Smoking just one pack a day can lead to 150 extra mutations in the lung cells a year. And it does even more damage to other parts of the body, causing huge numbers of mutations throughout other organs.

The study is the first to quantify the degree of damage that smoking does to our DNA. As such, it could help understand more about the kinds of problems that the habits causes, with each of those mutations potentially leading to cancer.

As well as those 150 mutations found in the lungs, scientists found that a pack-a-day habit for a year produced an average 97 mutations in each cell in the larynx (voice box), 39 mutations in the pharynx (top part of the throat), 23 in the mouth, 18 in the bladder and six in the liver. Smoking, which claims the lives of at least six million people worldwide each year, has been linked to at least 17 different types of human cancer.

The disease is triggered by mutations - changes in the genetic programming written in DNA - that can cause cells to become "immortal" and multiply uncontrollably. Cigarettes contain more than 7,000 different chemicals, of which 70 are known to be carcinogenic, the researchers said, pointing to the complexity of how smoke interacts with the body. "This study offers fresh insights into how tobacco smoke causes cancer," said Ludmil Alexandrov of the Los Alamos National Laboratory, one of the study's main co-authors. "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," he added. "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."