Table 1.

Specific issues in the toxicity testing of NPs.

Parameter	Specific issues with NPs
Exposure medium	Exposure medium is important because particle parameters are changed by medium composition (agglomeration)
Duration of exposure	Usually too short as NPs are metabolized to lower extent than conventional compounds
Monolayer culture	NPs cross cell layers by diffusion and paracellular transport to lower extent than conventional compounds
Monoculture	Cell uptake differs between phagocytes and non-phagocytes for NPs and less for conventional compounds
Absence of dynamics	NPs get in contact with cells by sedimentation, which does not play a role for conventional compounds
Low cell differentiation	Secretion of mucus hinders permeation of NPs to higher degree than conventional compounds due to the size exclusion effect a

^aSize exclusion means that NPs, due to their size, are sieved though the mucus mesh.