The prefix 'nano' means a very small size. A nanoparticle is a particle between 1 nm and 100 nm across, where 1 nm is one billionth of a metre (1 × 10-9 m). A nanoparticle is made of a few hundred to many thousands of atoms joined together.
An atom is much smaller. A typical atom has a diameter of about 0.1 nm (1 × 10-10 m), and small molecules such as water or oxygen are only a little larger than this. So a nanoparticle is about 10 to 1000 times wider than a single atom, and it is larger than a small molecule.
Particles of dust and pollution are grouped by size. Coarse particles are between 2500 nm and 10 000 nm across. Fine particles are between 100 nm and 2500 nm across. Nanoparticles are between 1 nm and 100 nm across, so they are the smallest group.
