BBC News: 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.
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.
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