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We investigated the effects of air purifiers and ventilation on the particulate matter PM 2.5 concentration of a semi-outdoor bus terminal using computational fluid dynamics (CFD) simulations. As per the numerical simulation results, the air purifiers alone significantly reduced the PM 2.5 concentration of the bus terminal. The effect of mechanical ventilation units on the PM 2.5 reduction is positive without air purifiers but negative with air purifiers. To understand the role of ventilation on the PM 2.5 reduction, a simple theoretical model was derived based on the mass conservation of the PM 2.5. This model considered the effects of traffic-related PM 2.5 emission, air purifiers, and ventilation. The PM 2.5 concentrations predicted by the proposed model showed good correlation with those from the CFD simulations. Using the model, we suggested that the ratio of the mass emission rate of PM 2.5 to the clean air delivery rate of air purifiers is an important value in determining the effect of ventilation. Moreover, when the background PM 2.5 concentration is higher than this value, the ventilation should be limited to reduce the PM 2.5 concentration of the bus terminal. An additional CFD simulation showed that limiting the ventilation using walls can reduce the PM 2.5 concentration of the bus terminal by 28.6% on average and up to a maximum of 44.2%. The present approach may be useful in improving the health of many commuters by designing openings of bus terminal and efficiently operating mechanical ventilation units. • Reduction of PM 2.5 concentration in a semi-outdoor bus terminal. • Combined effects of air purifiers and ventilation. • Strategies for an efficient operation of mechanical ventilation units. [ABSTRACT FROM AUTHOR]