In China, natural ventilation is a common way of improving indoor air quality (IAQ) in college classrooms. However, until now, the effects of both manual airing strategies and architectural factors on IAQ in classrooms have not been well explored. The present work aimed to investigate the effect of manual airing strategies, such as opening doors and opening exterior or interior windows, on the concentrations of both carbon dioxide (CO₂) and fine particulate matter (PM_2.5) in classrooms using field measurements. Through simulation, the effects of floor level, room orientation and the height of interior windows on CO₂ concentration were also analysed. The results of this study revealed that (1) simultaneously opening doors and exterior windows or opening the doors alone could effectively reduce the indoor CO₂ concentration, but the same effect could not be achieved by opening interior windows only; (2) the indoor PM_2.5 concentration was primarily affected by the level of outdoor PM_2.5, and it may exceed the recommended limit by 33% when the outdoor pollution level is high, even with closed doors and windows; and (3) in winter, both floor level and classroom orientation exerted a significant influence on the indoor CO₂ concentration, but the height of interior windows had no effect.