Due to the enclosed environment and large quantities of particulates in vehicle exhaust, shopping mall parking garages are a hotspot for exposure to pedestrians and staff. This study conducted two-week continuous field measurements focusing on traffic and pedestrian flows, thermal environment, and particle mass concentrations in a commercial garage in Dalian, China. The relationship between traffic flow and thermal environmental parameters, and its effect on particle mass concentration, was evaluated based on Pearson correlation analysis. The particle mass concentrations both inside and outside the garage, and the relative exposure in the garage compared to the rest of the day, were analyzed. Results demonstrated that hourly averaged PM2.5 mass concentration exceeded China's 35 μg/m3 long-term (one year) and the 70 μg/m3 short-term (24 h) exposure limits by 54.8% and 22.2%, respectively. For 92% of the time, the PM2.5 cumulative dose was higher inside the garage than outdoors. The available natural ventilation was insufficient to prevent unsafe levels of exposure to particulate matter in the garage.