Several PSP properties have been shown to be determinants of the biological activity (Section 2.2). Therefore a thorough physico-chemical characterisation of PSPs is very important. However, standardised test protocols for a number of important particle characteristics have not yet been developed yet, e.g. surface reactivity.
If it is assumed that volume of the particles retained in the lung is a relevant dose metric, the density becomes an important factor to be considered, since limit values are usually given in terms of mass concentration. Retained volume but not retained mass is related inversely to the density of the particles. Volume and mass are both related inversely to the density of the particles. It is still an open question which type of density (material density, envelope density, true density) has to be selected when the particles are suspended in biological fluids, particularly the macrophage fluid where particles are engulfed shortly after deposition, and how to correctly determine this density (in vitro).
Is it possible to distinguish between the biological effects of particles transferred to the interstitium and those of particles residing on the alveolar surface and interacting with the macrophages? According to the analysis of Kümpel (2001) translocation to the interstitium seems to be much higher in humans than in rats.
There is a need to consider the uncertainties in setting human equivalent mass concentrations that rely on the correctness of the particle density determination. This is particularly the case for powders composed of nanoscaled building blocks. In most cases these powders exist as agglomerates with variable degree of compactness and it is the envelope density of the respirable, airborne agglomerates that counts for the assessment of the aerosol volume based on the measurement of the aerosol mass, both, in inhalation experiments as well as for workplace measurements.