Most city trees in Nanjing are Platanus acerifolia and Populus nigra, which generate many whirling willow catkins in the air during the transitional season, yet little attention has been paid to the health risks, including itchy skin and respiratory infections, on occupants of roadside buildings. Since the air quality of these indoor spaces cannot meet WHO guidelines during the transitional season due to the influence of plant pollutants, a suitable ventilation scheme, together with air filtration measures, is urgently needed. Hence, four ventilation schemes were compared: natural ventilation, no ventilation with the door and windows closed, recirculating ventilation with a ceiling cassette fan-coil unit but no air filter, and recirculating ventilation with a ceiling cassette fan-coil unit and F7 filter. The performance of these modes was evaluated by comparing the effects of outdoor particulate matter on the indoor air quality. The results showed that the larger the particle size, the lower the I/O ratio. Furthermore, the influence of the occupants’ activities on indoor particle concentrations cannot be ignored, particularly for large particles, which varied more than small particles according to indoor human activity. Therefore, we suggest operating the ceiling cassette fan-coil unit with an air filter for this application, which can reduce the indoor particle concentration to an acceptable level and decrease the potential health risk posed by plant pollutants.