Researchers say they have cracked how air pollution leads to cancer, in a discovery that completely transforms our understanding of how tumours arise.

The team at the Francis Crick Institute in London showed that rather than causing damage, air pollution was waking up old damaged cells.

One of the world's leading experts, Prof Charles Swanton, said the breakthrough marked a "new era".

And it may now be possible to develop drugs that stop cancers forming.

The findings could explain how hundreds of cancer-causing substances act on the body.

The classical view of cancer starts with a healthy cell. It acquires more and more mutations in its genetic code, or DNA, until it reaches a tipping point. Then it becomes a cancer and grows uncontrollably.

But there are problems with this idea: cancerous mutations are found in seemingly healthy tissue, and many substances known to cause cancer - including air pollution - don't seem to damage people's DNA.

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