Characteristics of indoor air quality in underground metro stations: A critical review

This research paper presents a comprehensive review of different types of air pollutants, their concentration levels, sources contributing to air pollution, health impacts of pollutants, characteristics of air pollutants, influencing factors such as thermal characteristics and subway geometry, and Indoor Air Quality (IAQ) management in underground (UG) Metro stations. In the current study, actual levels of air pollutants were compared with guidelines specified by regulatory agencies to assess the risk of high exposure to toxic levels of air pollutants. It was found that exposure to particulate matter (PM), volatile organic compounds (VOCs), and bacteria were critical as levels were 2-10 times higher in UG metro stations than the permissible exposure limits specified by WHO, EPA, and ACGIH. Concentrations of aromatic VOCs were noted to be high in urban areas while concentrations of carbonyl VOCs were high in suburban areas. High bacterial concentrations were attributed to increased passenger traffic and saturation of air cleaning filters in HVAC systems. Several factors such as station design, depth of the tunnel, indoor thermal conditions, density of passengers, frequency of trains, station locality, and outdoor environmental conditions were found to significantly affect the IAQ of subway metro stations. On the basis of existing studies as well as the current review study, it is recommended that future research work could focus on the characterization of different gaseous and biological air pollutants, health risk assessment of exposure to various air pollutants, and field studies on the influence of thermal parameters on IAQ perception in UG metro stations.