Can portable air cleaners reconcile conflicting needs for open-door/window autonomy and indoor air quality for occupants in densely populated offices?

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, CO2 and PM2.5 concentrations, and occupant behaviors. We obtained 618 valid subjective responses and conducted a 14-day test to measure possible by-products, such as O3, 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 CO2 concentrations remained below 1000 ppm, the commonly accepted threshold, using PACs in the presence of OV. Second, PACs effectively reduced PM2.5 levels during outdoor PM2.5 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.