The metropolitan city of Seoul uses more energy than any other area in South Korea due to its high population density. It also has high emissions of air pollutants. Since an individual usually spends most of his/her working hours indoors, the ambient air quality refers to indoor air quality. In particular, PM 10concentration in the underground areas should be monitored to preserve the health of commuters in the subway system. Seoul Metro and Seoul Metropolitan Rapid Transit Corporation measure several air pollutants regularly. In this study, the accuracy of an instrument for PM measurement using the light scattering method was improved with the help of a linear regression analysis technique to continuously measure the concentrations in subway stations. In addition, an air quality monitoring system based on environmental sensors was implemented to display and record the data of PM 10, CO 2, temperature, and humidity. Through experimental studies, we found that ventilation fans could improve air quality and decrease PM 10concentrations in the tunnels effectively by increasing the air flow rate.