Even years after quitting, former smokers still have a raised risk of lung cancer - and now scientists believe they know why.
Smoking appears to permanently alter the activity of key genes, even though most cigarette damage is repaired over time.
Canadian researchers, writing in the journal BMC Genomics, looked at lung tissue of 24 people.
UK experts stressed that giving up still delivers massive health benefits.
It has been shown that the poisons in cigarette smoke can alter the activity of genes.
If you give up smoking, your risk of lung cancer falls significantly, but former smokers continue to have a slightly higher risk of lung cancer compared with someone who has never smoked.
"We know that giving up smoking massively reduces your chances of developing lung cancer"
The latest study from the British Columbia Cancer Research Centre in Vancouver suggests that some of these changes might be permanent.
They studied cell samples from the lungs of eight current smokers, 12 former smokers and four people who had never smoked.
Some gene changes appeared to be relatively short-lived, reversing after they had quit the habit for a year or more.
However, a small group of changes were more persistent, and some of these are thought to be involved in cancer susceptibility.
Self-repair
In particular three genes linked to the body's ability to repair DNA had reduced activity levels.
Raj Chari, who led the research, said: "Those genes and functions which do not revert to normal levels upon smoking cessation may provide insight into why former smokers still maintain a risk of developing lung cancer."
A spokesman for the pressure group Action on Smoking and Health (ASH) said that it was important for current smokers not to be put off trying to quit by the thought that genetic damage was irreversible.
She said: "We know that giving up smoking massively reduces your chances of developing lung cancer, and not only that, but your chances of heart disease and a number of other serious illnesses are also significantly reduced.
"Although former smokers do still have a slightly increased risk of lung cancer compared with someone who has never smoked, it is nowhere near as high as the risk of lung cancer to someone who is a current smoker."
Professor Stephen Spiro, from the British Lung Foundation, said: "This research involves a small number of people but nevertheless it adds to our understanding of how smoking can cause permanent genetic damage and helps to explain why some smokers remain at risk of lung cancer even after quitting."
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