Sizes at this level are measured in nanometres. One nanometre (nm) is 1 × 10-9 m, a billionth of a metre. An atom is about 0.1 nm across, and small molecules are around 1 nm or less.
Nanoparticles are between 1 and 100 nm across. They are therefore bigger than a single atom or a small molecule, but far smaller than the particles you can see in dust. Fine particles are between 100 and 2500 nm across. Coarse particles, such as dust, are between 2500 and 10 000 nm across, so they are larger than fine particles.
To compare sizes, divide one diameter by the other. A nanoparticle 10 nm across is 10 ÷ 0.1 = 100 times wider than an atom. The largest nanoparticle, at 100 nm, is 1000 times wider than an atom. In order of size from smallest to largest the sequence is atom, nanoparticles, fine particles, then coarse particles.
