The study from the Wellcome Sanger Institute and University College London found that compared to current smokers, people who had quit had more “genetically healthy” lung cells – with the organ growing new healthy cells to replenish the lining of their airways.
The cells in turn were less likely to develop into cancer in the future.
“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,” Dr Peter Campbell, from the Wellcome Sanger Institute and senior author on the study, said.
"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."
Of the 47,000 lung cancer cases reported in the UK every year, some 72 per cent are believed to be the result of smoking – a rate that makes up 21 per cent of total cancer deaths, according to Cancer Research UK.
The study, published in the journal Nature, saw researchers analyse lung biopsies from 16 people – a group that included smokers, ex-smokers, those who had never smoked, and children.
The results showed that nine out of every 10 lung cells in current smokers had up to 10,000 additional genetic mutations as a direct result of tobacco related chemicals compared to non-smokers.
Meanwhile, more than a quarter of those damaged cells had at least one cancer-driver mutation.
But in those who had quit smoking researchers discovered "a sizeable group of cells" lining the airways that had "escaped" genetic damage from cigarettes in the past.
Those cells were found to be on par with those who had never smoked, with ex-smokers retaining four times more healthy cells than their smoking counterparts.
However, researchers warned the risk of permanent damage deeper in the lung that can lead to chronic lung disease still remains.
Dr Kate Gowers, joint first author from UCL, said that even the healthy lung cells that contain thousands of genetic mutations could be "mini time-bombs waiting for the next hit that causes them to progress to cancer".
She added: "Further research with larger numbers of people is needed to understand how cancer develops from these damaged lung cells."
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