Air quality in underground metro station commuter platforms in Singapore: A cross-sectional analysis of factors influencing commuter exposure levels

The metro is a major component of public transport systems in urbanized cities and may expose commuters to indoor air pollutants. We examined how commuter exposure to indoor air pollutant levels varied with the characteristics of underground metro stations. We carried out a national study of 41 commuter platforms from 39 randomly selected underground stations in Singapore. We used generalized linear models to assess the variability of air pollutant concentrations with station characteristics. We computed commuter-weighted pollutant-specific station rankings to inform mitigation priority. PM2.5 (mean = 24.1 mu g/m(3)) and PM10 (mean = 26.9 mu g/m(3)) concentrations across commuter platforms were generally lower than those found outdoors while platform CO2 levels (mean = 608.1 ppm) were 1.7 times those outdoors. More stations exceeded the WHO 24-h mean exposure guidelines values for PM2.5 (n = 30, 73%) than for PM10 (n = 4, 10%). PM2.5, PM10 and CO2 levels were lowest in stations on the Circle, North-East and Downtown lines. PM2.5 and PM10 concentrations at platforms were positively associated with their outdoor concentrations. Mean CO2 concentration levels at platforms were the highest during commuter rush hours. Longer train arrival intervals were associated with lower CO2 levels. PM2.5 and PM10 levels in stations were negatively associated with the depth of the commuter platforms. Commuter volumes influenced the priority ranking of mitigation measures. Underground metro stations exhibited high variability and dependence on station characteristics as well as outdoor ambient pollutant levels. Our study findings suggest that any required improvements in ventilation and station design should be focused on PM2.5 exposure reduction and prioritized according to both indoor air pollutant levels and commuter volumes to provide the largest extent of commuter benefits.