Occupational exposure and personal exposure to hazardous air pollutants in underground metro stations and factors causing poor indoor air quality

In this comprehensive research study, the occupational exposure levels of subway personnel employed in underground metro stations and personal exposure levels of passengers commuting by metro trains to airborne pollutants were investigated. The factors leading to deteriorating indoor air quality due to greater concentrations of air pollutants were also identified. For this reason, a field study was conducted in Chennai, India during both dry and wet climatic conditions. Indoor air pollutants, particulate matter (PM10, PM2.5, and PM1), total volatile organic compounds (TVOC), carbon dioxide (CO2), carbon monoxide (CO), bacteria, and fungi along with thermal indicators, temperature (T), relative humidity (RH), and air velocity (VA) were monitored in 7 underground metro stations and inside trains. The occupational exposure to PM, CO2, and CO were within the ACGIH-TLV limits, whereas, subway staff's exposure to PM10, PM2.5, TVOC, bacteria, and fungi was found to be 2 – 4 times higher than the recommended health-based safe exposure threshold limits set by WHO and ISHRAE. In contrast, passengers' exposure was 1.5 – 3 times higher than recommended limits for PM2.5, TVOC, and bacteria. An increase in PM10, PM2.5, and TVOC exposure concentrations up to 50 µg/m3, 15 µg/m3, and 30 ppb was observed, respectively in the metro stations due to the continuous operation of trains and the movement of passengers. In subway stations, exposure to fine airborne particles in the range of 0.225 – 0.38 µm was dominant. Higher levels of exposure to PM were observed in the winter season than in summer. Higher levels of exposure to bioaerosols in metro stations during peak hours indicated the impact of increased passenger activity. Also, in metro platforms, T, RH, and VA exceeded the acceptable limits stipulated by ASHRAE. Thermal indicators were found to be a significant factor impacting concentrations of PM, TVOC, and bioaerosols. Finally, risk assessment indicated that 3 y (based on an 8 h shift for subway personnel) and 6 y (based on a 2 h commute for subway passengers) of continuous exposure to noxious air pollutants, PM10, PM2.5, TVOC, bacteria, and fungi would increase the likelihood of developing a lifetime average cancer risk.