What is nanoparticle concentration?

Nanoparticles are classified into three categories: natural, “anthropogenic” and manufactured nanoparticles. In ambient atmospheres (office, housing, …), the average concentration of nanoparticles is in the region of 103 to 5x104 part.cm-3 (for particles having a median diameter less than 2x102 nm). But these values vary widely in time and space.