P < 0.05). In rush hours, 44.0% of the samples met the requirements for CO₂ instantaneous concentration by GB 37488-2019; in non-rush hours, 93.0% of the samples met the requirements. In rush and non-rush hours, all samples met the requirements for CO₂ hourly average concentration level 1 by Practice Note. The median of CO₂ instantaneous concentration in carriages running underground (0.135%) was higher than that in carriages running on the ground (0.117%) (P < 0.05).

Conclusion At present, the concentration of CO₂ in carriages in the selected city does not fully meet the requirements of GB 37488-2019, but fully meet the requirements of Practice Note level 1. Both passenger flow and metro running environment affect the concentrations of CO₂ in carriages; therefore, it is suggested to take such CO₂ concentration control measures as peak passenger flow restriction, adjustment of ventilation capacity of carriages and tunnels, and installation of online monitoring and linkage control devices.