Smoking is bad for your health, and that includes genes too. This is the conclusion of one of the most comprehensive studies involving the effects of smoking and human DNA — and there are a few.

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The Harvard Medical School researchers tracked DNA modifications in 16,000 current and ex-smokers who had participated in various studies involving smoking, some of which go as far back as 1971. Besides filling questionnaires about smoking, diet, lifestyle and their health histories, the blood of each participant was collected and had its DNA extracted for sequencing.

The results suggest smokers have a pattern of methylation changes that affected more than 7,000 genes. Methylation modifies the function of a gene, either changing the way it functions or by (in)activating it.

“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.

With such a huge number of genes affected by smoking, it’s no wonder that smokers are highly at risk of developing heart disease and cancer, both caused by genetic damage.

Now, if all of this might sound highly concerning for those of you who stop smoking, there is some good news. The researchers say that not all DNA damage is permanent. In fact, most of the damage disappeared in people who had stopped smoking for at least five years. Some genes, including the TIAM2 gene linked to lymphoma, still had changes caused by smoking 30 years later, as reported in the journal Circulation: Cardiovascular Genetics.

[panel style=”panel-warning” title=”You should know” footer=””]Smoking is the most contributor to preventable illnesses, killing 480,000 Americans yearly and roughly six million people worldwide.

While smoking was very popular in the United States, the habit has been kicked by the nation. Only 15 percent of American adults and 11 percent of high school teenagers smoke nowadays. [/panel]

“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,” says Dr. Stephanie J. London, the deputy chief of the Epidemiology Branch at the National Institute of Environmental Health Sciences.

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