A new study, based on the lungs of ex-smokers, has confirmed that compared to current smokers, people who had stopped smoking had more genetically healthy lung cells, which have a much lower risk of developing into cancer.

The results highlight the benefits of stopping smoking completely, at any age, as it could do much more than just stopping further damage to the lungs. Researchers believe it could also allow new, healthy cells to actively replenish the lining of our airways, a shift which in proportion of healthy to damaged cells could help protect against cancer.

The researchers found that despite not being cancerous, more than nine out of every 10 lung cells in current smokers had up to 10,000 extra genetic changes – mutations – compared with non-smokers, and these mutations were caused directly by the chemicals in tobacco smoke.

Also, in people who had stopped smoking, there was a sizeable group of cells lining the airways that had escaped the genetic damage from their past smoking. Genetically, these cells were on par with those from people who had never smoked: they had much less genetic damage from smoking and would have a low risk of developing into cancer.

Ex-smokers had as many as four times more of these healthy cells than people who still smoked – representing up to 40% of the total lung cells in ex-smokers.

“People who have smoked heavily for 30, 40 or more years often say to me that it’s too late to stop smoking – the damage is already done” commented joint senior author Dr Peter Campbell, from the Wellcome Sanger Institute. “What is so exciting about our study is that it shows that it’s never too late to quit – some of the people in our study had smoked more than 15,000 packs of cigarettes over their life, but within a few years of quitting many of the cells lining their airways showed no evidence of damage from tobacco.”

Lung cancer is the most common cause of cancer death in the UK, accounting for 21% of all cancer deaths. Smoking tobacco damages DNA and hugely increases the risk of lung cancer, with around 72% of the 47,000 annual lung cancer cases in the UK caused by smoking.