Smoking causes a lasting effect on a person’s DNA that remains even after the person quits smoking, a new study reported.

The study, published in the journal Circulation: Cardiovascular Genetics, showed that smoking leaves a “footprint” on a person’s DNA through DNA methylation, a process that affects gene expression. This damage could eventually cause serious smoking-related illnesses proliferate.

The researchers said these findings indicate DNA methylation could give clues as to a person’s smoking history. They could also help researchers develop new therapies.

"These results are important because methylation, as one of the mechanisms of the regulation of gene expression, affects what genes are turned on, which has implications for the development of smoking-related diseases," study author Stephanie J. London and deputy chief of the Epidemiology Branch at the National Institute of Environmental Health Sciences, National Institutes of Health, said.

"Equally important is our finding that even after someone stops smoking, we still see the effects of smoking on their DNA,” she added.

Taking blood samples from almost 16,000 study participants, the researchers analyzed methylation sites in the human genome. They compared these with the methylation sites of people who have never smoked and those who have quit smoking.

The researchers discovered that the DNA methylation sites of individuals who smoked were linked with more than 7,000 genes, which is practically a third of all known human genes. In some instances, these methylation sites remained even when the individuals have stopped smoking for 30 years.

However, most of the methylation sites of those who have quit smoking became similar to those from the group that never smoked.

"Our study has found compelling evidence that smoking has a long-lasting impact on our molecular machinery, an impact that can last more than 30 years," first author Roby Joehanes, instructor at Harvard Medical School, said. "The encouraging news is that once you stop smoking, the majority of DNA methylation signals return to never smoker levels after five years, which means your body is trying to heal itself of the harmful impacts of tobacco smoking."

An important thing to note is that statistically significant methylation sites were associated with genes linked to smoking-related health conditions like cardiovascular diseases. The researchers emphasized the study was not meant to investigate the long-term effects.

Another study, published earlier this year, found that smoking during pregnancy chemically alters the DNA of a fetus. The researchers said patterns similar to those found in adult smokers were seen in the fetal DNA of smoking mothers, suggesting an association between smoking during pregnancy and child health complications.