New evidence reveals that smoking cigarettes can permanently change the human DNA; about 7,000 genes to be exact. These changes can lead to lung and respiratory diseases.

According to lead author Stephanie London of the U.S. National Institute of Environmental Health Sciences, the new study is based on the long-term residual effects of smoking. But the good news is the sooner you stop, the better off you will be.

London and her team reviewed blood samples from about 16,000 people from past studies. They discovered that people who stopped smoking possess genes that have "recovered" after five years of quitting. However, some genetic changes remain, even after 30 years of quitting cigarettes.

In this new study, researchers were able to study a process known as DNA methylation, which can yield no changes on the underlying DNA code but transform their expression.

Million of people are estimated to die every year from smoking-related diseases such as cancer, heart disease, and stroke.

Researchers suggest that even decades after quitting smoking, some people are still at risk of diseases due to DNA methylation.

During tests, scientists compared the DNA methylation sites of current and former smokers along with those of people who never smoked. Apart from detecting smoking-related changes in 7,000 genes (one-third of known human DNA), researchers also discovered sites that remain affected even after years of quitting.

London says that identifying these DNA changes can help to diagnose patients accurately. Further research can also provide new insight the development of potential treatments to repair the genes damaged by smoking.

This new study has been published in the journal Circulation: Cardiovascular Genetics.