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

Indoor and outdoor particle concentrations and ventilation rates were measured versus time ina large office building without tobacco smoking. Periodically, high efficiency filters replacedthe normal filters in air handling systems. For all particle sizes, indoor concentrations variedconsiderably with time. Even with the normal air filters, which have a low efficiency forsubmicron particles, number concentrations of submicron particles were a factor of three tosix smaller indoors compared to outdoors. The high efficiency filters reduced the indoor-outdoorparticle concentration ratio for submicron particles by 70% to 95%. For largerparticles, the decreases in indoor concentrations were substantially smaller. Comparisons ofmodel predictions with measured data indicate a large rate of removal of submicron indoorparticles by some process other than ventilation or air filtration, and also provide evidence ofsignificant indoor generation or resuspension of particles larger than 1 micrometer.