It's been proven beyond doubt that smoking tobacco is harmful to human health, but a new study has put some figures on how cigarettes cause cancer by altering smokers' DNA.
A team led by Ludmil Alexandrov, at Los Alamos National Laboratory in New Mexico, compared tumor DNA from 2,500 smokers and 1,000 non-smokers to identify which mutations that led to cancer were associated with smoking.
The study, which was published on Thursday, found that for every 50 cigarettes a person smokes, there is on average one DNA mutation per cell in that person's lungs.
"This study offers fresh insights into how tobacco smoke causes cancer," Dr. Alexandrov says. "Our analysis demonstrates that tobacco smoking causes mutations that lead to cancer by multiple distinct mechanisms.
"Tobacco smoking damages DNA in organs directly exposed to smoke as well as speeds up a mutational cellular clock in organs that are both directly and indirectly exposed to smoke."
In plain English, this means that not only does smoking damage organs directly affected by the smoke – such as the lungs and larynx – but can set off a chain reaction that can lead to cells in other organs potentially becoming cancerous.
The laboratory released this image which shows the DNA mutations that can occur to someone who smokes a pack of 20 cigarettes every day not only in the lungs, but other affected organs.
After deciphering these numbers, Dr. Alexandrov and his team will turn their attention to figuring out the chances that one of these smoking-related DNA mutations will turn into cancer, and which mutation types are likely to be more serious.
Smokers are playing 'Russian Roulette'
There is plenty of anecdotal evidence of people smoking all their lives but never contracting cancer, despite the thousands of mutations they've accrued, but Alexandrov, speaking to New Scientist, says that is down to pure luck.
"Smoking is like playing Russian roulette: the more you play, the higher the chance the mutations will hit the right genes and you will develop cancer," he said. "However, there will always be people who smoke a lot but the mutations do not hit the right genes."
He told the magazine that this should dispel the myth that social smoking is harmless, Alexandrov says. He also notes that quitting smoking, while preventing the risk of further mutations, doesn't reverse the ones already imprinted on your DNA.
The most recent Minnesota Adult Tobacco Survey found that in 2014, the state's smoking rate had declined to 14.4 percent – the lowest ever record – which was put down to a combination of higher tobacco prices, smoke-free policies and public health cessation programs.