The purpose of this study was to investigate the effects of size and phase composition on human exposure to airborne titanium dioxide (TiO(2)) nanoparticles (NPs) at workplaces. We reanalyzed published data of particle size distribution of airborne TiO(2) NPs during manufacturing activities and linked a physiologically based lung model to estimate size- and phase-specific TiO(2) NP burdens in target lung cells. We also adopted a cell model to simulate the exposure time-dependent size/phase-specific cell uptake of TiO(2) NPs in human dermal and lung cells. Combining laboratory, field, and modeling results, we proposed two major findings: (i) the estimated median effective anatase TiO(2) NP concentration (EC50) for cytotoxicity response on human dermal fibroblasts was estimated to be 24.84 (95% CI: 7.3-70.2) nmolmL(-1) and EC50 estimate for inflammatory response on human lung epithelial cells was 5414 (95% CI: 3370-7479) nmolmL(-1) and (ii) packers and surface treatment workers at the TiO(2) NP production workplaces are unlikely to pose substantial risk on lung inflammatory response. Nevertheless, our findings point out that TiO(2) NP production workers have significant risk on cytotoxicity response at relatively high airborne anatase TiO(2) NP concentrations at size range 10-30nm.