Indoor/outdoor particulate matter number and mass concentration in modern offices

Indoor/Outdoor (I/O) particulate mass concentration (PM10) and number concentrations were measured online in modern office environments with mechanical ventilation. The measurement took place during June 2014 in a building, which, belongs to the Norwegian Institute for Air Research, in Norway. Particle number size distribution was measured with an SMPS (0.014-0.7 μm) and an APS (0.5-18 μm) instruments, whereas, mass concentration was measured with a Dust-Trak II photometer. Two offices were selected to examine the outdoor contribution of particles and the influence of indoor sources. One office was fully occupied during working hours and the second one unoccupied at all times. The results suggested that human presence during the working hours affected considerably indoor particles in the occupied office both in terms of number and mass concentration compared to the non-working hours conditions. In the absence of any significant indoor source generating new particles (hardcopy devices), the indoor environment was influenced mainly from the presence of people with resuspension activities being the most important source for particle sizes larger than 1 μm. Moreover, indoor particle number and mass concentration was influenced substantially from outdoor sources. Generally, both indoor number and mass concentrations showed temporal fluctuations similar to those observed outdoors, suggesting that particle penetration was significant in both offices. However, low I/O ratio (90th percentile < 0.3 for both offices) indicated the efficient removal of particles from the air filtration system.