Taking samples from the respiratory tracts of 24 smokers, non-smokers and ex-smokers, Canadian researchers from the British Columbia Cancer Agency anaylsed gene activity using a powerful technique called "serial analysis of gene expression" (SAGE).
What they found is not encouraging for ex-puffers who thought they had escaped the dangers of lung cancer, the leading cause of cancer death in the world.
While certain undesirable genes changes triggered by tobacco were reversed, some DNA repair genes were permanently damaged by smoking, and others that have the potential to help combat lung cancer development remained switched off.
"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," noted lead author Raj Chari.
The team of researchers, led by Wan Lam and Stephen Lam, also identified a number of genes not previously associated with smoking that are switched on in active smokers.
CABYR -- a gene that helps sperm swim, and that is associated with brain tumours -- was found, for example, to play a role in bronchial repair.
Looking at genes involved in regeneration of the passageways used for breathing, the study, published in the journal BMC Genomics, identified genes falling into three categories.
Some, such as CABYR and TFF3, which encodes a structural component of mucus, are reversible, while others were only partially reversed.
Others, however, such as GSK3B -- associated in previous studies with lung cancer -- remained in their changed state even long after smokers had quit.
The findings were tested against a second group consisting, as did the first, of eight current, 12 former and four non-smokers, yielding the same results.
In total, more than 3 million SAGE tags representing over 110,000 potentially unique transcripts were generated in the study, forming the largest human SAGE study to date.
Tobacco smoking accounts for 85 percent of lung cancers, and former smokers account for half of those newly diagnoses with the disease.