Investigation of Indoor Air Quality and Passenger’s Exposure in Underground Rapid Transit System

A Metro system, also known as a subway/rapid transit system (RTS), is a high-capacity urban transport facility. The present study investigated indoor air quality and passengers’ exposure to PM, TVOC, CO2, and bacteria. Monitoring was conducted from Mar-Jun 2019. Four underground metro stations were selected. Data were collected for seven successive days in each station and inside trains. PM, TVOC, and bacteria concentrations were 2–5 folds the regulatory limits specified by WHO and EPA. In metro stations, the concentrations of PM10 and PM2.5 were in the range of 22.55 ± 3.21 to 74.23 ± 8.87 and 16.94 ± 1.89 to 49.39 ± 7.41 µg/m3, respectively. The concentrations of TVOC surpassed the regulatory limit in all stations with the range of 210 ± 41 to 614 ± 37 ppb. The maximum concentration of CO2 was 567 ± 32 ppm. Further, the concentrations of bacteria were substantially high in all stations in the range of 1487 ± 951 to 2573 ± 691 CFU/m3. Moreover, the personal exposure of passengers to air pollutants was 5–9 folds in underground and 2–3 folds in aboveground stations than in-train exposure. Passenger activities and infiltration were significant contributors to elevated pollutant concentrations. The lower concentrations of CO2 indicate a good amount of air circulation, while critical concentrations of other air pollutants indicate filthy air.