A new study helps explain how tobacco smoke causes changes to DNA. Reuters reports researchers have found tobacco smoke changes a chemical code on DNA, which can sometimes alter gene activity.

Some of the DNA changes reverse when a person quits smoking, but others do not, the researchers report in the journal Circulation: Cardiovascular Genetics.

“Many people think that after five years your health is mostly back to that of a nonsmoker, but that may not be the case,” lead study author Roby Joehanes of Beth Israel Deaconess Medical Center and Harvard Medical School in Boston told Reuters.