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14Particles released from biomass burning can contribute to severe air pollution. We monitored 15 indoor and outdoor particles in a mechanically ventilated and air-conditioned building during 16 and after the 2013 haze event in Singapore. Continuous monitoring of time-and size-resolved 17 particles in the diameter range 0.01-10 µm was conducted for two weeks in each sampling 18 campaign. During the haze event, the averaged size-resolved outdoor particle volume 19 concentrations (dV/d(logD p )) for diameters larger than 0.3 µm were considerably higher than 20 those during the post-haze days (9-185 µm 3 cm -3 versus 1-35 µm 3 cm -3 ). However, the 21 average number concentration of particles with diameters in the range 10-200 nm was 22 substantially lower on the hazy days than on the post-haze days (11,400 to 14,300 particles 23 cm -3 for hazy days, versus an average of 23,700 particles cm -3 on post-haze days). The 24 building mechanical ventilation system, equipped with MERV 7 filters, attenuated the 25 penetration and persistence of outdoor particles into the monitored building. The air conditioning and mechanical ventilation system with MERV 7 filters provided low 31 single-pass removal efficiency (less than ~ 30%) for particles with diameters of 0.01-1.0 µm. 32During the haze, for particles larger than ~ 0.2 µm, lower I/O ratios and higher removal 33 efficiencies occurred with the air conditioning operating as compared to with mechanical 34 ventilation only. This observation suggests the possibility of particle loss to air conditioning 35 system surfaces, possibly enhanced by thermophoretic or diffusiophoretic effects. 36