Experimental studies on indoor fine particles were performed in actual commercial offices in six Chinese cities: Beijing, Shanghai, Guangzhou, Tianjin, Shijiazhuang and Chengdu. Particle introduced through outdoor air supply (OA) system was the most significant contributor to indoor PM2.5. In most cases, the OA systems of the six office buildings could not provide enough protection for staff. The first reason was the insufficient filtration efficiency of the OA system for PM2.5 due to filter bypass and design shortage. The tested filtration efficiency of the OA system for PM2.5 ranged from 17.0 +/- 0.9% to 55.7 +/- 2.8%. The gaps between the tested and rated filtration efficiencies ranged from 4.4% to 26.3%. The gaps between the rated and required filtration efficiencies ranged from 7.4% to 64.5%. The second reason was the superfluous outdoor air supply rate. The outdoor air supply rates per staff could reach 7.5 L/s to 12.8 L/s with the operating OA systems, much higher than the ASHRAE guideline. Five control strategies (including higher level filters, minimum outdoor air supply rates, portable air cleaners and combinations of these options) were proposed for HVAC system to reduce indoor PM2.5 concentration level to 35 mu g/m(3). The reduced indoor PM2.5 exposure per power under strategy 2 (minimum outdoor air supply rate + higher level filters) was the highest in all cases. The total energy consumption of HVAC systems under strategy 2 was only 42%-71% of the current energy consumption. (C) 2017 Elsevier B.V. All rights reserved.