More than a quarter of a middle-aged person's skin may have already made the first steps towards cancer, according to "surprising" findings of a study in the UK.
Analysis of samples from 55- to 73-year-olds found more than 100 DNA mutations linked to cancer in every 1 sq cm of skin, scientists at the Sanger Institute, near Cambridge, said.
They told the BBC that the results were "surprising".
Skin cancer is one of the most common cancers. Ultraviolet-radiation from sunlight bombards our skin and transforms it from healthy to cancerous tissue.
Many of the mutations that culminate in skin cancer are already known, but the team wanted to know when they first started to appear.
The researchers analysed excess skin that had been removed from the eyelids of four patients. They then drilled down deeply into the skin's DNA to discover the very first steps being taken on the journey to cancer.
Dr Peter Campbell, the head of cancer genetics at Sanger, told the BBC News: "The most surprising thing is just the scale, that a quarter to a third of cells had these cancerous mutations is way higher than we'd expect, but these cells are functioning normally."
However, it would take multiple mutations - nobody is sure exactly how many - to culminate in a tumour.
The results, published in the journal Science, did show there were some subtle changes in the way the mildly mutated cells were behaving.
They were growing more quickly than other skin cells.
Dr Campbell said: "It certainly changes my sun worshipping, but I don't think we should be terrified.
"It drives home the message that these mutations accumulate throughout life, and the best prevention is a lifetime of attention to the damage from sun exposure."
The findings may be a warning to people trying to develop new cancer drugs, which often target the changes that make a cancer "unique".
"We have not really had any insight into whether those cancerous changes occur in normal cells as well, a treatment that kills 20-30 per cent of normal cells would potentially be a lot of collateral damage," Dr Campbell said.
