Submicron particle number doses in the human respiratory tract: implications for urban traffic and background environments.

The deposition of ambient submicron particles in the different parts of the human respiratory tract (HRT) was, for the first time, estimated for males and females from different age classes (children-adults-seniors) of urban population in the city of Thessaloniki, northern Greece, during the cold and the warm period of the year. Outdoor daily and hourly particle number doses in the different regions of the HRT, i.e., the extra-thoracic (ET), tracheobronchial (TB), and the acinar (AC) regions, were calculated by employing the Multiple-Path Particle Dosimetry (MPPD) model. Because of the absence of information being available for the hygroscopic properties of particles, three different particle hygroscopicity scenarios were considered: (i) non-hygroscopic (i.e., raw model estimations), (ii) nearly hydrophobic, and (iii) hygroscopic particles. When hygroscopic properties were considered, we found a remarkable reduction (up to ~ 55%) in the estimated total particle number doses in comparison to the non-hygroscopic particle scenario. Furthermore, we found that the size distribution pattern of the particle doses within the different parts of the HRT was strongly affected by particles' hygroscopic properties with the non-hygroscopic particle scenario significantly overestimating the particle doses in the sub-100-nm range, while underestimating the doses of larger particles. On the contrary, the deposition density appeared to be negligibly affected by the particles' hygroscopic properties, implying the existence of a possible threshold in the number of particles deposited per airway surface area. Similarly, the lobar particle number deposition fraction was unaffected by the hygroscopic properties of particles, as well as the ambient particle size distribution and the individuals' physiological parameters. The total particle number deposition doses estimated here are within the range of the corresponding values reported for other urban environments. It is hoped that our findings could contribute to better understanding of submicron particle exposure and add to the development of more sufficient methods to evaluate the related health impacts.