Recent research has found that smoking tobacco causes permanent damage to your DNA even years after quitting. Most of these affected genes have correlations with smoking-related illnesses like heart disease and cancer.

Doctors and medical researchers alike have had a rough understanding on how smoking tobacco can cause diseases like heart disease or cancer. Stress induced from the act of smoking, as well as by the toxins ingested, causes cells to place methyl groups on strands of DNA where they shouldn’t be. This type of damage is mutagenic and can lead to cancer. Research published in the journal Circulation: Cardiovascular Genetics has found that, while most of the damage repairs itself after quitting, some of this damage persists long-term.

The research used saved blood samples from almost 16,000 people that had taken part in studies going back to 1971. They used saved data on whether they had ever smoked, or if they had quit, and for how long. Smoking-associated DNA methylation occurred in 7,000 genes, nearly one third of the entire human genome.

“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,” said co-author on the study Dr. Stephanie London. “Equally important is our finding that even after someone stops smoking, we still see the effects of smoking on their DNA.”

It’s promising to note that the levels of DNA methylation return to almost normal after five years, but some damage persists for life. Damage to DNA puts ex-smokers at an increased risk of heart disease, stroke or cancer.

“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, Ph.D. “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.”

Quitting cigarettes will certainly improve short-term side effects of smoking (such as reduced lung capacity, teeth staining, vulnerability to minor respiratory conditions, slower healing of wounds, etc.), but it will also lower your overall risk to major illnesses like heart disease, stroke and cancer. However, it’s best to never start. Smoking will permanently and irreversibly damage your DNA for life.