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Conventional wisdom advises keeping windows and doors closed in air-conditioned or mechanically-ventilated space. However, this guideline often proves challenging in high-occupancy offices, where the constant influx and egress of individuals, coupled with a strong inclination toward open windows, trigger occupant-induced ventilation (OV) that can affect indoor air quality (IAQ) and energy consumption. To address this, we explored using portable air cleaners (PACs) as a cost-effective ventilation strategy to replace mechanical ventilation (MV) while maintaining IAQ and respecting occupant freedom. We conducted a single-blind trial in a 100 m2 office in Shanghai from December 2020 to March 2021, measuring 857 h of indoor and outdoor temperatures, CO 2 and PM 2.5 concentrations, and occupant behaviors. We obtained 618 valid subjective responses and conducted a 14-day test to measure possible by-products, such as O 3 , formaldehyde, and VOCs, during the operation of PACs. The results showed that, first, although MV achieved 2–3 times higher air change rates than PACs, indoor CO 2 concentrations remained below 1000 ppm, the commonly accepted threshold, using PACs in the presence of OV. Second, PACs effectively reduced PM 2.5 levels during outdoor PM 2.5 peaks. Third, occupants' perceived IAQ did not differ between the use of PACs and MV (p >0.05), although individuals with PACs in the afternoon reported slightly increased sleepiness (p <0.05). Fourth, in the presence of OV, PACs demonstrated significant energy-saving potential in improving IAQ. Our study showed PACs can be a cost-effective method to replace MV while maintaining IAQ and preserving occupant freedom. • Deploy portable air cleaners (PACs), enabling window and door opening by occupants. • A randomized, single-blind trial was conducted for two winters in an open office. • PACs can control indoor PM 2.5 in the presence of occupant-induced ventilation (OV). • Indoor CO 2 concentrations remained below 1000 ppm using PACs due to OV. • Measurements confirmed low risks associated with by-products of PACs. [ABSTRACT FROM AUTHOR]