PM_2.5 and Trace Elements in Underground Shopping Districts in the Seoul Metropolitan Area, Korea

We measured PM_2.5 in 41 underground shopping districts (USDs) in the Seoul metropolitan area from June to November 2017, and associated 18 trace elements to determine the sources and assess the respiratory risks. The PM_2.5 concentrations were 18.0 ± 8.0 μg/m³ inside USDs, which were lower than 25.2 ± 10.6 μg/m³ outside. We identified five sources such as indoor miscellanea, soil dust, vehicle exhaust/cooking, coal combustion, and road/subway dust, using factor analysis. Almost 67% of the total trace element concentration resulted from soil dust. Soil dust contribution increased with the number of stores because of fugitive dust emissions due to an increase in passers-by. Vehicle exhaust/cooking contribution was higher when the entrances of the USDs were closed, whereas coal combustion contribution was higher when the entrances of the USDs were open. Although miscellanea and coal combustion contributions were 3.4% and 0.7%, respectively, among five elements with cancer risk, Cr and Ni were included in miscellanea, and Pb, Cd, and As were included in coal combustion. The excess cancer risk (ECR) was the highest at 67 × 10^-6 for Cr, and the ECR for Pb was lower than 10^-6, a goal of the United States Environmental Protection Agency for hazardous air pollutants.