The subway system, also called underground, tube, and the metro is a high-capacity rapid urban transport facility consist of compact space, an airtight environment, and high passenger density per unit area. Due to the convenience of the least travel time, commuter footfalls have increased several folds in the number of metro stations. The ridership data shows that on average about 168 million passengers use the metro network in 182 cities within 56 countries regularly (Union Internationale des Transports Publics). In India, the metro system is relatively new and developing very fast during recent years. Chennai Metro is the third-largest metro network in India. Although subway metro stations have significantly helped to reduce outdoor air pollution; however, new challenges and issues of indoor air quality (IAQ) are likely to be far more severe than outdoor air quality.
Indoor air pollutants namely particulate matter (PM), volatile organic compounds (VOC), carbon dioxide (CO2), carbon monoxide (CO), oxides of nitrogen (NOx), ozone (O3), and bioaerosol accumulates in subway stations. Regular exposure to such hazardous air pollutants can cause serious health impacts. In the present research, IAQ characteristics of PM, gaseous, and biological air pollutants have been studied in Chennai metro stations (metro platforms and inside trains). Thermal characteristics and passengers' perceptions of the metro environment have also been studied. Based on the airborne PM and bioaerosol exposure, the deposition dose in the lungs of passengers and metro workers has been quantified to assess health risk. Results showed that the concentrations of air pollutants were high in metro platforms in comparison to metro trains. Exposure to PM, TVOC, and bacteria concentrations have exceeded up to 5 times the specified regulatory guidelines of WHO, AIHA, and ACGIH respectively. Concentrations of fine airborne particles of size 1 µm were found dominant in both metro platforms and metro trains. Recent studies have indicated the association between fine/ultrafine airborne particles and the spread of the COVID-19 virus and reported the highest association among indoor closed spaces. Subway metro stations feature underground and highly closed environments. Thus, they can be one of the hotspots of viral disease transmission, if an infected passenger travels. In this presentation, types of air pollutants, their characteristics, sources contributing, the influence of thermal parameters and station design, passengers' perception of IAQ, personal exposure, dose deposition, health impacts, and IAQ management in subway metro stations will be discussed.