Smoking damages DNA in clear patterns, researchers reported Tuesday. Most of the damage fades over time, they found — but not all of it.

Their study of 16,000 people found that while most of the disease-causing genetic footprints left by smoking fade after five years if people quit, some appear to stay there forever.

The marks are made in a process called methylation, which is an alteration of DNA that can inactivate a gene or change how it functions.

“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,” said Roby Joehanes of Hebrew SeniorLife and Harvard Medical School.