If you start a pack-a-day cigarette habit in January, every cell in your lungs will have 150 genetic mutations by December.
This is alarming because DNA mutations lead to cancer. They can all be passed on to your offspring.
For the first time, researchers have been able to show the average amount of cell mutations a smoker will have depending on how many cigarettes they've smoked in a lifetime.
The study, by researchers at the Wellcome Trust Sanger Institute and the Los Alamos National Laboratory, analysed more than 5,000 tumours and compared cancers from smokers with cancers from people who had never smoked to find the "molecular fingerprints" of smoking-based damage.
They then looked for these fingerprints in the DNA of 'healthy' smokers and found them in droves, especially in the lungs, larynx and mouth cavity.
Los Alamos National Laboratory author Ludmil Alexandrov said it was the missing link in understanding smoking-related cancer.
"Before now, we had a large body of epidemiological evidence linking smoking with cancer, but now we can actually observe and quantify the molecular changes in the DNA due to cigarette smoking," Alexandrov said.
"With this study, we have found that people who smoke a pack a day develop an average of 150 extra mutations in their lungs every year, which explains why smokers have such a higher risk of developing lung cancer."
In other smoking news, researchers at Harvard Medical School and Massachusetts General Hospital found people infected with HIV were more likely to die of a smoking-related illness than the disease itself.
The study found HIV-infected smokers over the age of 40 lived for an average of six fewer years than people with the disease who didn't smoke.