Commuter health risk and the protective effect of three typical metro environmental control systems in Beijing, China

The Beijing metro system consists of eighteen lines, and approximately 10 million commuters take the metro daily. Despite the presence of three different environmental control systems, the metro's air quality has caused widespread concern. This study investigated the particulate matter (PM) concentrations in the metro system, and assessed the protective effect of the three environmental control systems. The study evaluated the ratio of PM2.5 to PM10 to assess the relative proportion of PM2.5 and to investigate the influence of PMx on commuter health risk, using settlement theory and a comprehensive indoor air-quality index (CIAI), respectively. The average PM2.5 and PM10 concentrations on different system ranged from 25.4 to 145.5 (PM2.5) and from 26.2 to 165.0 μg/m3 (PM10). Moreover, the values of PM2.5/PM10 are greater than 83.5%; the PM2.5/PM10 ratio in the air was lower on the platform than inside the metro carriage. PM2.5 and PM10 concentrations are significantly correlated. PM concentrations inside carriages and on the platforms increased when the doors were open and when the metro approached. The CIAI values of PM2.5 were at levels classified as “unhealthy for sensitive groups,” exposing children and people with disabilities to health risks. The variation rate of PM2.5 and PM10 levels (1.94 and 6.24%) was lower when platform screen doors were present. Metro system air quality on the platform can be improved by installing platform screen doors to prevent dirty air from the tunnel from entering the platform and adjusting ventilation system fan speeds based on outdoor air quality.