How often does your skin replace itself?

The outer layer turns over in roughly a month. Cells are made at the base, are pushed upward while filling with keratin and shutting down, die, flatten into overlapping plates, and are shed - so what you can actually touch is a layer of dead cells, and a substantial share of household dust is the previous version of you.

That renewal is why superficial injuries leave nothing behind and deeper ones do. Scarring begins when the damage passes into the dermis below, which does not regenerate in the same way; it is patched with collagen laid down in parallel bundles instead of the original basket weave, which is why scar tissue looks different, has no hair or sweat glands, and is weaker.

The dermis is also what does not keep up. Collagen and elastin production falls with age and accumulated ultraviolet damage, which is most of what visible skin ageing is - and comparing a lifetime lorry driver's two cheeks makes the point better than any study. The classic photograph of a man with one side of his face far more lined than the other, from decades of sun through a side window, is a standard teaching image in dermatology.