Counterfactual behaviour of ultrafine particles in infiltration and lung deposition processes

摘要

A commonly used justification of concern for ultrafine aka nanoparticles is their high deposition probability in the alveolar region of human respiratory tract. This is often phrased as 'capability of penetrating deep in the lung'. While the higher probability of an ultrafine particle being deposited in the alveolar than other regions of the lung is well established, it is also clear - yet not as widely recognized - that the deposited mass in the alveolar region is almost certainly dominated by larger particles, as the particle masses increase much more rapidly as function of particle size than the deposition probability decreases. However, it is easily forgotten that infiltration process also significantly modifies the ambient particle size distribution due to the particle size driven differences in the penetration and deposition rates indoors. The objective of the current work is to quantitatively compare these processes using a previously developed and evaluated particle size dependent infiltration model and the ICRP (1994) model for respiratory tract uptake. The results show that time spent indoors substantially modifies the uptake of ultrafine particles. Human doses to ambient ultrafine particles are to a very large extent obtained outdoors and in traffic environments, while the PM2.5 uptakes are dominated by indoor environments. As a consequence time-activity variables are likely to be sufficient for qualitatively estimating the impact of ambient ultrafine particles on health in epidemiological settings. Indoors exposure and uptake of ultrafine particles takes place by accumulation mode carriers; i.e. the ultrafine particles are partly attached to larger particles, which in the accumulation mode size range effectively transports them indoors. Indoors mass uptake is dominated by accumulation mode as also coarse (super micron) particles are effectively removed from the breathing air by the building envelope and subsequent indoor deposition.