Tobacco smoking claims the lives of at least six million people every year.
People who smoke one packet of cigarettes a day may develop as many as 150 extra mutations in in their lungs every year which can eventually lead to cancer, a new study has warned.
Researchers at the Wellcome Trust Sanger Institute in the UK and Los Alamos National Laboratory in the US measured the catastrophic genetic damage caused by smoking in different organs of the body and identified several different mechanisms by which tobacco smoking causes mutations in DNA.
The study provides a direct link between the number of cigarettes smoked in a lifetime and the number of mutations in the tumour DNA.
The highest mutation rates were seen in the lung cancers but tumours in other parts of the body also contained these smoking-associated mutations, explaining how smoking causes many types of human cancer.
Tobacco smoking claims the lives of at least six million people every year and, if current trends continue, the World Health Organisation predicts more than 1 billion tobacco-related deaths in this century.
Smoking has been epidemiologically associated with at least 17 types of human cancer, but until now no-one has seen the mechanisms by which smoking causes many of these cancers.
Cancer is caused by mutations in the DNA of a cell. In the first comprehensive analysis of the DNA of cancers linked to smoking, researchers studied over 5,000 tumours, comparing cancers from smokers with cancers from people who had never smoked.
They found particular molecular fingerprints of DNA damage - called mutational signatures - in the smoker's DNA, and counted how many of these particular mutations were found in the different tumours.
The researchers found that, on average, smoking a pack of cigarettes a day led to 150 mutations in each lung cell every year.
These mutations represent individual potential start points for a cascade of genetic damage that can eventually lead to cancer.
The numbers of mutations within any cancer cell will vary between individuals, but this study shows the additional mutational load caused by tobacco.
"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," said Ludmil Alexandrov from Los Alamos.
"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," said Alexandrov.
Other organs were also affected, with the study showing that a pack a day led to an estimated average 97 mutations in each cell in the larynx, 39 mutations for the pharynx, 23 mutations for mouth, 18 mutations for bladder, and 6 mutations in every cell of the liver each year.
The study appears in the Journal Science.