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10Building activities are recognised to produce coarse particulate matter but less is 11 known about the release of airborne ultrafine particles (UFPs; those below 100 nm 12 in diameter). For the first time, this study has investigated the release of particles 13 in the 5-560 nm range from three simulated building activities: the crushing of 14 concrete cubes, the demolition of old concrete slabs, and the recycling of concrete 15 debris. A fast response differential mobility spectrometer (Cambustion DMS50) 16 was used to measure particle number concentrations (PNC) and size distributions 17(PNDs) at a sampling frequency of 10 Hz in a confined laboratory room providing 18 controlled environment and near-steady background PNCs. The sampling point 19 was intentionally kept close to the test samples so that the release of new UFPs 20 during these simulated processes can be quantified. Tri-modal particle size 21distributions were recorded for all cases, demonstrating different peak diameters 22 in fresh nuclei (<10 nm), nucleation (10-30 nm) and accumulation (30-300 nm) 23 modes for individual activities. The measured background size distributions 24showed modal peaks at about 13 and 49 nm with average background PNCs 25 ~1.4710 4 cm -3 . These background modal peaks shifted towards the larger sizes 26 during the work periods (i.e. actual experiments) and the total PNCs increased 27 between 2 and 17 times over the background PNCs for different activities. After 28 adjusting for background concentrations, the net release of PNCs during cube 29 crushing, slab demolition, and "dry" and "wet" recycling events were measured as 30 ~0.77, 19.1, 22.7 and 1.76 (×10 4 ) cm -3 , respectively. The PNDs were converted 31 into particle mass concentrations (PMCs). While majority of new PNC release 32 was below 100 nm (i.e. UFPs), the bulk of new PMC emissions were constituted 33 by the particles over 100 nm; ~95, 79, 73 and 90% of total PNCs, and ~71, 92, 93 34 and 91% of total PMCs, for cube crushing, slab demolition, dry recycling and wet 35 recycling, respectively. The results of this study firmly elucidate the release of 36UFPs and raise a need for further detailed studies and designing health and safety 37 related exposure guidelines for laboratory workplaces and operational building 38 sites. 39

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