LONDON • Scientists have found that smoking a pack of cigarettes daily can cause 150 damaging changes to a smoker's lung cells each year.
The findings come from a study of the devastating genetic damage, or mutations, caused by smoking in various organs in the body.
Published in the journal Science on Thursday, the findings showed a direct link between the number of cigarettes smoked in a lifetime and the number of mutations in the DNA of cancerous tumours.
The highest mutation rates were seen in lung cancers, but tumours in other parts of the body - including the bladder, liver and throat - also had smoking-associated mutations, they said. This explains why smoking also causes many other types of cancer beside lung cancer.
Smoking kills six million people a year globally and, if current trends continue, the World Health Organisation predicts more than 1 billion tobacco-related deaths this century.
Cancer is caused by mutations in the DNA - the carrier of genetic information - of a cell. Smoking has been linked with at least 17 types of cancer, but until now scientists were not clear on the mechanisms behind many of them.
Dr Ludmil Alexandrov of Los Alamos National Laboratory in the United States, one of those who carried out the research, explained that in particular, it had until now been difficult to explain how smoking increases the risk of cancer in parts of the body that do not come into direct contact with smoke.
"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."
This study analysed over 5,000 tumours, comparing cancers from smokers with those from people who had never smoked. It found certain molecular fingerprints of DNA damage - called mutational signatures - in the smokers' DNA, and the scientists counted how many of these were in different tumours.
In lung cells, they found that on average, smoking a pack of cigarettes a day led to 150 mutations in each cell every year. Each mutation is a potential start point for a "cascade of genetic damage" that can eventually lead to cancer, they said.