Nanoparticles sit between atoms and everyday particles

1. A nanoparticle measures between about $1$ and $100$ nanometres across.
2. A single atom is roughly a tenth of a nanometre across.
3. A nanoparticle therefore holds anything from a few tens of atoms to many millions.
4. It is far larger than an atom or a small molecule, though a large molecule such as a protein reaches this size.
5. It is far smaller than anything visible, including a speck of dust.

A large surface area to volume ratio changes behaviour

1. Dividing a solid into smaller pieces raises its surface area to volume ratio.
2. At nanoparticle size, that ratio becomes very large.
3. A large proportion of the atoms therefore sits at the surface.
4. Reactions happen at surfaces, so nanoparticles are far more reactive than the bulk material.
5. A much smaller mass can therefore do the same job.

Uses that depend on that behaviour

1. Sunscreens use nanoparticles of titanium dioxide or zinc oxide.
2. The particles block ultraviolet light while being too small to look white on the skin.
3. An older sunscreen using the bulk powder left a visible white layer.
4. Nanoparticle catalysts work with a much smaller mass of expensive metal.
5. Other uses include self-cleaning surfaces and stronger, lighter materials.

The possible risks

1. Some may be able to pass through the skin, and they are small enough to be breathed deep into the lungs.
2. Their high reactivity may cause effects that the bulk material does not.
3. The long-term effects on health are not yet fully known.
4. Particles washed off into rivers and the sea may affect wildlife.
5. Risk depends on which material the particles are made of, so a result for one says little about another.