The researchers have produced evidence of a different idea. The damage is already there in our cell's DNA, picked up as we grow and age, but something needs to pull the trigger that actually makes it cancerous.

The discovery came from exploring why non-smokers get lung cancer. The overwhelming majority of lung cancers are caused by smoking but still, one in 10 cases in the UK is down to air pollution.

The Crick scientists focused on a form of pollution called particulate matter 2.5 (known as PM2.5), which is far smaller than the diameter of a human hair.

Through a series of detailed human and animal experiments they showed:

Places with higher levels of air pollution had more lung cancers not caused by smoking

Breathing in PM2.5 leads to the release of a chemical alarm - interleukin-1-beta - in the lungs

This causes inflammation and activates cells in the lungs to help repair any damage

But around one in every 600,000 cells in the lungs of a 50-year-old already contains potentially cancerous mutations

These are acquired as we age but appear completely healthy until they are activated by the chemical alarm and become cancerous