Early theories about the toxicity of nanoparticles include dependency on surface area, particle size and surface activity. In this case, surface activity [a measure of the ionic state of the surface] was the key variable in relation to lung inflammation.
Evidence from:
DB Warheit et al. Toxicological Sciences (2007) Vol. 95(1) p 270 – 280
“Pulmonary Bioassay Studies with Nanoscale and Fine-Quartz Particles in Rats: Toxicity is Not Dependent upon Particle Size but on Surface Characteristics”
Surface activity can be measured in a standard way, but guidance as to safe or unsafe levels is not available.
Further detail: