Chronic exposure to cigarette smoke can change lung cells over time, making them more vulnerable to disease and priming them to develop cancer, US researchers said Monday.

The report in the journal Cancer Cell is based on lab experiments on lung cells that were exposed to chronic cigarette smoke -- the equivalent of a person smoking for 20 to 30 years.

After about 10 days, the cells began to change their gene expression, a process known as epigenetic change.

It took 10 months before these changes built up enough to boost the odds of cancer.

"When you're smoking, you are building up a substrate of epigenetic changes that we hypothesize are increasing your mathematics for developing lung cancer," said senior author Stephen Baylin, co-director of the Cancer Biology program Johns Hopkins University.

"Because if you're not a smoker, your risk of lung cancer is very low."

These epigenetic abnormalities essentially turn off multiple genes which are needed to help protect normal cells from developing cancer.

Epigenetic changes do not alter, or mutate, the basic DNA sequence of the gene, suggesting that there is hope for people who want to quit smoking.

"This work suggests the possibility that unlike mutations, which are harder to reverse, if you stop smoking at a certain time and duration, then you have a chance to decrease your mathematics that might be due to the buildup of epigenetic changes," said first author Michelle Vaz, a postdoctoral researcher at Johns Hopkins University School of Medicine. 

"The hypothesis is that there are potentially reversible changes that are contributing to a certain set of lung cancers."