Most of the subway stations are located underground and the indoor air quality can be very poor. IAQ tele-monitoring systems (TMS) have been installed at some of subway stations in Seoul to monitor indoor contaminants, such as carbon dioxide, particulate matters and nitrogen oxides. In this paper, we use CO₂, PM₁₀, NO₂ concentration data collected by TMS in one of the underground stations in Line 4. The correlations are analyzed between the concentrations measured at different locations, such as tunnel, waiting room, and platform to identify the source characteristics. The results indicate there are very weak correlations between CO₂-NO₂ and PM₁₀-NO₂ but strong correlation between CO₂-PM₁₀, because both of PM₁₀ and CO₂ are related to the number of passengers. The comparison of PM₁₀ concentrations before and after the installation of platform screen door (PSD) indicates that considerable amount of particulate matters are entrained from tunnel into platform area. The PSD exhibits positive effects on indoor air quality especially on particulate concentrations. In case of NO₂, there is a strong correlation between indoor and outdoor concentrations. The results presented in this paper can be used to control indoor air quality in subway stations more effectively.