This study measured the particle concentrations with an aerodynamic diameter smaller than 2.5 μm (PM2.5), nitrogen dioxide (NO2), and relative humidity (RH) at five metro subway stations in Suzhou's subway system (Lines 1 and 2). The real-time monitoring campaign was conducted from March 30th to April 10th and August 4th to August 21st, 2015. The monitoring practice was carried out during rush (7:00-9:00 AM and 17:00-19:00 PM) and regular hours (other times) at the ground and underground levels under different weather conditions with a purpose of obtaining representative data. The monitored results show that the concentrations of PM2.5 in the train carriages were lower than the concentrations at the underground platforms during both spring and summer. The mean PM2.5 concentrations at all the underground platforms in all the sub-stations monitored were significantly higher than those at the ground level. The human health impact was calculated to be 6300 annual DALYs (or 375 deaths) due to exposure to the subway system in Suzhou according to the UNEP-SETAC toxicity (USEtox) model. Linear regression models were applied to evaluate the relationships between the PM2.5, NO2 concentrations, and RH. We found that a 10% increment in RH from the current average level of 50-60% can lead to a 9.8 μg m-3 concentration decrease in PM2.5. This further results in the total human health impact being reduced to 2451 DALYs (150-4753 DALYs), representing a 20% decrease (1.2-38%).