In order to meet the national standard and public needs for Seoul Metro subway, air purification has been taken into a consistent consideration with the addition of high quality control facilities. This work focuses on the review of the cost effectiveness to improve the air filtration of an MVAC (mechanical ventilation & air conditioning) in a field subway. The inflow PM10 and PM2.5 into the test MVAC chamber were 19.4 kg/yr and 8.9 kg/yr respectively. It corresponds to the environmental cost of 5,857,000 won for PM10 and 6,608,000 won for PM2.5 every year. While the current filtration system collects approximately 55% of the inlet PM10 and 35% of PM2.5, a new lay-out showed 80% for PM10 and 55% for PM2.5. The new design presented the environmental benefit for PM10 with 1,464,000 won, whilst the additional cost required 3,192,744 won. In the future, however, the increase of environmental benefits for the indoor dust over the atmospheric aerosols will more or less compensate the potential cost. Keywords : Indoor air quality, Filtration, Environmental benefit, Subway, MVAC