Particle Concentrations and Sizes with Normal and High Efficiency Air Filtration in a Sealed Air-Conditioned Office Building

Abstract

During parts of 7 consecutive weeks, indoor and outdoor particle number concentrations and particle sizes were measured versus time in a large sealed air-conditioned office building without tobacco smoking. Building ventilation rates were also measured. During some periods, the normal filters in the building's air handling systems were replaced with high efficiency filters. A mass balance model was used to help interpret the study data. For all particle sizes, indoor number concentrations varied considerably between weeks and within a single work day. Even with the normal air filters, which have a low efficiency for submicron-size particles, indoor number concentrations of submicron particles were a factor of 3-6 smaller than outdoor particle number concentrations. For the range of particle sizes measured (> 0.3 μm), the indoor particle mass concentration was considerably less than outdoor particle mass concentration. The high efficiency filters dramatically reduced the indoor-outdoor particle concentration ratio for submicron particles (i.e., the decrease was 70% to 95%, depending on particle size). For larger particles, the decreases in indoor concentrations were substantially smaller. Comparisons of model predictions with measured data provide evidence of 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 1 μm.