Particle concentrations in air-conditioned office building with normal and high efficiency filtration

Indoor and outdoor particle concentrations and ventilation rates were measured versus time in a large office building without tobacco smoking. Periodically, high efficiency filters replaced the normal filters in air handling systems. For all particle sizes, indoor concentrations varied considerably with time. Even with the normal air filters, which have a low efficiency for submicron particles, number concentrations of submicron particles were a factor of three to six smaller indoors compared to outdoors. The high efficiency filters reduced the indooroutdoor particle concentration ratio for submicron particles by 70% to 95%. For larger particles, the decreases in indoor concentrations were substantially smaller. Comparisons of model predictions with measured data indicate a large rate of removal of submicron indoor particles by some process other than ventilation or air filtration, and also provide evidence of significant indoor generation or resuspension of particles larger than 111m.