This study developed a drift-flux model for particle movements in turbulent indoor airflows. To account for the process of particle deposition at solid boundaries in the numerical model, a semi-empirical deposition model was adopted in which the sizedependent deposition characteristics were well resolved. After validation against the experimental data, the drift-flux model was used to investigate human exposures to particles in three normally-used ventilation types: mixing ventilation (MV), displacement ventilation (DV), and under-floor air distribution (UFAD). The movements of submicron particles were like tracer gases while the gravitational settling effect should be taken into account for particles larger than 2.5 um. For particles released from an internal heat source, the concentration stratification of small particles (diameter <10 um) in the vertical direction appeared in DV and UFAD. It was found the advantageous principle for gaseous pollutants that a relatively lesspolluted occupied zone existed in DV and UFAD was also applicable to small particles.