Source of UFP may influence translocation
In both studies to investigate size, solubility and clearance from site of deposition (Oberdörster et al, 2002a; Kreyling et al, 2002), UFP particles were generated by spark discharge. Morfeld et al (2012) have questioned whether only primary nanoparticles or small aggregates/agglomerates generated for research purposes have the ability to translocate; and in the context of real life exposures (formation of microsize aggregates/agglomerates) what is the actual sub distribution of nanoparticles in the lung. For this question, the disintegration of the inhaled microsize aggregates/agglomerates into nanosized primary material was given special attention.
Nanosized TiO2 was tested in an inhalation study which also corrected for transmission electron microscopy (TEM) slicing bias; since particles are perceived as two dimensional surrogates of a three dimensional structure. A model was developed to estimate the expected numbers of particle diameters below 100 nm due to the TEM slicing bias. Comparisons of observed to expected values did not provide evidence in favour of the presence of nanoparticles in the rat lungs after correcting for TEM slicing bias. This indicated that nanostructured TiO2 aggregates/agglomerates do not disintegrate into smaller structures when exposed to fluids similar to the lung surfactant.
In the context of translocation, this suggests that contrary to spark generated UFP, nanostructured TiO2 generated via a different process show a different translocation behaviour. Manufactured nanostructured materials are often present as agglomerates due to van der Waals forces. Dispersing manufactured nanomaterials in a test atmosphere results in agglomerates up to several micrometres with only a minimal fraction present as primary particles. Morfeld et al (2012) indicated that the amount available for translocation is extremely low (below detection limit and about 0.0025% of the total deposition).